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Gtk+/Gnome Application Development
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Grabbing the Pointer

It is possible to grab the pointer, which means that all pointer events will go to the grab window for the duration of the grab. Normally pointer events go to the window the pointer is inside. You should grab the pointer, for example, if the user is using click-and-drag selection to select a rectangular area. If they click and then inadvertently drag the pointer outside the window, you should continue to track the pointer's location and change the selection accordingly. The grab also ensures that pointer events won't be sent to other applications.

To grab the pointer, call gdk_pointer_grab(), shown in Figure 8. The first argument to this function is the grab window; this window will receive events during the grab. The next argument should be TRUE or FALSE; it specifies whether events will go only to the grab window, or to its child windows as well. The confine_to argument specifies a window to confine the pointer to. The user will not be able to move the pointer outside this window. You can specify a different cursor for the duration of the grab; see the next section for details on creating a cursor. If you don't want to change the cursor, give NULL as the cursor argument. (Side note: it is safe to destroy the cursor immediately after calling gdk_pointer_grab() because it is a server-side resource and X will not deallocate it until the grab is over.)

The final argument, time, specifies when the grab should take effect, in server time. This is intended to resolve conflicts if two clients try to grab the pointer simultaneously; the time must be after the last grab time, and it must not be in the future. Usually, you will want to use the time field from the event you're processing, or the GDK_CURRENT_TIME macro. GDK_CURRENT_TIME is a magic constant that tells the X server to substitute the current time.

gdk_pointer_grab() returns TRUE if it succeeds. It is possible for it to fail if the grab window or confine_to window is hidden, another client has the grab already, or any of the arguments are invalid. Regrettably few applications check this return value, which is a bug (granted, a difficult-to-trigger one).

To ungrab the pointer, call gdk_pointer_ungrab(); the time argument is identical to the one in gdk_pointer_grab(). You can find out if the pointer is grabbed using gdk_pointer_is_grabbed(). You must ungrab the pointer when you're finished with it, because the user will be unable to use other applications while the pointer is grabbed.

Note that the GDK-level concept of grabbing the pointer is distinct from the GTK+-level grab concept. A GTK+ grab redirects certain events to a grabbing widget, creating a "modal" widget such as a dialog (see the section called Grabs in the chapter called GTK+ Basics). GTK+'s grab only affects the current application; only events that occur on one of the current application's widgets are redirected. The scope of a GDK grab is wider, encompassing the entire X server, not just your application.

#include <gdk/gdk.h>

gint gdk_pointer_grab(GdkWindow* window, gint owner_events, GdkWindow* confine_to, GdkCursor* cursor, guint32 time);

void gdk_pointer_ungrab(guint32 time);

gint gdk_pointer_is_grabbed(void);

Figure 8. Grabbing the Pointer

Gtk+/Gnome Application Development
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