X-Git-Url: https://git.saurik.com/wxWidgets.git/blobdiff_plain/8319fb52122dcbdf4c245ba4067ba5afc1f5eda4..7447d53c35249d42128d6243c90998f03882859a:/docs/doxygen/overviews/eventhandling.h?ds=sidebyside diff --git a/docs/doxygen/overviews/eventhandling.h b/docs/doxygen/overviews/eventhandling.h index 5d7385cb1e..10a1c6543a 100644 --- a/docs/doxygen/overviews/eventhandling.h +++ b/docs/doxygen/overviews/eventhandling.h @@ -1,727 +1,882 @@ -///////////////////////////////////////////////////////////////////////////// -// Name: eventhandling.h -// Purpose: topic overview -// Author: wxWidgets team -// RCS-ID: $Id$ -// Licence: wxWindows license -///////////////////////////////////////////////////////////////////////////// - -/** - -@page overview_eventhandling Event Handling - -Classes: wxEvtHandler, wxWindow, wxEvent - -@li @ref overview_eventhandling_introduction -@li @ref overview_eventhandling_eventtables -@li @ref overview_eventhandling_connect -@li @ref overview_eventhandling_processing -@li @ref overview_eventhandling_propagation -@li @ref overview_eventhandling_virtual -@li @ref overview_eventhandling_prog -@li @ref overview_eventhandling_pluggable -@li @ref overview_eventhandling_winid -@li @ref overview_eventhandling_custom -@li @ref overview_eventhandling_macros - - -
- - -@section overview_eventhandling_introduction Introduction - -There are two principal ways to handle events in wxWidgets. One of them uses -event table macros and allows you to define the connection between events -and their handlers only statically, i.e. during program compilation. The other -one uses wxEvtHandler::Connect() call and can be used to connect, and -disconnect, the handlers dynamically, i.e. during run-time depending on some -conditions. It also allows directly connecting the events of one object to a -handler method in another object while the static event tables can only handle -events in the object where they are defined so using Connect() is more flexible -than using the event tables. On the other hand, event tables are more succinct -and centralize all event handlers connection in one place. You can either -choose a single approach which you find preferable or freely combine both -methods in your program in different classes or even in one and the same class, -although this is probably sufficiently confusing to be a bad idea. - -But before you make this choice, let us discuss these two ways in some more -details: in the next section we provide a short introduction to handling the -events using the event tables, please see @ref overview_eventhandling_connect -for the discussion of Connect(). - -@section overview_eventhandling_eventtables Event Handling with Event Tables - -To use an event table you must first decide in which class you wish to -handle the events. The only requirement imposed by wxWidgets is that this class -must derive from wxEvtHandler and so, considering that wxWindow derives from -it, any classes representing windows can handle events. Simple events such as -menu commands are usually processed at the level of a top-level window -containing the menu, so let's suppose that you need to handle some events in @c -MyFrame class deriving from wxFrame. - -First thing to do is to define one or more event handlers. They -are just simple (non-virtual) methods of the class which take as a parameter a -reference to an object of wxEvent-derived class and have no return value (any -return information is passed via the argument, which is why it is non-const). -You also need to insert a macro - -@code -DECLARE_EVENT_TABLE() -@endcode - -somewhere in the class declaration. It doesn't matter where does it occur but -it's customary to put it at the end of it because the macro changes the access -type internally and so it's safest if there is nothing that follows it. So the -full class declaration might look like this: - -@code -class MyFrame : public wxFrame -{ -public: - MyFrame(...) : wxFrame(...) { } - - ... - -protected: - int m_whatever; - -private: - // notice that as the event handlers normally are not called from outside - // the class, they normally be private, in particular they don't need at - // all to be public - void OnExit(wxCommandEvent& event); - void OnButton1(wxCommandEvent& event); - void OnSize(wxSizeEvent& event); - - // it's common to call the event handlers OnSomething() but there is no - // obligation to it, this one is an event handler too: - void DoTest(wxCommandEvent& event); - - DECLARE_EVENT_TABLE() -}; -@endcode - -Next the event table must be defined and, as any definition, it must be placed -in an implementation file to tell. The event table tells wxWidgets how to map -events to member functions and in our example it could look like this: - -@code -BEGIN_EVENT_TABLE(MyFrame, wxFrame) - EVT_MENU(wxID_EXIT, MyFrame::OnExit) - EVT_MENU(DO_TEST, MyFrame::DoTest) - EVT_SIZE(MyFrame::OnSize) - EVT_BUTTON(BUTTON1, MyFrame::OnButton1) -END_EVENT_TABLE() -@endcode - -Notice that you must mention a method you want to use for the event handling in -the event table definition, just defining it in MyFrame class is @e not enough. - -Let us now look at the details of this definition: the first line means that we -are defining the event table for MyFrame class and that its base class is -wxFrame, so events not processed by MyFrame will, by default, be handled to -wxFrame. The next four lines define connections of individual events to their -handlers: the first two of them map menu commands from the items with the -identifiers specified as the first macro parameter to two different member -functions. In the next one, @c EVT_SIZE means that any changes in the size of -the frame will result in calling OnSize() method. Note that this macro doesn't -need a window identifier, since normally you are only interested in the current -window's size events. - -The EVT_BUTTON macro demonstrates that the originating event does not have to -come from the window class implementing the event table -- if the event source -is a button within a panel within a frame, this will still work, because event -tables are searched up through the hierarchy of windows for the command events -(but only command events, so you can't catch mouse move events in a child -control in the parent window in the same way because wxMouseEvent doesn't -derive from wxCommandEvent, see below for how you can do it). In this case, the -button's event table will be searched, then the parent panel's, then the -frame's. - -Finally, you need to implement the event handlers. As mentioned before, all -event handlers take a wxEvent-derived argument whose exact class differs -according to the type of event and the class of the originating window. For -size events, wxSizeEvent is used. For menu commands and most control commands -(such as button presses), wxCommandEvent is used. And when controls get more -complicated, more specific wxCommandEvent-derived event classes providing -additional control-specific information can be used, such as wxTreeEvent for -events from wxTreeCtrl windows. - -In the simplest possible case an event handler may not use the @c event -parameter at all, e.g. - -@code -void MyFrame::OnExit(wxCommandEvent&) -{ - // when the user selects "Exit" from the menu we should close - Close(true); -} -@endcode - -In other cases you may need some information carried by the @c event argument, -as in: - -@code -void MyFrame::OnSize(wxSizeEvent& event) -{ - wxSize size = event.GetSize(); - - ... update the frame using the new size ... -} -@endcode - -You will find the details about the event table macros and the corresponding -wxEvent-derived classes in the discussion of each control generating these -events. - - -@section overview_eventhandling_connect Dynamic Event Handling - -As with the event tables, you need to decide in which class do you intend to -handle the events first and, also as before, this class must still derive from -wxEvtHandler (usually indirectly via wxWindow), see the declaration of MyFrame -in the previous section. However the similarities end here and both the syntax -and the possibilities of this way of handling events in this way are rather -different. - -Let us start by looking at the syntax: the first obvious difference is that you -don't need to use neither @c DECLARE_EVENT_TABLE() nor @c BEGIN_EVENT_TABLE and -associated macros any more. Instead, in any place in your code, but usually in -the code of the class defining the handlers itself (and definitely not in the -global scope as with the event tables), you should call its Connect() method -like this: - -@code -MyFrame::MyFrame(...) -{ - Connect(wxID_EXIT, wxEVT_COMMAND_MENU_SELECTED, - wxCommandEventHandler(MyFrame::OnExit)); -} -@endcode - -This class should be self-explanatory except for wxCommandEventHandler part: -this is a macro which ensures that the method is of correct type by using -static_cast in the same way as event table macros do it inside them. - -Now let us describe the semantic differences: - - -To summarize, using Connect() requires slightly more typing but is much more -flexible than using static event tables so don't hesitate to use it when you -need this extra power. On the other hand, event tables are still perfectly fine -in simple situations where this extra flexibility is not needed. - - -@section overview_eventhandling_processing How Events are Processed - -The previous sections explain how to define event handlers but don't address -the question of how exactly does wxWidgets find the handler to call for the -given event. This section describes the algorithm used to do it in details. - -When an event is received from the windowing system, wxWidgets calls -wxEvtHandler::ProcessEvent() on the first event handler object belonging to the -window generating the event. The normal order of event table searching by -ProcessEvent() is as follows, with the event processing stopping as soon as a -handler is found (unless the handler calls wxEvent::Skip() in which case it -doesn't count as having handled the event): -
    -
  1. - Before anything else happens, wxApp::FilterEvent() is called. If it returns - anything but -1 (default), the event handling stops immediately. -
    2. - -
  2. - If this event handler is disabled via a call to - wxEvtHandler::SetEvtHandlerEnabled() the next three steps are skipped and - the event handler resumes at step (5). - - If the object is a wxWindow and has an associated validator, wxValidator - gets a chance to process the event. -
    3. - -
  3. - The list of dynamically connected event handlers, i.e. those for which - Connect() was called, is consulted. Notice that this is done before - checking the static event table entries, so if both a dynamic and a static - event handler match the same event, the static one is never going to be - used. -
    4. - -
  4. - The event table containing all the handlers defined using the event table - macros in this class and its base classes is examined. Notice that this - means that any event handler defined in a base class will be executed at - this step. -
    5. - -
  5. - The event is passed to the next event handler, if any, in the event handler - chain. This chain can be formed using wxEvtHandler::SetNextHandler() or - wxWindow::PushEventHandler() but usually there is no next event handler and - chaining event handlers using these functions is much less useful now that - Connect() exists so this step will almost never do anything. -
    6. - -
  6. - If the object is a wxWindow and the event is set to set to propagate (by - default only wxCommandEvent-derived events are set to propagate), then the - processing restarts from the step (1) (and excluding the step (7)) for the - parent window. -
    7. - -
  7. - Finally, i.e. if the event is still not processed, the wxApp object itself - gets a last chance to process it. -
    8. -
- -Please pay close attention to step 6!. People often overlook or get -confused by this powerful feature of the wxWidgets event processing system. The -details of event propagation upwards the window hierarchy are described in the -next section. - -Also please notice that there are additional steps in the event handling for -the windows making part of wxWidgets document-view framework, i.e. -wxDocParentFrame, wxDocChildFrame and their MDI equivalents wxDocMDIParentFrame -and wxDocMDIChildFrame. The parent frame classes modify the step (2) above to -send the events received by them to wxDocManager object first. This object, in -turn, sends the event to the current view and the view itself lets its -associated document to process the event first. The child frame classes send -the event directly to the associated view which still forwards it to its -document object. Notice that to avoid remembering the exact order in which the -events are processed in the document-view frame, the simplest, and recommended, -solution is to only handle the events at the view classes level, but not in the -document or document manager classes - - -@section overview_eventhandling_propagation How Events Propagate Upwards - -As mentioned in the previous section, the events of the classes deriving from -wxCommandEvent are propagated by default to the parent window if they are not -processed in this window itself. But although by default only the command -events are propagated like this, other events can be propagated as well because -the event handling code uses wxEvent::ShouldPropagate() to check for whether an -event should be propagated. It is also possible to propagate the event only a -limited number of times and not until it is processed (or a top level parent -window is reached). - -Finally, there is another additional complication (which, in fact, simplifies -life of wxWidgets programmers significantly): when propagating the command -events upwards to the parent window, the event propagation stops when it -reaches the parent dialog, if any. This means that you don't risk to get -unexpected events from the dialog controls (which might be left unprocessed by -the dialog itself because it doesn't care about them) when a modal dialog is -popped up. The events do propagate beyond the frames, however. The rationale -for this choice is that there are only a few frames in a typical application -and their parent-child relation are well understood by the programmer while it -may be very difficult, if not impossible, to track down all the dialogs which -may be popped up in a complex program (remember that some are created -automatically by wxWidgets). If you need to specify a different behaviour for -some reason, you can use wxWindow::SetExtraStyle(wxWS_EX_BLOCK_EVENTS) -explicitly to prevent the events from being propagated beyond the given window -or unset this flag for the dialogs which have it on by default. - -Typically events that deal with a window as a window (size, motion, -paint, mouse, keyboard, etc.) are sent only to the window. Events -that have a higher level of meaning and/or are generated by the window -itself, (button click, menu select, tree expand, etc.) are command -events and are sent up to the parent to see if it is interested in the event. - -As mentioned above, only command events are recursively applied to the parents -event handler in the library itself. As this quite often causes confusion for -users, here is a list of system events which will @em not get sent to the -parent's event handler: - -@li wxEvent: The event base class -@li wxActivateEvent: A window or application activation event -@li wxCloseEvent: A close window or end session event -@li wxEraseEvent: An erase background event -@li wxFocusEvent: A window focus event -@li wxKeyEvent: A keypress event -@li wxIdleEvent: An idle event -@li wxInitDialogEvent: A dialog initialisation event -@li wxJoystickEvent: A joystick event -@li wxMenuEvent: A menu event -@li wxMouseEvent: A mouse event -@li wxMoveEvent: A move event -@li wxPaintEvent: A paint event -@li wxQueryLayoutInfoEvent: Used to query layout information -@li wxSetCursorEvent: Used for special cursor processing based on current mouse position -@li wxSizeEvent: A size event -@li wxScrollWinEvent: A scroll event sent by a scrolled window (not a scroll bar) -@li wxSysColourChangedEvent: A system colour change event - -In some cases, it might be desired by the programmer to get a certain number -of system events in a parent window, for example all key events sent to, but not -used by, the native controls in a dialog. In this case, a special event handler -will have to be written that will override ProcessEvent() in order to pass -all events (or any selection of them) to the parent window. - - -@section overview_eventhandling_virtual Event Handlers vs Virtual Methods - -It may be noted that wxWidgets' event processing system implements something -close to virtual methods in normal C++ in spirit: both of these mechanisms -allow to alter the behaviour of the base class by defining the event handling -functions in the derived classes. - -There is however an important difference between the two mechanisms when you -want to invoke the default behaviour, as implemented by the base class, from a -derived class handler. With the virtual functions, you need to call the base -class function directly and you can do it either in the beginning of the -derived class handler function (to post-process the event) or at its end (to -pre-process the event). With the event handlers, you only have the option of -pre-processing the events and in order to still let the default behaviour to -happen you must call wxEvent::Skip() and @em not call the base class event -handler directly. In fact, the event handler probably doesn't even exist in the -base class as the default behaviour is often implemented in platform-specific -code by the underlying toolkit or OS itself. But even if it does exist at -wxWidgets level, it should never be called directly as the event handler are -not part of wxWidgets API and should never be called directly. - -Finally, please notice that the event handlers themselves shouldn't be virtual. -They should always be non-virtual and usually private (as there is no need to -make them public) methods of a wxEvtHandler-derived class. - - -@section overview_eventhandling_prog User Generated Events vs Programmatically Generated Events - -While generically wxEvents can be generated both by user -actions (e.g. resize of a wxWindow) and by calls to functions -(e.g. wxWindow::SetSize), wxWidgets controls normally send wxCommandEvent-derived -events only for the user-generated events. The only @b exceptions to this rule are: - -@li wxNotebook::AddPage: No event-free alternatives -@li wxNotebook::AdvanceSelection: No event-free alternatives -@li wxNotebook::DeletePage: No event-free alternatives -@li wxNotebook::SetSelection: Use wxNotebook::ChangeSelection instead, as - wxNotebook::SetSelection is deprecated -@li wxTreeCtrl::Delete: No event-free alternatives -@li wxTreeCtrl::DeleteAllItems: No event-free alternatives -@li wxTreeCtrl::EditLabel: No event-free alternatives -@li All wxTextCtrl methods - -wxTextCtrl::ChangeValue can be used instead of wxTextCtrl::SetValue but the other -functions, such as wxTextCtrl::Replace or wxTextCtrl::WriteText don't have event-free -equivalents. - - - -@section overview_eventhandling_pluggable Pluggable Event Handlers - -In fact, you don't have to derive a new class from a window class -if you don't want to. You can derive a new class from wxEvtHandler instead, -defining the appropriate event table, and then call wxWindow::SetEventHandler -(or, preferably, wxWindow::PushEventHandler) to make this -event handler the object that responds to events. This way, you can avoid -a lot of class derivation, and use instances of the same event handler class (but different -objects as the same event handler object shouldn't be used more than once) to -handle events from instances of different widget classes. - -If you ever have to call a window's event handler -manually, use the GetEventHandler function to retrieve the window's event handler and use that -to call the member function. By default, GetEventHandler returns a pointer to the window itself -unless an application has redirected event handling using SetEventHandler or PushEventHandler. - -One use of PushEventHandler is to temporarily or permanently change the -behaviour of the GUI. For example, you might want to invoke a dialog editor -in your application that changes aspects of dialog boxes. You can -grab all the input for an existing dialog box, and edit it 'in situ', -before restoring its behaviour to normal. So even if the application -has derived new classes to customize behaviour, your utility can indulge -in a spot of body-snatching. It could be a useful technique for on-line -tutorials, too, where you take a user through a serious of steps and -don't want them to diverge from the lesson. Here, you can examine the events -coming from buttons and windows, and if acceptable, pass them through to -the original event handler. Use PushEventHandler/PopEventHandler -to form a chain of event handlers, where each handler processes a different -range of events independently from the other handlers. - - - -@section overview_eventhandling_winid Window Identifiers - -Window identifiers are integers, and are used to -uniquely determine window identity in the event system (though you can use it -for other purposes). In fact, identifiers do not need to be unique -across your entire application just so long as they are unique within a -particular context you're interested in, such as a frame and its children. You -may use the @c wxID_OK identifier, for example, on any number of dialogs so -long as you don't have several within the same dialog. - -If you pass @c wxID_ANY to a window constructor, an identifier will be -generated for you automatically by wxWidgets. This is useful when you don't -care about the exact identifier either because you're not going to process the -events from the control being created at all or because you process the events -from all controls in one place (in which case you should specify @c wxID_ANY -in the event table or wxEvtHandler::Connect call -as well. The automatically generated identifiers are always negative and so -will never conflict with the user-specified identifiers which must be always -positive. - -See @ref page_stdevtid for the list of standard identifiers available. -You can use wxID_HIGHEST to determine the number above which it is safe to -define your own identifiers. Or, you can use identifiers below wxID_LOWEST. -Finally, you can allocate identifiers dynamically using wxNewId() function to. -If you use wxNewId() consistently in your application, you can be sure that -the your identifiers don't conflict accidentally. - - -@section overview_eventhandling_custom Custom Event Summary - -@subsection overview_eventhandling_custom_general General approach - -Since version 2.2.x of wxWidgets, each event type is identified by ID which -is given to the event type @e at runtime which makes it possible to add -new event types to the library or application without risking ID clashes -(two different event types mistakingly getting the same event ID). This -event type ID is stored in a struct of type @b const wxEventType. - -In order to define a new event type, there are principally two choices. -One is to define a entirely new event class (typically deriving from -wxEvent or wxCommandEvent. - -The other is to use the existing event classes and give them an new event -type. You'll have to define and declare a new event type using either way, -and this is done using the following macros: - -@code -// in the header of the source file -BEGIN_DECLARE_EVENT_TYPES() -DECLARE_EVENT_TYPE(name, value) -END_DECLARE_EVENT_TYPES() - -// in the implementation -DEFINE_EVENT_TYPE(name) -@endcode - -You can ignore the @e value parameter of the DECLARE_EVENT_TYPE macro -since it is used only for backwards compatibility with wxWidgets 2.0.x based -applications where you have to give the event type ID an explicit value. -See also the @ref page_samples_event for an example of code -defining and working with the custom event types. - - -@subsection overview_eventhandling_custom_existing Using Existing Event Classes - -If you just want to use a wxCommandEvent with -a new event type, you can then use one of the generic event table macros -listed below, without having to define a new macro yourself. This also -has the advantage that you won't have to define a new wxEvent::Clone() -method for posting events between threads etc. This could look like this -in your code: - -@code -DECLARE_EVENT_TYPE(wxEVT_MY_EVENT, -1) -DEFINE_EVENT_TYPE(wxEVT_MY_EVENT) - -// user code intercepting the event - -BEGIN_EVENT_TABLE(MyFrame, wxFrame) -EVT_MENU (wxID_EXIT, MyFrame::OnExit) -// .... -EVT_COMMAND (ID_MY_WINDOW, wxEVT_MY_EVENT, MyFrame::OnMyEvent) -END_EVENT_TABLE() - -void MyFrame::OnMyEvent( wxCommandEvent ) -{ - // do something - wxString text = event.GetText(); -} - - -// user code sending the event - -void MyWindow::SendEvent() -{ - wxCommandEvent event( wxEVT_MY_EVENT, GetId() ); - event.SetEventObject( this ); - // Give it some contents - event.SetText( wxT("Hallo") ); - // Send it - GetEventHandler()->ProcessEvent( event ); -} -@endcode - - -@subsection overview_eventhandling_custom_generic Generic Event Table Macros - -@beginTable -@row2col{EVT_CUSTOM(event\, id\, func), - Allows you to add a custom event table - entry by specifying the event identifier (such as wxEVT_SIZE), - the window identifier, and a member function to call.} -@row2col{EVT_CUSTOM_RANGE(event\, id1\, id2\, func), - The same as EVT_CUSTOM, but responds to a range of window identifiers.} -@row2col{EVT_COMMAND(id\, event\, func), - The same as EVT_CUSTOM, but expects a member function with a - wxCommandEvent argument.} -@row2col{EVT_COMMAND_RANGE(id1\, id2\, event\, func), - The same as EVT_CUSTOM_RANGE, but - expects a member function with a wxCommandEvent argument.} -@row2col{EVT_NOTIFY(event\, id\, func), - The same as EVT_CUSTOM, but - expects a member function with a wxNotifyEvent argument.} -@row2col{EVT_NOTIFY_RANGE(event\, id1\, id2\, func), - The same as EVT_CUSTOM_RANGE, but - expects a member function with a wxNotifyEvent argument.} -@endTable - - -@subsection overview_eventhandling_custom_ownclass Defining Your Own Event Class - -Under certain circumstances, it will be required to define your own event -class e.g. for sending more complex data from one place to another. Apart -from defining your event class, you will also need to define your own -event table macro (which is quite long). Watch out to put in enough -casts to the inherited event function. Here is an example: - -@code -// code defining event - -class wxPlotEvent: public wxNotifyEvent -{ -public: - wxPlotEvent( wxEventType commandType = wxEVT_NULL, int id = 0 ); - - // accessors - wxPlotCurve *GetCurve() - { return m_curve; } - - // required for sending with wxPostEvent() - virtual wxEvent *Clone() const; - -private: - wxPlotCurve *m_curve; -}; - -DECLARE_EVENT_TYPE( wxEVT_PLOT_ACTION, -1 ) - -typedef void (wxEvtHandler::*wxPlotEventFunction)(wxPlotEvent&); - -#define EVT_PLOT(id, fn) \ - DECLARE_EVENT_TABLE_ENTRY( wxEVT_PLOT_ACTION, id, -1, \ - (wxObjectEventFunction) (wxEventFunction) (wxCommandEventFunction) (wxNotifyEventFunction) \ - wxStaticCastEvent( wxPlotEventFunction, &fn ), (wxObject *) NULL ), - - -// code implementing the event type and the event class - -DEFINE_EVENT_TYPE( wxEVT_PLOT_ACTION ) - -wxPlotEvent::wxPlotEvent( ... - - -// user code intercepting the event - -BEGIN_EVENT_TABLE(MyFrame, wxFrame) -EVT_PLOT (ID_MY_WINDOW, MyFrame::OnPlot) -END_EVENT_TABLE() - -void MyFrame::OnPlot( wxPlotEvent &event ) -{ - wxPlotCurve *curve = event.GetCurve(); -} - - -// user code sending the event - -void MyWindow::SendEvent() -{ - wxPlotEvent event( wxEVT_PLOT_ACTION, GetId() ); - event.SetEventObject( this ); - event.SetCurve( m_curve ); - GetEventHandler()->ProcessEvent( event ); -} -@endcode - - -@section overview_eventhandling_macros Event Handling Summary - -For the full list of event classes, please see the -@ref group_class_events "event classes group page". - - -@todo for all controls state clearly when calling a member function results in an - event being generated and when it doesn't (possibly updating also the - 'Events generated by the user vs programmatically generated events' paragraph - of the 'Event handling overview' with the list of the functions which break - that rule). - -*/ - +///////////////////////////////////////////////////////////////////////////// +// Name: eventhandling.h +// Purpose: topic overview +// Author: wxWidgets team +// RCS-ID: $Id$ +// Licence: wxWindows licence +///////////////////////////////////////////////////////////////////////////// + +/** + +@page overview_events Events and Event Handling + +Related classes: wxEvtHandler, wxWindow, wxEvent + +@li @ref overview_events_introduction +@li @ref overview_events_eventhandling +@li @ref overview_events_processing +@li @ref overview_events_custom +@li @ref overview_events_misc + + +
+ + +@section overview_events_introduction Introduction to Events + +Like with all the other GUI frameworks, the control of flow in wxWidgets +applications is event-based: the program normally performs most of its actions +in response to the events generated by the user. These events can be triggered +by using the input devices (such as keyboard, mouse, joystick) directly or, +more commonly, by a standard control which synthesizes such input events into +higher level events: for example, a wxButton can generate a click event when +the user presses the left mouse button on it and then releases it without +pressing @c Esc in the meanwhile. There are also events which don't directly +correspond to the user actions, such as wxTimerEvent or wxSocketEvent. + +But in all cases wxWidgets represents these events in a uniform way and allows +you to handle them in the same way wherever they originate from. And while the +events are normally generated by wxWidgets itself, you can also do this, which +is especially useful when using custom events (see @ref overview_events_custom). + +To be more precise, each event is described by: + - Event type: this is simply a value of type wxEventType which + uniquely identifies the type of the event. For example, clicking on a button, + selecting an item from a list box and pressing a key on the keyboard all + generate events with different event types. + - Event class carried by the event: each event has some information + associated with it and this data is represented by an object of a class + derived from wxEvent. Events of different types can use the same event class, + for example both button click and listbox selection events use wxCommandEvent + class (as do all the other simple control events), but the key press event + uses wxKeyEvent as the information associated with it is different. + - Event source: wxEvent stores the object which generated the event + and, for windows, its identifier (see @ref overview_events_winid). As it is + common to have more than one object generating events of the same type (e.g. a + typical window contains several buttons, all generating the same button click + event), checking the event source object or its id allows to distinguish + between them. + + +@section overview_events_eventhandling Event Handling + +There are two principal ways to handle events in wxWidgets. One of them uses +event table macros and allows you to define the binding between events +and their handlers only statically, i.e., during program compilation. The other +one uses wxEvtHandler::Bind<>() call and can be used to bind and +unbind, the handlers dynamically, i.e. during run-time depending on some +conditions. It also allows the direct binding of events to: +@li A handler method in another object. +@li An ordinary function like a static method or a global function. +@li An arbitrary functor like boost::function<>. + +The static event tables can only handle events in the object where they are +defined so using Bind<>() is more flexible than using the event tables. On the +other hand, event tables are more succinct and centralize all event handler +bindings in one place. You can either choose a single approach that you find +preferable or freely combine both methods in your program in different classes +or even in one and the same class, although this is probably sufficiently +confusing to be a bad idea. + +Also notice that most of the existing wxWidgets tutorials and discussions use +the event tables because they historically preceded the apparition of dynamic +event handling in wxWidgets. But this absolutely doesn't mean that using the +event tables is the preferred way: handling events dynamically is better in +several aspects and you should strongly consider doing it if you are just +starting with wxWidgets. On the other hand, you still need to know about the +event tables if only because you are going to see them in many samples and +examples. + +So before you make the choice between static event tables and dynamically +connecting the event handlers, let us discuss these two ways in more detail. In +the next section we provide a short introduction to handling the events using +the event tables. Please see @ref overview_events_bind for the discussion of +Bind<>(). + +@subsection overview_events_eventtables Event Handling with Event Tables + +To use an event table you must first decide in which class you wish to +handle the events. The only requirement imposed by wxWidgets is that this class +must derive from wxEvtHandler and so, considering that wxWindow derives from +it, any classes representing windows can handle events. Simple events such as +menu commands are usually processed at the level of a top-level window +containing the menu, so let's suppose that you need to handle some events in @c +MyFrame class deriving from wxFrame. + +First define one or more event handlers. They +are just simple methods of the class that take as a parameter a +reference to an object of a wxEvent-derived class and have no return value (any +return information is passed via the argument, which is why it is non-const). +You also need to insert a macro + +@code +wxDECLARE_EVENT_TABLE() +@endcode + +somewhere in the class declaration. It doesn't matter where it appears but +it's customary to put it at the end because the macro changes the access +type internally so it's safest if nothing follows it. The +full class declaration might look like this: + +@code +class MyFrame : public wxFrame +{ +public: + MyFrame(...) : wxFrame(...) { } + + ... + +protected: + int m_whatever; + +private: + // Notice that as the event handlers normally are not called from outside + // the class, they normally are private. In particular they don't need + // to be public. + void OnExit(wxCommandEvent& event); + void OnButton1(wxCommandEvent& event); + void OnSize(wxSizeEvent& event); + + // it's common to call the event handlers OnSomething() but there is no + // obligation to do that; this one is an event handler too: + void DoTest(wxCommandEvent& event); + + wxDECLARE_EVENT_TABLE() +}; +@endcode + +Next the event table must be defined and, as with any definition, it must be +placed in an implementation file. The event table tells wxWidgets how to map +events to member functions and in our example it could look like this: + +@code +wxBEGIN_EVENT_TABLE(MyFrame, wxFrame) + EVT_MENU(wxID_EXIT, MyFrame::OnExit) + EVT_MENU(DO_TEST, MyFrame::DoTest) + EVT_SIZE(MyFrame::OnSize) + EVT_BUTTON(BUTTON1, MyFrame::OnButton1) +wxEND_EVENT_TABLE() +@endcode + +Notice that you must mention a method you want to use for the event handling in +the event table definition; just defining it in MyFrame class is @e not enough. + +Let us now look at the details of this definition: the first line means that we +are defining the event table for MyFrame class and that its base class is +wxFrame, so events not processed by MyFrame will, by default, be handled by +wxFrame. The next four lines define bindings of individual events to their +handlers: the first two of them map menu commands from the items with the +identifiers specified as the first macro parameter to two different member +functions. In the next one, @c EVT_SIZE means that any changes in the size of +the frame will result in calling OnSize() method. Note that this macro doesn't +need a window identifier, since normally you are only interested in the current +window's size events. + +The @c EVT_BUTTON macro demonstrates that the originating event does not have to +come from the window class implementing the event table -- if the event source +is a button within a panel within a frame, this will still work, because event +tables are searched up through the hierarchy of windows for the command events. +(But only command events, so you can't catch mouse move events in a child +control in the parent window in the same way because wxMouseEvent doesn't +derive from wxCommandEvent. See below for how you can do it.) In this case, the +button's event table will be searched, then the parent panel's, then the +frame's. + +Finally, you need to implement the event handlers. As mentioned before, all +event handlers take a wxEvent-derived argument whose exact class differs +according to the type of event and the class of the originating window. For +size events, wxSizeEvent is used. For menu commands and most control commands +(such as button presses), wxCommandEvent is used. When controls get more +complicated, more specific wxCommandEvent-derived event classes providing +additional control-specific information can be used, such as wxTreeEvent for +events from wxTreeCtrl windows. + +In the simplest possible case an event handler may not use the @c event +parameter at all. For example, + +@code +void MyFrame::OnExit(wxCommandEvent& WXUNUSED(event)) +{ + // when the user selects "Exit" from the menu we should close + Close(true); +} +@endcode + +In other cases you may need some information carried by the @c event argument, +as in: + +@code +void MyFrame::OnSize(wxSizeEvent& event) +{ + wxSize size = event.GetSize(); + + ... update the frame using the new size ... +} +@endcode + +You will find the details about the event table macros and the corresponding +wxEvent-derived classes in the discussion of each control generating these +events. + + +@subsection overview_events_bind Dynamic Event Handling + +@see @ref overview_cpp_rtti_disabled + +The possibilities of handling events in this way are rather different. +Let us start by looking at the syntax: the first obvious difference is that you +need not use wxDECLARE_EVENT_TABLE() nor wxBEGIN_EVENT_TABLE() and the +associated macros. Instead, in any place in your code, but usually in +the code of the class defining the handler itself (and definitely not in the +global scope as with the event tables), call its Bind<>() method like this: + +@code +MyFrame::MyFrame(...) +{ + Bind(wxEVT_COMMAND_MENU_SELECTED, &MyFrame::OnExit, this, wxID_EXIT); +} +@endcode + +Note that @c this pointer must be specified here. + +Now let us describe the semantic differences: + + +Let us now look at more examples of how to use different event handlers using +the two overloads of Bind() function: first one for the object methods and the +other one for arbitrary functors (callable objects, including simple functions): + +In addition to using a method of the object generating the event itself, you +can use a method from a completely different object as an event handler: + +@code +void MyFrameHandler::OnFrameExit( wxCommandEvent & ) +{ + // Do something useful. +} + +MyFrameHandler myFrameHandler; + +MyFrame::MyFrame() +{ + Bind( wxEVT_COMMAND_MENU_SELECTED, &MyFrameHandler::OnFrameExit, + &myFrameHandler, wxID_EXIT ); +} +@endcode + +Note that @c MyFrameHandler doesn't need to derive from wxEvtHandler. But +keep in mind that then the lifetime of @c myFrameHandler must be greater than +that of @c MyFrame object -- or at least it needs to be unbound before being +destroyed. + + +To use an ordinary function or a static method as an event handler you would +write something like this: + +@code +void HandleExit( wxCommandEvent & ) +{ + // Do something useful +} + +MyFrame::MyFrame() +{ + Bind( wxEVT_COMMAND_MENU_SELECTED, &HandleExit, wxID_EXIT ); +} +@endcode + +And finally you can bind to an arbitrary functor and use it as an event +handler: + +@code + +struct MyFunctor +{ + void operator()( wxCommandEvent & ) + { + // Do something useful + } +}; + +MyFunctor myFunctor; + +MyFrame::MyFrame() +{ + Bind( wxEVT_COMMAND_MENU_SELECTED, &myFunctor, wxID_EXIT ); +} +@endcode + +A common example of a functor is boost::function<>: + +@code +using namespace boost; + +void MyHandler::OnExit( wxCommandEvent & ) +{ + // Do something useful +} + +MyHandler myHandler; + +MyFrame::MyFrame() +{ + function< void ( wxCommandEvent & ) > exitHandler( bind( &MyHandler::OnExit, &myHandler, _1 )); + + Bind( wxEVT_COMMAND_MENU_SELECTED, exitHandler, wxID_EXIT ); +} +@endcode + + +With the aid of boost::bind<>() you can even use methods or functions which +don't quite have the correct signature: + +@code +void MyHandler::OnExit( int exitCode, wxCommandEvent &, wxString goodByeMessage ) +{ + // Do something useful +} + +MyHandler myHandler; + +MyFrame::MyFrame() +{ + function< void ( wxCommandEvent & ) > exitHandler( + bind( &MyHandler::OnExit, &myHandler, EXIT_FAILURE, _1, "Bye" )); + + Bind( wxEVT_COMMAND_MENU_SELECTED, exitHandler, wxID_EXIT ); +} +@endcode + + +To summarize, using Bind<>() requires slightly more typing but is much more +flexible than using static event tables so don't hesitate to use it when you +need this extra power. On the other hand, event tables are still perfectly fine +in simple situations where this extra flexibility is not needed. + + +@section overview_events_processing How Events are Processed + +The previous sections explain how to define event handlers but don't address +the question of how exactly wxWidgets finds the handler to call for the +given event. This section describes the algorithm used in detail. Notice that +you may want to run the @ref page_samples_event while reading this section and +look at its code and the output when the button which can be used to test the +event handlers execution order is clicked to understand it better. + +When an event is received from the windowing system, wxWidgets calls +wxEvtHandler::ProcessEvent() on the first event handler object belonging to the +window generating the event. The normal order of event table searching by +ProcessEvent() is as follows, with the event processing stopping as soon as a +handler is found (unless the handler calls wxEvent::Skip() in which case it +doesn't count as having handled the event and the search continues): +
    +
  1. + Before anything else happens, wxApp::FilterEvent() is called. If it returns + anything but -1 (default), the event handling stops immediately. +
    2. + +
  2. + If this event handler is disabled via a call to + wxEvtHandler::SetEvtHandlerEnabled() the next three steps are skipped and + the event handler resumes at step (5). +
    3. + +
  3. + If the object is a wxWindow and has an associated validator, wxValidator + gets a chance to process the event. +
    4. + +
  4. + The list of dynamically bound event handlers, i.e., those for which + Bind<>() was called, is consulted. Notice that this is done before + checking the static event table entries, so if both a dynamic and a static + event handler match the same event, the static one is never going to be + used unless wxEvent::Skip() is called in the dynamic one. +
    5. + +
  5. + The event table containing all the handlers defined using the event table + macros in this class and its base classes is examined. Notice that this + means that any event handler defined in a base class will be executed at + this step. +
    6. + +
  6. + The event is passed to the next event handler, if any, in the event handler + chain, i.e., the steps (1) to (4) are done for it. Usually there is no next + event handler so the control passes to the next step but see @ref + overview_events_nexthandler for how the next handler may be defined. +
    7. + +
  7. + If the object is a wxWindow and the event is set to propagate (by default + only wxCommandEvent-derived events are set to propagate), then the + processing restarts from the step (1) (and excluding the step (7)) for the + parent window. If this object is not a window but the next handler exists, + the event is passed to its parent if it is a window. This ensures that in a + common case of (possibly several) non-window event handlers pushed on top + of a window, the event eventually reaches the window parent. +
    8. + +
  8. + Finally, i.e., if the event is still not processed, the wxApp object itself + (which derives from wxEvtHandler) gets a last chance to process it. +
    9. +
+ +Please pay close attention to step 6! People often overlook or get +confused by this powerful feature of the wxWidgets event processing system. The +details of event propagation up the window hierarchy are described in the +next section. + +Also please notice that there are additional steps in the event handling for +the windows-making part of wxWidgets document-view framework, i.e., +wxDocParentFrame, wxDocChildFrame and their MDI equivalents wxDocMDIParentFrame +and wxDocMDIChildFrame. The parent frame classes modify step (2) above to +send the events received by them to wxDocManager object first. This object, in +turn, sends the event to the current view and the view itself lets its +associated document process the event first. The child frame classes send +the event directly to the associated view which still forwards it to its +document object. Notice that to avoid remembering the exact order in which the +events are processed in the document-view frame, the simplest, and recommended, +solution is to only handle the events at the view classes level, and not in the +document or document manager classes + + +@subsection overview_events_propagation How Events Propagate Upwards + +As mentioned above, the events of the classes deriving from wxCommandEvent are +propagated by default to the parent window if they are not processed in this +window itself. But although by default only the command events are propagated +like this, other events can be propagated as well because the event handling +code uses wxEvent::ShouldPropagate() to check whether an event should be +propagated. It is also possible to propagate the event only a limited number of +times and not until it is processed (or a top level parent window is reached). + +Finally, there is another additional complication (which, in fact, simplifies +life of wxWidgets programmers significantly): when propagating the command +events up to the parent window, the event propagation stops when it +reaches the parent dialog, if any. This means that you don't risk getting +unexpected events from the dialog controls (which might be left unprocessed by +the dialog itself because it doesn't care about them) when a modal dialog is +popped up. The events do propagate beyond the frames, however. The rationale +for this choice is that there are only a few frames in a typical application +and their parent-child relation are well understood by the programmer while it +may be difficult, if not impossible, to track down all the dialogs that +may be popped up in a complex program (remember that some are created +automatically by wxWidgets). If you need to specify a different behaviour for +some reason, you can use wxWindow::SetExtraStyle(wxWS_EX_BLOCK_EVENTS) +explicitly to prevent the events from being propagated beyond the given window +or unset this flag for the dialogs that have it on by default. + +Typically events that deal with a window as a window (size, motion, +paint, mouse, keyboard, etc.) are sent only to the window. Events +that have a higher level of meaning or are generated by the window +itself (button click, menu select, tree expand, etc.) are command +events and are sent up to the parent to see if it is interested in the event. +More precisely, as said above, all event classes @b not deriving from wxCommandEvent +(see the wxEvent inheritance map) do @b not propagate upward. + +In some cases, it might be desired by the programmer to get a certain number +of system events in a parent window, for example all key events sent to, but not +used by, the native controls in a dialog. In this case, a special event handler +will have to be written that will override ProcessEvent() in order to pass +all events (or any selection of them) to the parent window. + + +@subsection overview_events_nexthandler Event Handlers Chain + +The step 4 of the event propagation algorithm checks for the next handler in +the event handler chain. This chain can be formed using +wxEvtHandler::SetNextHandler(): + @image html overview_events_chain.png +(referring to the image, if @c A->ProcessEvent is called and it doesn't handle + the event, @c B->ProcessEvent will be called and so on...). + +Additionally, in the case of wxWindow you can build a stack (implemented using +wxEvtHandler double-linked list) using wxWindow::PushEventHandler(): + @image html overview_events_winstack.png +(referring to the image, if @c W->ProcessEvent is called, it immediately calls + @c A->ProcessEvent; if nor @c A nor @c B handle the event, then the wxWindow +itself is used -- i.e. the dynamically bind event handlers and static event +table entries of wxWindow are looked as the last possibility, after all pushed +event handlers were tested). + +By default the chain is empty, i.e. there is no next handler. + + +@section overview_events_custom Custom Event Summary + +@subsection overview_events_custom_general General approach + +As each event is uniquely defined by its event type, defining a custom event +starts with defining a new event type for it. This is done using +wxDEFINE_EVENT() macro. As an event type is a variable, it can also be +declared using wxDECLARE_EVENT() if necessary. + +The next thing to do is to decide whether you need to define a custom event +class for events of this type or if you can reuse an existing class, typically +either wxEvent (which doesn't provide any extra information) or wxCommandEvent +(which contains several extra fields and also propagates upwards by default). +Both strategies are described in details below. See also the @ref +page_samples_event for a complete example of code defining and working with the +custom event types. + +Finally, you will need to generate and post your custom events. +Generation is as simple as instancing your custom event class and initializing +its internal fields. +For posting events to a certain event handler there are two possibilities: +using wxEvtHandler::AddPendingEvent or using wxEvtHandler::QueueEvent. +Basically you will need to use the latter when doing inter-thread communication; +when you use only the main thread you can also safely use the former. +Last, note that there are also two simple global wrapper functions associated +to the two wxEvtHandler mentioned functions: wxPostEvent() and wxQueueEvent(). + + +@subsection overview_events_custom_existing Using Existing Event Classes + +If you just want to use a wxCommandEvent with a new event type, use one of the +generic event table macros listed below, without having to define a new event +class yourself. + +Example: + +@code +// this is typically in a header: it just declares MY_EVENT event type +wxDECLARE_EVENT(MY_EVENT, wxCommandEvent); + +// this is a definition so can't be in a header +wxDEFINE_EVENT(MY_EVENT, wxCommandEvent); + +// example of code handling the event with event tables +wxBEGIN_EVENT_TABLE(MyFrame, wxFrame) + EVT_MENU (wxID_EXIT, MyFrame::OnExit) + ... + EVT_COMMAND (ID_MY_WINDOW, MY_EVENT, MyFrame::OnMyEvent) +wxEND_EVENT_TABLE() + +void MyFrame::OnMyEvent(wxCommandEvent& event) +{ + // do something + wxString text = event.GetString(); +} + +// example of code handling the event with Bind<>(): +MyFrame::MyFrame() +{ + Bind(MY_EVENT, &MyFrame::OnMyEvent, this, ID_MY_WINDOW); +} + +// example of code generating the event +void MyWindow::SendEvent() +{ + wxCommandEvent event(MY_EVENT, GetId()); + event.SetEventObject(this); + + // Give it some contents + event.SetString("Hello"); + + // Do send it + ProcessWindowEvent(event); +} +@endcode + + +@subsection overview_events_custom_ownclass Defining Your Own Event Class + +Under certain circumstances, you must define your own event class e.g., for +sending more complex data from one place to another. Apart from defining your +event class, you also need to define your own event table macro if you want to +use event tables for handling events of this type. + +Here is an example: + +@code +// define a new event class +class MyPlotEvent: public wxEvent +{ +public: + MyPlotEvent(wxEventType eventType, int winid, const wxPoint& pos) + : wxEvent(winid, eventType), + m_pos(pos) + { + } + + // accessors + wxPoint GetPoint() const { return m_pos; } + + // implement the base class pure virtual + virtual wxEvent *Clone() const { return new MyPlotEvent(*this); } + +private: + const wxPoint m_pos; +}; + +// we define a single MY_PLOT_CLICKED event type associated with the class +// above but typically you are going to have more than one event type, e.g. you +// could also have MY_PLOT_ZOOMED or MY_PLOT_PANNED &c -- in which case you +// would just add more similar lines here +wxDEFINE_EVENT(MY_PLOT_CLICKED, MyPlotEvent); + + +// if you want to support old compilers you need to use some ugly macros: +typedef void (wxEvtHandler::*MyPlotEventFunction)(MyPlotEvent&); +#define MyPlotEventHandler(func) wxEVENT_HANDLER_CAST(MyPlotEventFunction, func) + +// if your code is only built using reasonably modern compilers, you could just +// do this instead: +#define MyPlotEventHandler(func) (&func) + +// finally define a macro for creating the event table entries for the new +// event type +// +// remember that you don't need this at all if you only use Bind<>() and that +// you can replace MyPlotEventHandler(func) with just &func unless you use a +// really old compiler +#define MY_EVT_PLOT_CLICK(id, func) \ + wx__DECLARE_EVT1(MY_PLOT_CLICKED, id, MyPlotEventHandler(func)) + + +// example of code handling the event (you will use one of these methods, not +// both, of course): +wxBEGIN_EVENT_TABLE(MyFrame, wxFrame) + EVT_PLOT(ID_MY_WINDOW, MyFrame::OnPlot) +wxEND_EVENT_TABLE() + +MyFrame::MyFrame() +{ + Bind(MY_PLOT_CLICKED, &MyFrame::OnPlot, this, ID_MY_WINDOW); +} + +void MyFrame::OnPlot(MyPlotEvent& event) +{ + ... do something with event.GetPoint() ... +} + + +// example of code generating the event: +void MyWindow::SendEvent() +{ + MyPlotEvent event(MY_PLOT_CLICKED, GetId(), wxPoint(...)); + event.SetEventObject(this); + ProcessWindowEvent(event); +} +@endcode + + + +@section overview_events_misc Miscellaneous Notes + +@subsection overview_events_virtual Event Handlers vs Virtual Methods + +It may be noted that wxWidgets' event processing system implements something +close to virtual methods in normal C++ in spirit: both of these mechanisms +allow you to alter the behaviour of the base class by defining the event handling +functions in the derived classes. + +There is however an important difference between the two mechanisms when you +want to invoke the default behaviour, as implemented by the base class, from a +derived class handler. With the virtual functions, you need to call the base +class function directly and you can do it either in the beginning of the +derived class handler function (to post-process the event) or at its end (to +pre-process the event). With the event handlers, you only have the option of +pre-processing the events and in order to still let the default behaviour +happen you must call wxEvent::Skip() and @em not call the base class event +handler directly. In fact, the event handler probably doesn't even exist in the +base class as the default behaviour is often implemented in platform-specific +code by the underlying toolkit or OS itself. But even if it does exist at +wxWidgets level, it should never be called directly as the event handlers are +not part of wxWidgets API and should never be called directly. + + + +@subsection overview_events_prog User Generated Events vs Programmatically Generated Events + +While generically wxEvents can be generated both by user +actions (e.g., resize of a wxWindow) and by calls to functions +(e.g., wxWindow::SetSize), wxWidgets controls normally send wxCommandEvent-derived +events only for the user-generated events. The only @b exceptions to this rule are: + +@li wxNotebook::AddPage: No event-free alternatives +@li wxNotebook::AdvanceSelection: No event-free alternatives +@li wxNotebook::DeletePage: No event-free alternatives +@li wxNotebook::SetSelection: Use wxNotebook::ChangeSelection instead, as + wxNotebook::SetSelection is deprecated +@li wxTreeCtrl::Delete: No event-free alternatives +@li wxTreeCtrl::DeleteAllItems: No event-free alternatives +@li wxTreeCtrl::EditLabel: No event-free alternatives +@li All wxTextCtrl methods + +wxTextCtrl::ChangeValue can be used instead of wxTextCtrl::SetValue but the other +functions, such as wxTextCtrl::Replace or wxTextCtrl::WriteText don't have event-free +equivalents. + + + +@subsection overview_events_pluggable Pluggable Event Handlers + +TODO: Probably deprecated, Bind() provides a better way to do this + +In fact, you don't have to derive a new class from a window class +if you don't want to. You can derive a new class from wxEvtHandler instead, +defining the appropriate event table, and then call wxWindow::SetEventHandler +(or, preferably, wxWindow::PushEventHandler) to make this +event handler the object that responds to events. This way, you can avoid +a lot of class derivation, and use instances of the same event handler class (but different +objects as the same event handler object shouldn't be used more than once) to +handle events from instances of different widget classes. + +If you ever have to call a window's event handler +manually, use the GetEventHandler function to retrieve the window's event handler and use that +to call the member function. By default, GetEventHandler returns a pointer to the window itself +unless an application has redirected event handling using SetEventHandler or PushEventHandler. + +One use of PushEventHandler is to temporarily or permanently change the +behaviour of the GUI. For example, you might want to invoke a dialog editor +in your application that changes aspects of dialog boxes. You can +grab all the input for an existing dialog box, and edit it 'in situ', +before restoring its behaviour to normal. So even if the application +has derived new classes to customize behaviour, your utility can indulge +in a spot of body-snatching. It could be a useful technique for on-line +tutorials, too, where you take a user through a serious of steps and +don't want them to diverge from the lesson. Here, you can examine the events +coming from buttons and windows, and if acceptable, pass them through to +the original event handler. Use PushEventHandler/PopEventHandler +to form a chain of event handlers, where each handler processes a different +range of events independently from the other handlers. + + + +@subsection overview_events_winid Window Identifiers + +Window identifiers are integers, and are used to +uniquely determine window identity in the event system (though you can use it +for other purposes). In fact, identifiers do not need to be unique +across your entire application as long they are unique within the +particular context you're interested in, such as a frame and its children. You +may use the @c wxID_OK identifier, for example, on any number of dialogs +as long as you don't have several within the same dialog. + +If you pass @c wxID_ANY to a window constructor, an identifier will be +generated for you automatically by wxWidgets. This is useful when you don't +care about the exact identifier either because you're not going to process the +events from the control being created or because you process the events +from all controls in one place (in which case you should specify @c wxID_ANY +in the event table or wxEvtHandler::Bind call +as well). The automatically generated identifiers are always negative and so +will never conflict with the user-specified identifiers which must be always +positive. + +See @ref page_stdevtid for the list of standard identifiers available. +You can use wxID_HIGHEST to determine the number above which it is safe to +define your own identifiers. Or, you can use identifiers below wxID_LOWEST. +Finally, you can allocate identifiers dynamically using wxNewId() function too. +If you use wxNewId() consistently in your application, you can be sure that +your identifiers don't conflict accidentally. + + +@subsection overview_events_custom_generic Generic Event Table Macros + +@beginTable +@row2col{EVT_CUSTOM(event\, id\, func), + Allows you to add a custom event table + entry by specifying the event identifier (such as wxEVT_SIZE), + the window identifier, and a member function to call.} +@row2col{EVT_CUSTOM_RANGE(event\, id1\, id2\, func), + The same as EVT_CUSTOM, but responds to a range of window identifiers.} +@row2col{EVT_COMMAND(id\, event\, func), + The same as EVT_CUSTOM, but expects a member function with a + wxCommandEvent argument.} +@row2col{EVT_COMMAND_RANGE(id1\, id2\, event\, func), + The same as EVT_CUSTOM_RANGE, but + expects a member function with a wxCommandEvent argument.} +@row2col{EVT_NOTIFY(event\, id\, func), + The same as EVT_CUSTOM, but + expects a member function with a wxNotifyEvent argument.} +@row2col{EVT_NOTIFY_RANGE(event\, id1\, id2\, func), + The same as EVT_CUSTOM_RANGE, but + expects a member function with a wxNotifyEvent argument.} +@endTable + + + +@subsection overview_events_list List of wxWidgets events + +For the full list of event classes, please see the +@ref group_class_events "event classes group page". + + +*/ +