23.2. Event Handling
The GTK signals we have already discussed are for high-level actions, such as a menu item being selected. However, sometimes it is useful to learn about lower-level occurrences, such as the mouse being moved, or a key being pressed. There are also GTK signals corresponding to these low-level events. The handlers for these signals have an extra parameter which is a GdkEvent object containing information about the event. For instance, motion event handlers are passed a GdkEvent object containing EventMotion information which has (in part) attributes like:
type window time x y ... state ... |
window is the window in which the event occurred.
x and y give the coordinates of the event.
type will be set to the event
type, in this case
MOTION_NOTIFY.
The types are defined in GDK.py:
NOTHING a special code to indicate a null event. DELETE the window manager has requested that the toplevel window be hidden or destroyed, usually when the user clicks on a special icon in the title bar. DESTROY the window has been destroyed. EXPOSE all or part of the window has become visible and needs to be redrawn. MOTION_NOTIFY the pointer (usually a mouse) has moved. BUTTON_PRESS a mouse button has been pressed. _2BUTTON_PRESS a mouse button has been double-clicked (clicked twice within a short period of time). Note that each click also generates a BUTTON_PRESS event. _3BUTTON_PRESS a mouse button has been clicked 3 times in a short period of time. Note that each click also generates a BUTTON_PRESS event. BUTTON_RELEASE a mouse button has been released. KEY_PRESS a key has been pressed. KEY_RELEASE a key has been released. ENTER_NOTIFY the pointer has entered the window. LEAVE_NOTIFY the pointer has left the window. FOCUS_CHANGE the keyboard focus has entered or left the window. CONFIGURE the size, position or stacking order of the window has changed. Note that GTK discards these events for GDK_WINDOW_CHILD windows. MAP the window has been mapped. UNMAP the window has been unmapped. PROPERTY_NOTIFY a property on the window has been changed or deleted. SELECTION_CLEAR the application has lost ownership of a selection. SELECTION_REQUEST another application has requested a selection. SELECTION_NOTIFY a selection has been received. PROXIMITY_IN an input device has moved into contact with a sensing surface (e.g. a touchscreen or graphics tablet). PROXIMITY_OUT an input device has moved out of contact with a sensing surface. DRAG_ENTER the mouse has entered the window while a drag is in progress. DRAG_LEAVE the mouse has left the window while a drag is in progress. DRAG_MOTION the mouse has moved in the window while a drag is in progress. DRAG_STATUS the status of the drag operation initiated by the window has changed. DROP_START a drop operation onto the window has started. DROP_FINISHED the drop operation initiated by the window has completed. CLIENT_EVENT a message has been received from another application. VISIBILITY_NOTIFY the window visibility status has changed. NO_EXPOSE indicates that the source region was completely available when parts of a drawable were copied. This is not very useful. |
state specifies the modifier
state when the event occurred (that is, it specifies which modifier keys
and mouse buttons were pressed). It is the bitwise OR of some of the following
(defined in GDK.py):
SHIFT_MASK LOCK_MASK CONTROL_MASK MOD1_MASK MOD2_MASK MOD3_MASK MOD4_MASK MOD5_MASK BUTTON1_MASK BUTTON2_MASK BUTTON3_MASK BUTTON4_MASK BUTTON5_MASK |
As for other signals, to determine what
happens when an event occurs we call the connect() method.
But we also need to let GTK know which events we want to be notified about.
To do this, we call the method:
widget.set_events(events) |
The events
argument specifies the events we are interested in. It is the bitwise OR
of constants that specify different types of events. For future reference,
the event types (defined in GDK.py) are:
EXPOSURE_MASK POINTER_MOTION_MASK POINTER_MOTION_HINT_MASK BUTTON_MOTION_MASK BUTTON1_MOTION_MASK BUTTON2_MOTION_MASK BUTTON3_MOTION_MASK BUTTON_PRESS_MASK BUTTON_RELEASE_MASK KEY_PRESS_MASK KEY_RELEASE_MASK ENTER_NOTIFY_MASK LEAVE_NOTIFY_MASK FOCUS_CHANGE_MASK STRUCTURE_MASK PROPERTY_CHANGE_MASK VISIBILITY_NOTIFY_MASK PROXIMITY_IN_MASK PROXIMITY_OUT_MASK SUBSTRUCTURE_MASK |
There are a few subtle points that have
to be observed when calling the set_events()
method. First, it must be called before the X window for a PyGTK widget
is created. In practical terms, this means you should call it immediately
after creating the widget. Second, the widget must be one which will be
realized with an associated X window. For efficiency, many widget types
do not have their own window, but draw in their parent's window. These
widgets include:
GtkAlignment GtkArrow GtkBin GtkBox GtkImage GtkItem GtkLabel GtkLayout GtkPixmap GtkScrolledWindow GtkSeparator GtkTable GtkAspectFrame GtkFrame GtkVBox GtkHBox GtkVSeparator GtkHSeparator |
To capture events for these widgets, you need to use an EventBox widget. See the section on the EventBox widget for details.
The event attributes that are set by PyGTK for each type of event are:
every event type window send_event NOTHING DELETE DESTROY # no additional attributes EXPOSE area count MOTION_NOTIFY time x y pressure xtilt ytilt state is_hint source deviceid x_root y_root BUTTON_PRESS _2BUTTON_PRESS _3BUTTON_PRESS BUTTON_RELEASE time x y pressure xtilt ytilt state button source deviceid x_root y_root KEY_PRESS KEY_RELEASE time state keyval string ENTER_NOTIFY LEAVE_NOTIFY subwindow time x y x_root y_root mode detail focus state FOCUS_CHANGE _in CONFIGURE x y width height MAP UNMAP # no additional attributes PROPERTY_NOTIFY atom time state SELECTION_CLEAR SELECTION_REQUEST SELECTION_NOTIFY selection target property requestor time PROXIMITY_IN PROXIMITY_OUT time source deviceid DRAG_ENTER DRAG_LEAVE DRAG_MOTION DRAG_STATUS DROP_START DROP_FINISHED context time x_root y_root CLIENT_EVENT message_type data_format data VISIBILTY_NOTIFY state NO_EXPOSE # no additional attributes |
23.2.1 Scribble - Event Handling
For our drawing program, we want to know when the mouse button is pressed and when the mouse is moved, so we specify POINTER_MOTION_MASK and BUTTON_PRESS_MASK. We also want to know when we need to redraw our window, so we specify EXPOSURE_MASK. Although we want to be notified via a Configure event when our window size changes, we don't have to specify the corresponding STRUCTURE_MASK flag, because it is automatically specified for all windows.It turns out, however, that there is a problem with just specifying POINTER_MOTION_MASK. This will cause the server to add a new motion event to the event queue every time the user moves the mouse. Imagine that it takes us 0.1 seconds to handle a motion event, but the X server queues a new motion event every 0.05 seconds. We will soon get way behind the users drawing. If the user draws for 5 seconds, it will take us another 5 seconds to catch up after they release the mouse button! What we would like is to only get one motion event for each event we process. The way to do this is to specify POINTER_MOTION_HINT_MASK.
When we specify POINTER_MOTION_HINT_MASK,
the server sends us a motion event the first time the pointer moves after
entering our window, or after a button press or release event. Subsequent
motion events will be suppressed until we explicitly ask for the position
of the pointer using the GdkWindow attribute pointer:
x, y = window.pointer |
The state of the modifier keys and mouse buttons at the time
of the event can be retrieved using the attribute pointer_state:
state = window.pointer_state |
window is a GdkWindow object.
x and y
are the coordinates of the pointer and state
is the modifier mask to detect which keys are pressed.
(There is a GtkWidget method, get_pointer()
which provides the same information as the GdkWIndow pointer
attribute but there is no GtkWidget method corresponding to the pointer_stateattribute.)
The scribblesimple.py example program demonstrates the basic use of events and event handlers. Figure 23.1 illustrates the program in action:
Figure 23.1 Simple Scribble Example
91 # Signals used to handle backing pixmap
92 drawing_area.connect("expose_event", expose_event)
93 drawing_area.connect("configure_event", configure_event)
94
95 # Event signals
96 drawing_area.connect("motion_notify_event", motion_notify_event)
97 drawing_area.connect("button_press_event", button_press_event)
98
99 drawing_area.set_events(GDK.EXPOSURE_MASK
100 | GDK.LEAVE_NOTIFY_MASK
101 | GDK.BUTTON_PRESS_MASK
102 | GDK.POINTER_MOTION_MASK
103 | GDK.POINTER_MOTION_HINT_MASK)
|
The button_press_event()
and motion_notify_event()
event handlers in scribblesimple.py
are:
55 def button_press_event(widget, event): 56 if event.button == 1 and pixmap != None: 57 draw_brush(widget, event.x, event.y) 58 return gtk.TRUE 59 60 def motion_notify_event(widget, event): 61 if event.is_hint: 62 x, y, = event.window.pointer 63 state = event.window.pointer_state 64 else: 65 x = event.x 66 y = event.y 67 state = event.state 68 69 if state & GDK.BUTTON1_MASK and pixmap != None: 70 draw_brush(widget, x, y) 71 72 return gtk.TRUE |
The expose_event() and configure_event() handlers will be described later.