## Event handling overview

Classes: wxEvtHandler, wxWindow, wxEvent

Introduction
How events are processed
Pluggable event handlers
Window identifiers
Event macros summary

### Introduction

Before version 2.0 of wxWindows, events were handled by the application either by supplying callback functions, or by overriding virtual member functions such as OnSize.

From wxWindows 2.0, event tables are used instead, with a few exceptions.

An event table is placed in an implementation file to tell wxWindows how to map events to member functions. These member functions are not virtual functions, but they are all similar in form: they take a single wxEvent-derived argument, and have a void return type.

Here's an example of an event table.

    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()

The first two entries map menu commands to two different member functions. The EVT_SIZE macro doesn't need a window identifier, since normally you are only interested in the current window's size events. (In fact you could intercept a particular window's size event by using EVT_CUSTOM(wxEVT_SIZE, id, func).)

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. In this case, the button's event table will be searched, then the parent panel's, then the frame's.

As mentioned before, the member functions that handle events do not have to be virtual. Indeed, the member functions should not be virtual as the event handler ignores that the functions are virtual, i.e. overriding a virtual member function in a derived class will not have any effect. These member functions take an event argument, and the class of event 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, then specific event classes are used, such as wxTreeEvent for events from wxTreeCtrl windows.

As well as the event table in the implementation file, there must be a DECLARE_EVENT_TABLE macro in the class definition. For example:

    class MyFrame: public wxFrame {

      DECLARE_DYNAMIC_CLASS(MyFrame)

    public:
      ...
      void OnExit(wxCommandEvent& event);
      void OnSize(wxSizeEvent& event);
    protected:
      int       m_count;
      ...
      DECLARE_EVENT_TABLE()
    };

### How events are processed

When an event is received from the windowing system, wxWindows calls wxEvtHandler::ProcessEvent on the first event handler object belonging to the window generating the event.

It may be noted that wxWindows' event processing system implements something very close to virtual methods in normal C++, i.e. it is possible to alter the behaviour of a class by overriding its event handling functions. In many cases this works even for changing the behaviour of native controls. For example it is possible to filter out a number of key events sent by the system to a native text control by overriding wxTextCtrl and defining a handler for key events using EVT_KEY_DOWN. This would indeed prevent any key events from being sent to the native control - which might not be what is desired. In this case the event handler function has to call Skip() so as to indicate that the search for the event handler should continue.

To summarize, instead of explicitly calling the base class version as you would have done with C++ virtual functions (i.e. wxTextCtrl::OnChar()), you should instead call Skip.

In practice, this would look like this if the derived text control only accepts 'a' to 'z' and 'A' to 'Z':

    void MyTextCtrl::OnChar(wxKeyEvent& event)
    {
        if ( isalpha( event.KeyCode() ) )
        {
           // key code is within legal range. we call event.Skip() so the
           // event can be processed either in the base wxWindows class
           // or the native control.

           event.Skip();
        }
        else
        {
           // illegal key hit. we don't call event.Skip() so the
           // event is not processed anywhere else.

           wxBell();
        }
    }

The normal order of event table searching by ProcessEvent is as follows:

1. If the object is disabled (via a call to wxEvtHandler::SetEvtHandlerEnabled) the function skips to step (6).
2. If the object is a wxWindow, ProcessEvent is recursively called on the window's wxValidator. If this returns TRUE, the function exits.
3. SearchEventTable is called for this event handler. If this fails, the base class table is tried, and so on until no more tables exist or an appropriate function was found, in which case the function exits.
4. The search is applied down the entire chain of event handlers (usually the chain has a length of one). If this succeeds, the function exits.
5. If the object is a wxWindow and the event is a wxCommandEvent, ProcessEvent is recursively applied to the parent window's event handler. If this returns TRUE, the function exits.
6. Finally, ProcessEvent is called on the wxApp object.

Pay close attention to Step 5. People often overlook or get confused by this powerful feature of the wxWindows event processing system. To put it a different way, events derived either directly or indirectly from wxCommandEvent will travel up the containment hierarchy from child to parent until an event handler is found that doesn't call event.Skip(). Events not derived from wxCommandEvent are sent only to the window they occurred in and then stop.

Finally, there is another additional complication (which, in fact, simplifies life of wxWindows 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 wxWindows). If you need to specify a different behaviour for some reason, you can use 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.

Note that your application may wish to override ProcessEvent to redirect processing of events. This is done in the document/view framework, for example, to allow event handlers to be defined in the document or view. To test for command events (which will probably be the only events you wish to redirect), you may use wxEvent::IsCommandEvent for efficiency, instead of using the slower run-time type system.

As mentioned above, only command events are recursively applied to the parents event handler. As this quite often causes confusion for users, here is a list of system events which will NOT get sent to the parent's event handler:

wxEvent | The event base class
wxActivateEvent | A window or application activation event
wxCloseEvent | A close window or end session event
wxEraseEvent | An erase background event
wxFocusEvent | A window focus event
wxKeyEvent | A keypress event
wxIdleEvent | An idle event
wxInitDialogEvent | A dialog initialisation event
wxJoystickEvent | A joystick event
wxMenuEvent | A menu event
wxMouseEvent | A mouse event
wxMoveEvent | A move event
wxPaintEvent | A paint event
wxQueryLayoutInfoEvent | Used to query layout information
wxSizeEvent | A size event
wxScrollWinEvent | A scroll event sent by a scrolled window (not a scroll bar)
wxSysColourChangedEvent | A system colour change event
wxUpdateUIEvent | A user interface update 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.

### 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 the same event handler object to handle events from instances of different 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.

### 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 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 -1 to a window constructor, an identifier will be generated for you, but beware: if things don't respond in the way they should, it could be because of an id conflict. It is safer to supply window ids at all times. Automatic generation of identifiers starts at 1 so may well conflict with your own identifiers.

The following standard identifiers are supplied. 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.

    #define wxID_LOWEST             4999

    #define wxID_OPEN               5000
    #define wxID_CLOSE              5001
    #define wxID_NEW                5002
    #define wxID_SAVE               5003
    #define wxID_SAVEAS             5004
    #define wxID_REVERT             5005
    #define wxID_EXIT               5006
    #define wxID_UNDO               5007
    #define wxID_REDO               5008
    #define wxID_HELP               5009
    #define wxID_PRINT              5010
    #define wxID_PRINT_SETUP        5011
    #define wxID_PREVIEW            5012
    #define wxID_ABOUT              5013
    #define wxID_HELP_CONTENTS      5014
    #define wxID_HELP_COMMANDS      5015
    #define wxID_HELP_PROCEDURES    5016
    #define wxID_HELP_CONTEXT       5017

    #define wxID_CUT                5030
    #define wxID_COPY               5031
    #define wxID_PASTE              5032
    #define wxID_CLEAR              5033
    #define wxID_FIND               5034
    #define wxID_DUPLICATE          5035
    #define wxID_SELECTALL          5036

    #define wxID_FILE1              5050
    #define wxID_FILE2              5051
    #define wxID_FILE3              5052
    #define wxID_FILE4              5053
    #define wxID_FILE5              5054
    #define wxID_FILE6              5055
    #define wxID_FILE7              5056
    #define wxID_FILE8              5057
    #define wxID_FILE9              5058

    #define wxID_OK                 5100
    #define wxID_CANCEL             5101
    #define wxID_APPLY              5102
    #define wxID_YES                5103
    #define wxID_NO                 5104
    #define wxID_STATIC             5105

    #define wxID_HIGHEST            5999

### Event macros summary

Generic event table macros

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.

Macros listed by event class

The documentation for specific event macros is organised by event class. Please refer to these sections for details.

wxActivateEvent | The EVT_ACTIVATE and EVT_ACTIVATE_APP macros intercept activation and deactivation events.
wxCommandEvent | A range of commonly-used control events.
wxCloseEvent | The EVT_CLOSE macro handles window closure called via wxWindow::Close.
wxDropFilesEvent | The EVT_DROP_FILES macros handles file drop events.
wxEraseEvent | The EVT_ERASE_BACKGROUND macro is used to handle window erase requests.
wxFocusEvent | The EVT_SET_FOCUS and EVT_KILL_FOCUS macros are used to handle keyboard focus events.
wxKeyEvent | EVT_CHAR, EVT_KEY_DOWN and EVT_KEY_UP macros handle keyboard input for any window.
wxIdleEvent | The EVT_IDLE macro handle application idle events (to process background tasks, for example).
wxInitDialogEvent | The EVT_INIT_DIALOG macro is used to handle dialog initialisation.
wxListEvent | These macros handle wxListCtrl events.
wxMenuEvent | These macros handle special menu events (not menu commands).
wxMouseEvent | Mouse event macros can handle either individual mouse events or all mouse events.
wxMoveEvent | The EVT_MOVE macro is used to handle a window move.
wxPaintEvent | The EVT_PAINT macro is used to handle window paint requests.
wxScrollEvent | These macros are used to handle scroll events from wxScrollBar, wxSlider,and wxSpinButton.
wxSizeEvent | The EVT_SIZE macro is used to handle a window resize.
wxSplitterEvent | The EVT_SPLITTER_SASH_POS_CHANGED, EVT_SPLITTER_UNSPLIT and EVT_SPLITTER_DCLICK macros are used to handle the various splitter window events.
wxSysColourChangedEvent | The EVT_SYS_COLOUR_CHANGED macro is used to handle events informing the application that the user has changed the system colours (Windows only).
wxTreeEvent | These macros handle wxTreeCtrl events.
wxUpdateUIEvent | The EVT_UPDATE_UI macro is used to handle user interface update pseudo-events, which are generated to give the application the chance to update the visual state of menus, toolbars and controls.