+/**
+
+@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
+
+
+<hr>
+
+
+@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:
+ - <em>Event type</em>: 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.
+ - <em>Event class</em> 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.
+ - <em>Event source</em>: 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
+<em>event table</em> 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 <em>event table</em> 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 <em>event handlers</em>. 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
+DECLARE_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);
+
+ DECLARE_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
+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 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
+
+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 DECLARE_EVENT_TABLE() nor BEGIN_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:
+<ul>
+ <li>
+ Event handlers can be bound at any moment. For example, it's possible
+ to do some initialization first and only bind the handlers if and when
+ it succeeds. This can avoid the need to test that the object was properly
+ initialized in the event handlers themselves. With Bind<>() they
+ simply won't be called if it wasn't correctly initialized.
+ </li>
+
+ <li>
+ As a slight extension of the above, the handlers can also be unbound at
+ any time with Unbind<>() (and maybe rebound later). Of course,
+ it's also possible to emulate this behaviour with the classic
+ static (i.e., bound via event tables) handlers by using an internal
+ flag indicating whether the handler is currently enabled and returning
+ from it if it isn't, but using dynamically bind handlers requires
+ less code and is also usually more clear.
+ </li>
+
+ <li>
+ Almost last but very, very far from least is the increased flexibility
+ which allows to bind an event to:
+ @li A method in another object.
+ @li An ordinary function like a static method or a global function.
+ @li An arbitrary functor like boost::function<>.
+
+ This is impossible to do with the event tables because it is not
+ possible to specify these handlers to dispatch the event to, so it
+ necessarily needs to be sent to the same object which generated the
+ event. Not so with Bind<>() which can be used to specify these handlers
+ which will handle the event. To give a quick example, a common question
+ is how to receive the mouse movement events happening when the mouse is
+ in one of the frame children in the frame itself. Doing it in a naive
+ way doesn't work:
+ <ul>
+ <li>
+ A @c EVT_LEAVE_WINDOW(MyFrame::OnMouseLeave) line in the frame
+ event table has no effect as mouse move (including entering and
+ leaving) events are not propagated up to the parent window
+ (at least not by default).
+ </li>
+
+ <li>
+ Putting the same line in a child event table will crash during
+ run-time because the MyFrame method will be called on a wrong
+ object -- it's easy to convince oneself that the only object
+ that can be used here is the pointer to the child, as
+ wxWidgets has nothing else. But calling a frame method with the
+ child window pointer instead of the pointer to the frame is, of
+ course, disastrous.
+ </li>
+ </ul>
+
+ However writing
+ @code
+ MyFrame::MyFrame(...)
+ {
+ m_child->Bind(wxEVT_LEAVE_WINDOW, &MyFrame::OnMouseLeave, this);
+ }
+ @endcode
+ will work exactly as expected. Note that you can get the object that
+ generated the event -- and that is not the same as the frame -- via
+ wxEvent::GetEventObject() method of @c event argument passed to the
+ event handler.
+ </li>
+
+ <li>
+ Really last point is the consequence of the previous one: because of
+ increased flexibility of Bind(), it is also safer as it is impossible
+ to accidentally use a method of another class. Instead of run-time
+ crashes you will get compilation errors in this case when using Bind().
+ </li>
+</ul>
+
+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.
+
+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):
+<ol>
+ <li value="0">
+ Before anything else happens, wxApp::FilterEvent() is called. If it returns
+ anything but -1 (default), the event handling stops immediately.
+ </li>
+
+ <li value="1">
+ 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).
+ </li>
+
+ <li value="2">
+ If the object is a wxWindow and has an associated validator, wxValidator
+ gets a chance to process the event.
+ </li>
+
+ <li value="3">
+ The list of dynamically bind 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.
+ </li>
+
+ <li value="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.
+ </li>
+
+ <li value="5">
+ 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. 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...).
+ 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).
+ Note however that usually there are no wxEvtHandler chains nor wxWindows stacks
+ so this step will usually do anything.
+ </li>
+
+ <li value="6">
+ 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.
+ </li>
+
+ <li value="7">
+ 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.
+ </li>
+</ol>
+
+<em>Please pay close attention to step 6!</em> 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.
+
+
+@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.
+
+
+@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
+BEGIN_EVENT_TABLE(MyFrame, wxFrame)
+ EVT_MENU (wxID_EXIT, MyFrame::OnExit)
+ ...
+ EVT_COMMAND (ID_MY_WINDOW, MY_EVENT, MyFrame::OnMyEvent)
+END_EVENT_TABLE()
+
+void MyFrame::OnMyEvent(wxCommandEvent& event)
+{
+ // do something
+ wxString text = event.GetText();
+}
+
+// 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.SetText("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;
+};
projects / wxWidgets.git / blobdiff