rest_substitutions/overviews/events_overview.rst

.. include:: headings.inc


.. _events and event handling:

==============================================
|phoenix_title|  **Events and Event Handling**
==============================================


Like with all the other GUI frameworks, the control of flow in
wxPython 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 :ref:`wx.Button` can generate a click event when
the user presses the left mouse button on it and then releases it
without pressing ``Esc`` in the meanwhile. There are also events which
don't directly correspond to the user actions, such as
:ref:`wx.TimerEvent`.

But in all cases wxPython 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 wxPython itself,
you can also do this, which is especially useful when using custom
events (see :ref:`Custom Event Summary <custom event summary>`).

To be more precise, each event is described by:

- **Event type**: this is simply a value of type `EventType` 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.

- **Event class**: 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 :ref:`wx.Event`. Events of different types can
  use the same event class, for example both button click and listbox
  selection events use :ref:`wx.CommandEvent` class (as do all the
  other simple control events), but the key press event uses
  :ref:`wx.KeyEvent` as the information associated with it is
  different.

- **Event source**: :ref:`wx.Event` stores the object which generated the
  event and, for windows, its identifier (see :ref:`Window Identifiers
  <window identifiers>`). 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.


.. _event handling:

Event Handling
--------------

There is one principal way to handle events in wxPython, which uses
the :meth:`wx.EvtHandler.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:

- A handler method in the same or another object.
- An ordinary function like a static method or a global function.
- An arbitrary callable object.


.. _dynamic event handling:

Dynamic Event Handling
----------------------

Let us start by looking at the syntax: in any place in your code, but
usually in the code of the class defining the handler itself, call its
``Bind()`` method like this::

	class MyFrame(wx.Frame):
	    def __init__(self, parent):
	        wx.Frame.__init__(self, parent)

                # Other initialization code...

                self.Bind(wx.EVT_MENU, self.OnExit, id=wx.ID_EXIT)


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.

As a slight extension of the above, the handlers can also be unbound
at any time with :meth:`wx.EvtHandler.Unbind` (and maybe rebound
later).

Almost last but very, very far from least is the flexibility which
allows to bind an event to:

- A method in another object.
- An ordinary function like a static method or a global function.
- An arbitrary function.


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::

	def OnFrameExit(event):
	    # Do something useful.
	    pass

	class MyFrame(wx.Frame):
	    def __init__(self, parent):
	        wx.Frame.__init__(self, parent)

                # Other initialization code...

                self.Bind(wx.EVT_MENU, OnFrameExit, id=wx.ID_EXIT)


Note that `MyFrameHandler` doesn't need to derive from
:ref:`wx.EvtHandler`.


.. _how events are processed:

How Events are Processed
------------------------

The previous sections explain how to define event handlers but don't
address the question of how exactly wxPython 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, wxPython calls
:meth:`wx.EvtHandler.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 :meth:`wx.Event.Skip` in which case it doesn't count as
having handled the event and the search continues):

1. Before anything else happens, :meth:`wx.AppConsole.FilterEvent` is
   called. If it returns anything but -1 (default), the event handling
   stops immediately.

2. If this event handler is disabled via a call to
   :meth:`wx.EvtHandler.SetEvtHandlerEnabled` the next three steps are
   skipped and the event handler resumes at step (5).

3. If the object is a :ref:`wx.Window` and has an associated validator,
   :ref:`wx.Validator` gets a chance to process the event.

4. The list of dynamically bound event handlers, i.e., those for which
   ``Bind()`` was called, is consulted.

5. 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.

6. 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. Usually
   there is no next event handler so the control passes to the next
   step but see :ref:`Event Handlers Chain <event handlers chain>` for
   how the next handler may be defined.

7. If the object is a :ref:`wx.Window` and the event is set to
   propagate (by default only event types derived from
   :ref:`wx.CommandEvent` 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.

8. Finally, i.e., if the event is still not processed, the
   :ref:`wx.App` object itself (which derives from
   :ref:`wx.EvtHandler`) gets a last chance to process it.


**Please pay close attention to step 6!** People often overlook or get
confused by this powerful feature of the wxPython event processing
system. The details of event propagation up the window hierarchy are
described in the next section.


.. _how events propagate upwards:

How Events Propagate Upwards
^^^^^^^^^^^^^^^^^^^^^^^^^^^^

As mentioned above, the events of the classes deriving from
:ref:`wx.CommandEvent` 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
:meth:`wx.Event.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 wxPython 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 wxPython).  If you need to specify a
different behaviour for some reason, you can use
:meth:`wx.Window.SetExtraStyle` (``wx.WS_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 :ref:`wx.CommandEvent` (see the :ref:`wx.Event` inheritance
diagram) 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.


.. _event handlers chain:

Event Handlers Chain
^^^^^^^^^^^^^^^^^^^^

Step 4 of the event propagation algorithm checks for the next
handler in the event handler chain. This chain can be formed using
:meth:`wx.EvtHandler.SetNextHandler`:

.. figure:: _static/images/overviews/overview_events_chain.png
   :align: center

|

(Referring to the image, if ``A.ProcessEvent`` is called and it doesn't
handle the event, ``B.ProcessEvent`` will be called and so on...).

Additionally, in the case of :ref:`wx.Window` you can build a stack
(implemented using :ref:`wx.EvtHandler` double-linked list) using
:meth:`wx.Window.PushEventHandler`:

.. figure:: _static/images/overviews/overview_events_winstack.png
   :align: center

|


(Referring to the image, if ``W.ProcessEvent`` is called, it
immediately calls ``A.ProcessEvent``; if nor A nor B handle the event,
then the :ref:`wx.Window` itself is used -- i.e. the dynamically bind
event handlers and static event table entries of :ref:`wx.Window` are
looked as the last possibility, after all pushed event handlers were
tested).

By default the chain is empty, i.e. there is no next handler.


.. _custom event summary:

Custom Event Summary
--------------------

General approach
^^^^^^^^^^^^^^^^

Custom event classes allow you to create more polished-seeming
controls by allowing the control's user to process updates without
needing to sub-class the control. However, to effectively use events,
you normally need to create custom event classes.

This recipe gives you some boilerplate code for creating your own
custom event classes::

	import wx
	import wx.lib.newevent

	SomeNewEvent, EVT_SOME_NEW_EVENT = wx.lib.newevent.NewEvent()
	SomeNewCommandEvent, EVT_SOME_NEW_COMMAND_EVENT = wx.lib.newevent.NewCommandEvent()


You can bind the events normally via either binding syntax::

	self.Bind(EVT_SOME_NEW_EVENT, self.handler)
	EVT_SOME_NEW_EVENT(self, self.handler)


You can also attach arbitrary data to the event during its creation,
then post it to whatever window you choose::

	    # Create the event
	    evt = SomeNewEvent(attr1="hello", attr2=654)
	    # Post the event
	    wx.PostEvent(target, evt)


When handling events with such arbitrary data, you can fetch the data
via attributes, named the same as the names passed in during the event
instance creation. That is, given the two keyword arguments passed to
``SomeNewEvent`` above::


	    def handler(self, evt):
		# Given the above constructed event, the following is true
		evt.attr1 == "hello"
		evt.attr2 == 654


Miscellaneous Notes
-------------------


.. _user generated events vs programmatically generated events:

User Generated Events vs Programmatically Generated Events
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

While generically a :ref:`wx.Event` can be generated both by user actions
(e.g., resize of a :ref:`wx.Window`) and by calls to functions (e.g.,
:meth:`wx.Window.SetSize`), wxPython controls normally send
:ref:`wx.CommandEvent` -derived events only for the user-generated
events. The only exceptions to this rule are:

- :meth:`wx.BookCtrlBase.AddPage` No event-free alternatives

- :meth:`wx.BookCtrlBase.AdvanceSelection` No event-free alternatives

- :meth:`wx.BookCtrlBase.DeletePage` No event-free alternatives

- :meth:`wx.Notebook.SetSelection`: Use :meth:`wx.Notebook.ChangeSelection`
  instead, as :meth:`wx.Notebook.SetSelection` is deprecated

- :meth:`wx.TreeCtrl.Delete`: No event-free alternatives

- :meth:`wx.TreeCtrl.DeleteAllItems`: No event-free alternatives

- :meth:`wx.TreeCtrl.EditLabel`: No event-free alternatives

- All :ref:`wx.TextCtrl` methods


:meth:`wx.TextEntry.ChangeValue` can be used instead of
:meth:`wx.TextEntry.SetValue` but the other functions, such as
:meth:`wx.TextEntry.Replace` or :meth:`wx.TextCtrl.WriteText` don't
have event-free equivalents.


.. _window identifiers:

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 as long they are unique within the particular
context you're interested in, such as a frame and its children. You
may use the ``wx.ID_OK`` identifier, for example, on any number of
dialogs as long as you don't have several within the same dialog.

If you pass ``wx.ID_ANY`` or -1 to a window constructor, an identifier
will be generated for you automatically by wxPython. 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 ``wx.ID_ANY`` in the
:meth:`wx.EvtHandler.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.

.. seealso:: See :ref:`Standard event identifiers <standard event identifiers>`
   for the list of standard identifiers available.


You can use ``wx.ID_HIGHEST`` to determine the number above which it is
safe to define your own identifiers. Or, you can use identifiers below
``wx.ID_LOWEST``.  Finally, you can allocate identifiers dynamically
using :func:`wx.NewIdRef` function too. If you use :func:`wx.NewIdRef`
consistently in your application, you can be sure that your
identifiers don't conflict accidentally.