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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 Custom Event Summary).
To be more precise, each event is described by:
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 Dynamic Event Handling for the discussion of Bind<>().
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
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:
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() };
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:
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()
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 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, 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. 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 event
parameter at all. For example,
void MyFrame::OnExit(wxCommandEvent& WXUNUSED(event)) { // when the user selects "Exit" from the menu we should close Close(true); }
In other cases you may need some information carried by the event
argument, as in:
void MyFrame::OnSize(wxSizeEvent& event) { wxSize size = event.GetSize(); ... update the frame using the new size ... }
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.
MyFrame::MyFrame(...)
{
Bind(wxEVT_COMMAND_MENU_SELECTED, &MyFrame::OnExit, this, wxID_EXIT);
}
Note that this
pointer must be specified here.
Now let us describe the semantic differences:
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).
However writing
MyFrame::MyFrame(...)
{
m_child->Bind(wxEVT_LEAVE_WINDOW, &MyFrame::OnMouseLeave, this);
}
event
argument passed to the event handler.
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:
void MyFrameHandler::OnFrameExit( wxCommandEvent & ) { // Do something useful. } MyFrameHandler myFrameHandler; MyFrame::MyFrame() { Bind( wxEVT_COMMAND_MENU_SELECTED, &MyFrameHandler::OnFrameExit, &myFrameHandler, wxID_EXIT ); }
Note that MyFrameHandler
doesn't need to derive from wxEvtHandler. But keep in mind that then the lifetime of myFrameHandler
must be greater than that of 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:
void HandleExit( wxCommandEvent & ) { // Do something useful } MyFrame::MyFrame() { Bind( wxEVT_COMMAND_MENU_SELECTED, &HandleExit, wxID_EXIT ); }
And finally you can bind to an arbitrary functor and use it as an event handler:
struct MyFunctor { void operator()( wxCommandEvent & ) { // Do something useful } }; MyFunctor myFunctor; MyFrame::MyFrame() { Bind( wxEVT_COMMAND_MENU_SELECTED, &myFunctor, wxID_EXIT ); }
A common example of a functor is boost::function<>:
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 ); }
With the aid of boost::bind<>() you can even use methods or functions which don't quite have the correct signature:
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 ); }
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.
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):
A->ProcessEvent
is called and it doesn't handle the event, 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(): W->ProcessEvent
is called, it immediately calls A->ProcessEvent
; if nor A
nor 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.
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
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 not deriving from wxCommandEvent (see the wxEvent inheritance map) do 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.
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 Event Sample for a complete example of code defining and working with the custom event types.
Example:
// 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); }
Here is an example:
// 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): BEGIN_EVENT_TABLE(MyFrame, wxFrame) EVT_PLOT(ID_MY_WINDOW, MyFrame::OnPlot) END_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); }
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 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.
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.
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 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 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 Standard event identifiers 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.
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. |
EVT_CUSTOM_RANGE(event, id1, id2, func) | The same as EVT_CUSTOM, but responds to a range of window identifiers. |
EVT_COMMAND(id, event, func) | The same as EVT_CUSTOM, but expects a member function with a wxCommandEvent argument. |
EVT_COMMAND_RANGE(id1, id2, event, func) | The same as EVT_CUSTOM_RANGE, but expects a member function with a wxCommandEvent argument. |
EVT_NOTIFY(event, id, func) | The same as EVT_CUSTOM, but expects a member function with a wxNotifyEvent argument. |
EVT_NOTIFY_RANGE(event, id1, id2, func) | The same as EVT_CUSTOM_RANGE, but expects a member function with a wxNotifyEvent argument. |
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