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.

ymasuda 平成17年11月19日