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git.saurik.com Git - wxWidgets.git/blob - docs/doxygen/overviews/eventhandling.h
1 /////////////////////////////////////////////////////////////////////////////
2 // Name: eventhandling.h
3 // Purpose: topic overview
4 // Author: wxWidgets team
6 // Licence: wxWindows license
7 /////////////////////////////////////////////////////////////////////////////
11 @page overview_events Events and Event Handling
13 Related classes: wxEvtHandler, wxWindow, wxEvent
15 @li @ref overview_events_introduction
16 @li @ref overview_events_eventhandling
17 @li @ref overview_events_processing
18 @li @ref overview_events_custom
19 @li @ref overview_events_misc
25 @section overview_events_introduction Introduction to Events
27 Like with all the other GUI frameworks, the control of flow in wxWidgets
28 applications is event-based: the program normally performs most of its actions
29 in response to the events generated by the user. These events can be triggered
30 by using the input devices (such as keyboard, mouse, joystick) directly or,
31 more commonly, by a standard control which synthesizes such input events into
32 higher level events: for example, a wxButton can generate a click event when
33 the user presses the left mouse button on it and then releases it without
34 pressing @c Esc in the meanwhile. There are also events which don't directly
35 correspond to the user actions, such as wxTimerEvent or wxSocketEvent.
37 But in all cases wxWidgets represents these events in a uniform way and allows
38 you to handle them in the same way wherever they originate from. And while the
39 events are normally generated by wxWidgets itself, you can also do this, which
40 is especially useful when using custom events (see @ref overview_events_custom).
42 To be more precise, each event is described by:
43 - <em>Event type</em>: this is simply a value of type wxEventType which
44 uniquely identifies the type of the event. For example, clicking on a button,
45 selecting an item from a list box and pressing a key on the keyboard all
46 generate events with different event types.
47 - <em>Event class</em> carried by the event: each event has some information
48 associated with it and this data is represented by an object of a class
49 derived from wxEvent. Events of different types can use the same event class,
50 for example both button click and listbox selection events use wxCommandEvent
51 class (as do all the other simple control events), but the key press event
52 uses wxKeyEvent as the information associated with it is different.
53 - <em>Event source</em>: wxEvent stores the object which generated the event
54 and, for windows, its identifier (see @ref overview_events_winid). As it is
55 common to have more than one object generating events of the same type (e.g. a
56 typical window contains several buttons, all generating the same button click
57 event), checking the event source object or its id allows to distinguish
61 @section overview_events_eventhandling Event Handling
63 There are two principal ways to handle events in wxWidgets. One of them uses
64 <em>event table</em> macros and allows you to define the binding between events
65 and their handlers only statically, i.e., during program compilation. The other
66 one uses wxEvtHandler::Bind<>() call and can be used to bind and
67 unbind, the handlers dynamically, i.e. during run-time depending on some
68 conditions. It also allows the direct binding of events to:
69 @li A handler method in another object.
70 @li An ordinary function like a static method or a global function.
71 @li An arbitrary functor like boost::function<>.
73 The static event tables can only handle events in the object where they are
74 defined so using Bind<>() is more flexible than using the event tables. On the
75 other hand, event tables are more succinct and centralize all event handler
76 bindings in one place. You can either choose a single approach that you find
77 preferable or freely combine both methods in your program in different classes
78 or even in one and the same class, although this is probably sufficiently
79 confusing to be a bad idea.
81 Also notice that most of the existing wxWidgets tutorials and discussions use
82 the event tables because they historically preceded the apparition of dynamic
83 event handling in wxWidgets. But this absolutely doesn't mean that using the
84 event tables is the preferred way: handling events dynamically is better in
85 several aspects and you should strongly consider doing it if you are just
86 starting with wxWidgets. On the other hand, you still need to know about the
87 event tables if only because you are going to see them in many samples and
90 So before you make the choice between static event tables and dynamically
91 connecting the event handlers, let us discuss these two ways in more detail. In
92 the next section we provide a short introduction to handling the events using
93 the event tables. Please see @ref overview_events_bind for the discussion of
96 @subsection overview_events_eventtables Event Handling with Event Tables
98 To use an <em>event table</em> you must first decide in which class you wish to
99 handle the events. The only requirement imposed by wxWidgets is that this class
100 must derive from wxEvtHandler and so, considering that wxWindow derives from
101 it, any classes representing windows can handle events. Simple events such as
102 menu commands are usually processed at the level of a top-level window
103 containing the menu, so let's suppose that you need to handle some events in @c
104 MyFrame class deriving from wxFrame.
106 First define one or more <em>event handlers</em>. They
107 are just simple methods of the class that take as a parameter a
108 reference to an object of a wxEvent-derived class and have no return value (any
109 return information is passed via the argument, which is why it is non-const).
110 You also need to insert a macro
113 wxDECLARE_EVENT_TABLE()
116 somewhere in the class declaration. It doesn't matter where it appears but
117 it's customary to put it at the end because the macro changes the access
118 type internally so it's safest if nothing follows it. The
119 full class declaration might look like this:
122 class MyFrame : public wxFrame
125 MyFrame(...) : wxFrame(...) { }
133 // Notice that as the event handlers normally are not called from outside
134 // the class, they normally are private. In particular they don't need
136 void OnExit(wxCommandEvent& event);
137 void OnButton1(wxCommandEvent& event);
138 void OnSize(wxSizeEvent& event);
140 // it's common to call the event handlers OnSomething() but there is no
141 // obligation to do that; this one is an event handler too:
142 void DoTest(wxCommandEvent& event);
144 DECLARE_EVENT_TABLE()
148 Next the event table must be defined and, as with any definition, it must be
149 placed in an implementation file. The event table tells wxWidgets how to map
150 events to member functions and in our example it could look like this:
153 wxBEGIN_EVENT_TABLE(MyFrame, wxFrame)
154 EVT_MENU(wxID_EXIT, MyFrame::OnExit)
155 EVT_MENU(DO_TEST, MyFrame::DoTest)
156 EVT_SIZE(MyFrame::OnSize)
157 EVT_BUTTON(BUTTON1, MyFrame::OnButton1)
161 Notice that you must mention a method you want to use for the event handling in
162 the event table definition; just defining it in MyFrame class is @e not enough.
164 Let us now look at the details of this definition: the first line means that we
165 are defining the event table for MyFrame class and that its base class is
166 wxFrame, so events not processed by MyFrame will, by default, be handled by
167 wxFrame. The next four lines define bindings of individual events to their
168 handlers: the first two of them map menu commands from the items with the
169 identifiers specified as the first macro parameter to two different member
170 functions. In the next one, @c EVT_SIZE means that any changes in the size of
171 the frame will result in calling OnSize() method. Note that this macro doesn't
172 need a window identifier, since normally you are only interested in the current
173 window's size events.
175 The @c EVT_BUTTON macro demonstrates that the originating event does not have to
176 come from the window class implementing the event table -- if the event source
177 is a button within a panel within a frame, this will still work, because event
178 tables are searched up through the hierarchy of windows for the command events.
179 (But only command events, so you can't catch mouse move events in a child
180 control in the parent window in the same way because wxMouseEvent doesn't
181 derive from wxCommandEvent. See below for how you can do it.) In this case, the
182 button's event table will be searched, then the parent panel's, then the
185 Finally, you need to implement the event handlers. As mentioned before, all
186 event handlers take a wxEvent-derived argument whose exact class differs
187 according to the type of event and the class of the originating window. For
188 size events, wxSizeEvent is used. For menu commands and most control commands
189 (such as button presses), wxCommandEvent is used. When controls get more
190 complicated, more specific wxCommandEvent-derived event classes providing
191 additional control-specific information can be used, such as wxTreeEvent for
192 events from wxTreeCtrl windows.
194 In the simplest possible case an event handler may not use the @c event
195 parameter at all. For example,
198 void MyFrame::OnExit(wxCommandEvent& WXUNUSED(event))
200 // when the user selects "Exit" from the menu we should close
205 In other cases you may need some information carried by the @c event argument,
209 void MyFrame::OnSize(wxSizeEvent& event)
211 wxSize size = event.GetSize();
213 ... update the frame using the new size ...
217 You will find the details about the event table macros and the corresponding
218 wxEvent-derived classes in the discussion of each control generating these
222 @subsection overview_events_bind Dynamic Event Handling
224 @see @ref overview_cpp_rtti_disabled
226 The possibilities of handling events in this way are rather different.
227 Let us start by looking at the syntax: the first obvious difference is that you
228 need not use DECLARE_EVENT_TABLE() nor BEGIN_EVENT_TABLE() and the
229 associated macros. Instead, in any place in your code, but usually in
230 the code of the class defining the handler itself (and definitely not in the
231 global scope as with the event tables), call its Bind<>() method like this:
234 MyFrame::MyFrame(...)
236 Bind(wxEVT_COMMAND_MENU_SELECTED, &MyFrame::OnExit, this, wxID_EXIT);
240 Note that @c this pointer must be specified here.
242 Now let us describe the semantic differences:
245 Event handlers can be bound at any moment. For example, it's possible
246 to do some initialization first and only bind the handlers if and when
247 it succeeds. This can avoid the need to test that the object was properly
248 initialized in the event handlers themselves. With Bind<>() they
249 simply won't be called if it wasn't correctly initialized.
253 As a slight extension of the above, the handlers can also be unbound at
254 any time with Unbind<>() (and maybe rebound later). Of course,
255 it's also possible to emulate this behaviour with the classic
256 static (i.e., bound via event tables) handlers by using an internal
257 flag indicating whether the handler is currently enabled and returning
258 from it if it isn't, but using dynamically bind handlers requires
259 less code and is also usually more clear.
263 Almost last but very, very far from least is the increased flexibility
264 which allows to bind an event to:
265 @li A method in another object.
266 @li An ordinary function like a static method or a global function.
267 @li An arbitrary functor like boost::function<>.
269 This is impossible to do with the event tables because it is not
270 possible to specify these handlers to dispatch the event to, so it
271 necessarily needs to be sent to the same object which generated the
272 event. Not so with Bind<>() which can be used to specify these handlers
273 which will handle the event. To give a quick example, a common question
274 is how to receive the mouse movement events happening when the mouse is
275 in one of the frame children in the frame itself. Doing it in a naive
279 A @c EVT_LEAVE_WINDOW(MyFrame::OnMouseLeave) line in the frame
280 event table has no effect as mouse move (including entering and
281 leaving) events are not propagated up to the parent window
282 (at least not by default).
286 Putting the same line in a child event table will crash during
287 run-time because the MyFrame method will be called on a wrong
288 object -- it's easy to convince oneself that the only object
289 that can be used here is the pointer to the child, as
290 wxWidgets has nothing else. But calling a frame method with the
291 child window pointer instead of the pointer to the frame is, of
298 MyFrame::MyFrame(...)
300 m_child->Bind(wxEVT_LEAVE_WINDOW, &MyFrame::OnMouseLeave, this);
303 will work exactly as expected. Note that you can get the object that
304 generated the event -- and that is not the same as the frame -- via
305 wxEvent::GetEventObject() method of @c event argument passed to the
310 Really last point is the consequence of the previous one: because of
311 increased flexibility of Bind(), it is also safer as it is impossible
312 to accidentally use a method of another class. Instead of run-time
313 crashes you will get compilation errors in this case when using Bind().
317 Let us now look at more examples of how to use different event handlers using
318 the two overloads of Bind() function: first one for the object methods and the
319 other one for arbitrary functors (callable objects, including simple functions):
321 In addition to using a method of the object generating the event itself, you
322 can use a method from a completely different object as an event handler:
325 void MyFrameHandler::OnFrameExit( wxCommandEvent & )
327 // Do something useful.
330 MyFrameHandler myFrameHandler;
334 Bind( wxEVT_COMMAND_MENU_SELECTED, &MyFrameHandler::OnFrameExit,
335 &myFrameHandler, wxID_EXIT );
339 Note that @c MyFrameHandler doesn't need to derive from wxEvtHandler. But
340 keep in mind that then the lifetime of @c myFrameHandler must be greater than
341 that of @c MyFrame object -- or at least it needs to be unbound before being
345 To use an ordinary function or a static method as an event handler you would
346 write something like this:
349 void HandleExit( wxCommandEvent & )
351 // Do something useful
356 Bind( wxEVT_COMMAND_MENU_SELECTED, &HandleExit, wxID_EXIT );
360 And finally you can bind to an arbitrary functor and use it as an event
367 void operator()( wxCommandEvent & )
369 // Do something useful
377 Bind( wxEVT_COMMAND_MENU_SELECTED, &myFunctor, wxID_EXIT );
381 A common example of a functor is boost::function<>:
384 using namespace boost;
386 void MyHandler::OnExit( wxCommandEvent & )
388 // Do something useful
395 function< void ( wxCommandEvent & ) > exitHandler( bind( &MyHandler::OnExit, &myHandler, _1 ));
397 Bind( wxEVT_COMMAND_MENU_SELECTED, exitHandler, wxID_EXIT );
402 With the aid of boost::bind<>() you can even use methods or functions which
403 don't quite have the correct signature:
406 void MyHandler::OnExit( int exitCode, wxCommandEvent &, wxString goodByeMessage )
408 // Do something useful
415 function< void ( wxCommandEvent & ) > exitHandler(
416 bind( &MyHandler::OnExit, &myHandler, EXIT_FAILURE, _1, "Bye" ));
418 Bind( wxEVT_COMMAND_MENU_SELECTED, exitHandler, wxID_EXIT );
423 To summarize, using Bind<>() requires slightly more typing but is much more
424 flexible than using static event tables so don't hesitate to use it when you
425 need this extra power. On the other hand, event tables are still perfectly fine
426 in simple situations where this extra flexibility is not needed.
429 @section overview_events_processing How Events are Processed
431 The previous sections explain how to define event handlers but don't address
432 the question of how exactly wxWidgets finds the handler to call for the
433 given event. This section describes the algorithm used in detail. Notice that
434 you may want to run the @ref page_samples_event while reading this section and
435 look at its code and the output when the button which can be used to test the
436 event handlers execution order is clicked to understand it better.
438 When an event is received from the windowing system, wxWidgets calls
439 wxEvtHandler::ProcessEvent() on the first event handler object belonging to the
440 window generating the event. The normal order of event table searching by
441 ProcessEvent() is as follows, with the event processing stopping as soon as a
442 handler is found (unless the handler calls wxEvent::Skip() in which case it
443 doesn't count as having handled the event and the search continues):
446 Before anything else happens, wxApp::FilterEvent() is called. If it returns
447 anything but -1 (default), the event handling stops immediately.
451 If this event handler is disabled via a call to
452 wxEvtHandler::SetEvtHandlerEnabled() the next three steps are skipped and
453 the event handler resumes at step (5).
457 If the object is a wxWindow and has an associated validator, wxValidator
458 gets a chance to process the event.
462 The list of dynamically bound event handlers, i.e., those for which
463 Bind<>() was called, is consulted. Notice that this is done before
464 checking the static event table entries, so if both a dynamic and a static
465 event handler match the same event, the static one is never going to be
466 used unless wxEvent::Skip() is called in the dynamic one.
470 The event table containing all the handlers defined using the event table
471 macros in this class and its base classes is examined. Notice that this
472 means that any event handler defined in a base class will be executed at
477 The event is passed to the next event handler, if any, in the event handler
478 chain, i.e., the steps (1) to (4) are done for it. Usually there is no next
479 event handler so the control passes to the next step but see @ref
480 overview_events_nexthandler for how the next handler may be defined.
484 If the object is a wxWindow and the event is set to propagate (by default
485 only wxCommandEvent-derived events are set to propagate), then the
486 processing restarts from the step (1) (and excluding the step (7)) for the
487 parent window. If this object is not a window but the next handler exists,
488 the event is passed to its parent if it is a window. This ensures that in a
489 common case of (possibly several) non-window event handlers pushed on top
490 of a window, the event eventually reaches the window parent.
494 Finally, i.e., if the event is still not processed, the wxApp object itself
495 (which derives from wxEvtHandler) gets a last chance to process it.
499 <em>Please pay close attention to step 6!</em> People often overlook or get
500 confused by this powerful feature of the wxWidgets event processing system. The
501 details of event propagation up the window hierarchy are described in the
504 Also please notice that there are additional steps in the event handling for
505 the windows-making part of wxWidgets document-view framework, i.e.,
506 wxDocParentFrame, wxDocChildFrame and their MDI equivalents wxDocMDIParentFrame
507 and wxDocMDIChildFrame. The parent frame classes modify step (2) above to
508 send the events received by them to wxDocManager object first. This object, in
509 turn, sends the event to the current view and the view itself lets its
510 associated document process the event first. The child frame classes send
511 the event directly to the associated view which still forwards it to its
512 document object. Notice that to avoid remembering the exact order in which the
513 events are processed in the document-view frame, the simplest, and recommended,
514 solution is to only handle the events at the view classes level, and not in the
515 document or document manager classes
518 @subsection overview_events_propagation How Events Propagate Upwards
520 As mentioned above, the events of the classes deriving from wxCommandEvent are
521 propagated by default to the parent window if they are not processed in this
522 window itself. But although by default only the command events are propagated
523 like this, other events can be propagated as well because the event handling
524 code uses wxEvent::ShouldPropagate() to check whether an event should be
525 propagated. It is also possible to propagate the event only a limited number of
526 times and not until it is processed (or a top level parent window is reached).
528 Finally, there is another additional complication (which, in fact, simplifies
529 life of wxWidgets programmers significantly): when propagating the command
530 events up to the parent window, the event propagation stops when it
531 reaches the parent dialog, if any. This means that you don't risk getting
532 unexpected events from the dialog controls (which might be left unprocessed by
533 the dialog itself because it doesn't care about them) when a modal dialog is
534 popped up. The events do propagate beyond the frames, however. The rationale
535 for this choice is that there are only a few frames in a typical application
536 and their parent-child relation are well understood by the programmer while it
537 may be difficult, if not impossible, to track down all the dialogs that
538 may be popped up in a complex program (remember that some are created
539 automatically by wxWidgets). If you need to specify a different behaviour for
540 some reason, you can use <tt>wxWindow::SetExtraStyle(wxWS_EX_BLOCK_EVENTS)</tt>
541 explicitly to prevent the events from being propagated beyond the given window
542 or unset this flag for the dialogs that have it on by default.
544 Typically events that deal with a window as a window (size, motion,
545 paint, mouse, keyboard, etc.) are sent only to the window. Events
546 that have a higher level of meaning or are generated by the window
547 itself (button click, menu select, tree expand, etc.) are command
548 events and are sent up to the parent to see if it is interested in the event.
549 More precisely, as said above, all event classes @b not deriving from wxCommandEvent
550 (see the wxEvent inheritance map) do @b not propagate upward.
552 In some cases, it might be desired by the programmer to get a certain number
553 of system events in a parent window, for example all key events sent to, but not
554 used by, the native controls in a dialog. In this case, a special event handler
555 will have to be written that will override ProcessEvent() in order to pass
556 all events (or any selection of them) to the parent window.
559 @subsection overview_events_nexthandler Event Handlers Chain
561 The step 4 of the event propagation algorithm checks for the next handler in
562 the event handler chain. This chain can be formed using
563 wxEvtHandler::SetNextHandler():
564 @image html overview_events_chain.png
565 (referring to the image, if @c A->ProcessEvent is called and it doesn't handle
566 the event, @c B->ProcessEvent will be called and so on...).
568 Additionally, in the case of wxWindow you can build a stack (implemented using
569 wxEvtHandler double-linked list) using wxWindow::PushEventHandler():
570 @image html overview_events_winstack.png
571 (referring to the image, if @c W->ProcessEvent is called, it immediately calls
572 @c A->ProcessEvent; if nor @c A nor @c B handle the event, then the wxWindow
573 itself is used -- i.e. the dynamically bind event handlers and static event
574 table entries of wxWindow are looked as the last possibility, after all pushed
575 event handlers were tested).
577 By default the chain is empty, i.e. there is no next handler.
580 @section overview_events_custom Custom Event Summary
582 @subsection overview_events_custom_general General approach
584 As each event is uniquely defined by its event type, defining a custom event
585 starts with defining a new event type for it. This is done using
586 wxDEFINE_EVENT() macro. As an event type is a variable, it can also be
587 declared using wxDECLARE_EVENT() if necessary.
589 The next thing to do is to decide whether you need to define a custom event
590 class for events of this type or if you can reuse an existing class, typically
591 either wxEvent (which doesn't provide any extra information) or wxCommandEvent
592 (which contains several extra fields and also propagates upwards by default).
593 Both strategies are described in details below. See also the @ref
594 page_samples_event for a complete example of code defining and working with the
597 Finally, you will need to generate and post your custom events.
598 Generation is as simple as instancing your custom event class and initializing
600 For posting events to a certain event handler there are two possibilities:
601 using wxEvtHandler::AddPendingEvent or using wxEvtHandler::QueueEvent.
602 Basically you will need to use the latter when doing inter-thread communication;
603 when you use only the main thread you can also safely use the former.
604 Last, note that there are also two simple global wrapper functions associated
605 to the two wxEvtHandler mentioned functions: wxPostEvent() and wxQueueEvent().
608 @subsection overview_events_custom_existing Using Existing Event Classes
610 If you just want to use a wxCommandEvent with a new event type, use one of the
611 generic event table macros listed below, without having to define a new event
617 // this is typically in a header: it just declares MY_EVENT event type
618 wxDECLARE_EVENT(MY_EVENT, wxCommandEvent);
620 // this is a definition so can't be in a header
621 wxDEFINE_EVENT(MY_EVENT, wxCommandEvent);
623 // example of code handling the event with event tables
624 BEGIN_EVENT_TABLE(MyFrame, wxFrame)
625 EVT_MENU (wxID_EXIT, MyFrame::OnExit)
627 EVT_COMMAND (ID_MY_WINDOW, MY_EVENT, MyFrame::OnMyEvent)
630 void MyFrame::OnMyEvent(wxCommandEvent& event)
633 wxString text = event.GetText();
636 // example of code handling the event with Bind<>():
639 Bind(MY_EVENT, &MyFrame::OnMyEvent, this, ID_MY_WINDOW);
642 // example of code generating the event
643 void MyWindow::SendEvent()
645 wxCommandEvent event(MY_EVENT, GetId());
646 event.SetEventObject(this);
648 // Give it some contents
649 event.SetText("Hello");
652 ProcessWindowEvent(event);
657 @subsection overview_events_custom_ownclass Defining Your Own Event Class
659 Under certain circumstances, you must define your own event class e.g., for
660 sending more complex data from one place to another. Apart from defining your
661 event class, you also need to define your own event table macro if you want to
662 use event tables for handling events of this type.
667 // define a new event class
668 class MyPlotEvent: public wxEvent
671 MyPlotEvent(wxEventType eventType, int winid, const wxPoint& pos)
672 : wxEvent(winid, eventType),
678 wxPoint GetPoint() const { return m_pos; }
680 // implement the base class pure virtual
681 virtual wxEvent *Clone() const { return new MyPlotEvent(*this); }
687 // we define a single MY_PLOT_CLICKED event type associated with the class
688 // above but typically you are going to have more than one event type, e.g. you
689 // could also have MY_PLOT_ZOOMED or MY_PLOT_PANNED &c -- in which case you
690 // would just add more similar lines here
691 wxDEFINE_EVENT(MY_PLOT_CLICKED, MyPlotEvent);
694 // if you want to support old compilers you need to use some ugly macros:
695 typedef void (wxEvtHandler::*MyPlotEventFunction)(MyPlotEvent&);
696 #define MyPlotEventHandler(func) wxEVENT_HANDLER_CAST(MyPlotEventFunction, func)
698 // if your code is only built using reasonably modern compilers, you could just
700 #define MyPlotEventHandler(func) (&func)
702 // finally define a macro for creating the event table entries for the new
705 // remember that you don't need this at all if you only use Bind<>() and that
706 // you can replace MyPlotEventHandler(func) with just &func unless you use a
707 // really old compiler
708 #define MY_EVT_PLOT_CLICK(id, func) \
709 wx__DECLARE_EVT1(MY_PLOT_CLICKED, id, MyPlotEventHandler(func))
712 // example of code handling the event (you will use one of these methods, not
714 BEGIN_EVENT_TABLE(MyFrame, wxFrame)
715 EVT_PLOT(ID_MY_WINDOW, MyFrame::OnPlot)
720 Bind(MY_PLOT_CLICKED, &MyFrame::OnPlot, this, ID_MY_WINDOW);
723 void MyFrame::OnPlot(MyPlotEvent& event)
725 ... do something with event.GetPoint() ...
729 // example of code generating the event:
730 void MyWindow::SendEvent()
732 MyPlotEvent event(MY_PLOT_CLICKED, GetId(), wxPoint(...));
733 event.SetEventObject(this);
734 ProcessWindowEvent(event);
740 @section overview_events_misc Miscellaneous Notes
742 @subsection overview_events_virtual Event Handlers vs Virtual Methods
744 It may be noted that wxWidgets' event processing system implements something
745 close to virtual methods in normal C++ in spirit: both of these mechanisms
746 allow you to alter the behaviour of the base class by defining the event handling
747 functions in the derived classes.
749 There is however an important difference between the two mechanisms when you
750 want to invoke the default behaviour, as implemented by the base class, from a
751 derived class handler. With the virtual functions, you need to call the base
752 class function directly and you can do it either in the beginning of the
753 derived class handler function (to post-process the event) or at its end (to
754 pre-process the event). With the event handlers, you only have the option of
755 pre-processing the events and in order to still let the default behaviour
756 happen you must call wxEvent::Skip() and @em not call the base class event
757 handler directly. In fact, the event handler probably doesn't even exist in the
758 base class as the default behaviour is often implemented in platform-specific
759 code by the underlying toolkit or OS itself. But even if it does exist at
760 wxWidgets level, it should never be called directly as the event handlers are
761 not part of wxWidgets API and should never be called directly.
765 @subsection overview_events_prog User Generated Events vs Programmatically Generated Events
767 While generically wxEvents can be generated both by user
768 actions (e.g., resize of a wxWindow) and by calls to functions
769 (e.g., wxWindow::SetSize), wxWidgets controls normally send wxCommandEvent-derived
770 events only for the user-generated events. The only @b exceptions to this rule are:
772 @li wxNotebook::AddPage: No event-free alternatives
773 @li wxNotebook::AdvanceSelection: No event-free alternatives
774 @li wxNotebook::DeletePage: No event-free alternatives
775 @li wxNotebook::SetSelection: Use wxNotebook::ChangeSelection instead, as
776 wxNotebook::SetSelection is deprecated
777 @li wxTreeCtrl::Delete: No event-free alternatives
778 @li wxTreeCtrl::DeleteAllItems: No event-free alternatives
779 @li wxTreeCtrl::EditLabel: No event-free alternatives
780 @li All wxTextCtrl methods
782 wxTextCtrl::ChangeValue can be used instead of wxTextCtrl::SetValue but the other
783 functions, such as wxTextCtrl::Replace or wxTextCtrl::WriteText don't have event-free
788 @subsection overview_events_pluggable Pluggable Event Handlers
790 <em>TODO: Probably deprecated, Bind() provides a better way to do this</em>
792 In fact, you don't have to derive a new class from a window class
793 if you don't want to. You can derive a new class from wxEvtHandler instead,
794 defining the appropriate event table, and then call wxWindow::SetEventHandler
795 (or, preferably, wxWindow::PushEventHandler) to make this
796 event handler the object that responds to events. This way, you can avoid
797 a lot of class derivation, and use instances of the same event handler class (but different
798 objects as the same event handler object shouldn't be used more than once) to
799 handle events from instances of different widget classes.
801 If you ever have to call a window's event handler
802 manually, use the GetEventHandler function to retrieve the window's event handler and use that
803 to call the member function. By default, GetEventHandler returns a pointer to the window itself
804 unless an application has redirected event handling using SetEventHandler or PushEventHandler.
806 One use of PushEventHandler is to temporarily or permanently change the
807 behaviour of the GUI. For example, you might want to invoke a dialog editor
808 in your application that changes aspects of dialog boxes. You can
809 grab all the input for an existing dialog box, and edit it 'in situ',
810 before restoring its behaviour to normal. So even if the application
811 has derived new classes to customize behaviour, your utility can indulge
812 in a spot of body-snatching. It could be a useful technique for on-line
813 tutorials, too, where you take a user through a serious of steps and
814 don't want them to diverge from the lesson. Here, you can examine the events
815 coming from buttons and windows, and if acceptable, pass them through to
816 the original event handler. Use PushEventHandler/PopEventHandler
817 to form a chain of event handlers, where each handler processes a different
818 range of events independently from the other handlers.
822 @subsection overview_events_winid Window Identifiers
824 Window identifiers are integers, and are used to
825 uniquely determine window identity in the event system (though you can use it
826 for other purposes). In fact, identifiers do not need to be unique
827 across your entire application as long they are unique within the
828 particular context you're interested in, such as a frame and its children. You
829 may use the @c wxID_OK identifier, for example, on any number of dialogs
830 as long as you don't have several within the same dialog.
832 If you pass @c wxID_ANY to a window constructor, an identifier will be
833 generated for you automatically by wxWidgets. This is useful when you don't
834 care about the exact identifier either because you're not going to process the
835 events from the control being created or because you process the events
836 from all controls in one place (in which case you should specify @c wxID_ANY
837 in the event table or wxEvtHandler::Bind call
838 as well). The automatically generated identifiers are always negative and so
839 will never conflict with the user-specified identifiers which must be always
842 See @ref page_stdevtid for the list of standard identifiers available.
843 You can use wxID_HIGHEST to determine the number above which it is safe to
844 define your own identifiers. Or, you can use identifiers below wxID_LOWEST.
845 Finally, you can allocate identifiers dynamically using wxNewId() function too.
846 If you use wxNewId() consistently in your application, you can be sure that
847 your identifiers don't conflict accidentally.
850 @subsection overview_events_custom_generic Generic Event Table Macros
853 @row2col{EVT_CUSTOM(event\, id\, func),
854 Allows you to add a custom event table
855 entry by specifying the event identifier (such as wxEVT_SIZE),
856 the window identifier, and a member function to call.}
857 @row2col{EVT_CUSTOM_RANGE(event\, id1\, id2\, func),
858 The same as EVT_CUSTOM, but responds to a range of window identifiers.}
859 @row2col{EVT_COMMAND(id\, event\, func),
860 The same as EVT_CUSTOM, but expects a member function with a
861 wxCommandEvent argument.}
862 @row2col{EVT_COMMAND_RANGE(id1\, id2\, event\, func),
863 The same as EVT_CUSTOM_RANGE, but
864 expects a member function with a wxCommandEvent argument.}
865 @row2col{EVT_NOTIFY(event\, id\, func),
866 The same as EVT_CUSTOM, but
867 expects a member function with a wxNotifyEvent argument.}
868 @row2col{EVT_NOTIFY_RANGE(event\, id1\, id2\, func),
869 The same as EVT_CUSTOM_RANGE, but
870 expects a member function with a wxNotifyEvent argument.}
875 @subsection overview_events_list List of wxWidgets events
877 For the full list of event classes, please see the
878 @ref group_class_events "event classes group page".