Event propagation

In order to receive events, a gesture needs to either set a propagation phase through gtk_event_controller_set_propagation_phase(), or feed those manually through gtk_event_controller_handle_event().

In the capture phase, events are propagated from the toplevel down to the target widget, and gestures that are attached to containers above the widget get a chance to interact with the event before it reaches the target.

After the capture phase, GTK+ emits the traditional “button-press-event”, “button-release-event”, “touch-event”, etc signals. Gestures with the GTK_PHASE_TARGET phase are fed events from the default “event” handlers.

In the bubble phase, events are propagated up from the target widget to the toplevel, and gestures that are attached to containers above the widget get a chance to interact with events that have not been handled yet.

States of a sequence

Whenever input interaction happens, a single event may trigger a cascade of GtkGestures, both across the parents of the widget receiving the event and in parallel within an individual widget. It is a responsibility of the widgets using those gestures to set the state of touch sequences accordingly in order to enable cooperation of gestures around the GdkEventSequences triggering those.

Within a widget, gestures can be grouped through gtk_gesture_group(), grouped gestures synchronize the state of sequences, so calling gtk_gesture_set_sequence_state() on one will effectively propagate the state throughout the group.

By default, all sequences start out in the GTK_EVENT_SEQUENCE_NONE state, sequences in this state trigger the gesture event handler, but event propagation will continue unstopped by gestures.

If a sequence enters into the GTK_EVENT_SEQUENCE_DENIED state, the gesture group will effectively ignore the sequence, letting events go unstopped through the gesture, but the "slot" will still remain occupied while the touch is active.

If a sequence enters in the GTK_EVENT_SEQUENCE_CLAIMED state, the gesture group will grab all interaction on the sequence, by:

Note: if a sequence is set early to GTK_EVENT_SEQUENCE_CLAIMED on GDK_TOUCH_BEGIN/GDK_BUTTON_PRESS (so those events are captured before reaching the event widget, this implies GTK_PHASE_CAPTURE), one similar event will emulated if the sequence changes to GTK_EVENT_SEQUENCE_DENIED. This way event coherence is preserved before event propagation is unstopped again.

Sequence states can't be changed freely, see gtk_gesture_set_sequence_state() to know about the possible lifetimes of a GdkEventSequence.

Touchpad gestures

On the platforms that support it, GtkGesture will handle transparently touchpad gesture events. The only precautions users of GtkGesture should do to enable this support are:

gtk_gesture_get_device ()

GdkDevice *
gtk_gesture_get_device (GtkGesture *gesture);

Returns the master GdkDevice that is currently operating on gesture , or NULL if the gesture is not being interacted.

Since: 3.14

gtk_gesture_get_window ()

GdkWindow *
gtk_gesture_get_window (GtkGesture *gesture);

Returns the user-defined window that receives the events handled by gesture . See gtk_gesture_set_window() for more information.

Since: 3.14

gtk_gesture_set_window ()

void
gtk_gesture_set_window (GtkGesture *gesture,
                        GdkWindow *window);

Sets a specific window to receive events about, so gesture will effectively handle only events targeting window , or a child of it. window must pertain to gtk_event_controller_get_widget().

Since: 3.14

gtk_gesture_is_active ()

gboolean
gtk_gesture_is_active (GtkGesture *gesture);

Returns TRUE if the gesture is currently active. A gesture is active meanwhile there are touch sequences interacting with it.

Since: 3.14

gtk_gesture_is_recognized ()

gboolean
gtk_gesture_is_recognized (GtkGesture *gesture);

Returns TRUE if the gesture is currently recognized. A gesture is recognized if there are as many interacting touch sequences as required by gesture , and “check” returned TRUE for the sequences being currently interpreted.

Since: 3.14

gtk_gesture_get_sequence_state ()

GtkEventSequenceState
gtk_gesture_get_sequence_state (GtkGesture *gesture,
                                GdkEventSequence *sequence);

Returns the sequence state, as seen by gesture .

Since: 3.14

gtk_gesture_set_sequence_state ()

gboolean
gtk_gesture_set_sequence_state (GtkGesture *gesture,
                                GdkEventSequence *sequence,
                                GtkEventSequenceState state);

Sets the state of sequence in gesture . Sequences start in state GTK_EVENT_SEQUENCE_NONE, and whenever they change state, they can never go back to that state. Likewise, sequences in state GTK_EVENT_SEQUENCE_DENIED cannot turn back to a not denied state. With these rules, the lifetime of an event sequence is constrained to the next four:

Note: Due to event handling ordering, it may be unsafe to set the state on another gesture within a “begin” signal handler, as the callback might be executed before the other gesture knows about the sequence. A safe way to perform this could be:

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static void
first_gesture_begin_cb (GtkGesture       *first_gesture,
                        GdkEventSequence *sequence,
                        gpointer          user_data)
{
  gtk_gesture_set_sequence_state (first_gesture, sequence, GTK_EVENT_SEQUENCE_CLAIMED);
  gtk_gesture_set_sequence_state (second_gesture, sequence, GTK_EVENT_SEQUENCE_DENIED);
}

static void
second_gesture_begin_cb (GtkGesture       *second_gesture,
                         GdkEventSequence *sequence,
                         gpointer          user_data)
{
  if (gtk_gesture_get_sequence_state (first_gesture, sequence) == GTK_EVENT_SEQUENCE_CLAIMED)
    gtk_gesture_set_sequence_state (second_gesture, sequence, GTK_EVENT_SEQUENCE_DENIED);
}

If both gestures are in the same group, just set the state on the gesture emitting the event, the sequence will be already be initialized to the group's global state when the second gesture processes the event.

Since: 3.14

gtk_gesture_set_state ()

gboolean
gtk_gesture_set_state (GtkGesture *gesture,
                       GtkEventSequenceState state);

Sets the state of all sequences that gesture is currently interacting with. See gtk_gesture_set_sequence_state() for more details on sequence states.

Since: 3.14

gtk_gesture_get_sequences ()

GList *
gtk_gesture_get_sequences (GtkGesture *gesture);

Returns the list of GdkEventSequences currently being interpreted by gesture .

Since: 3.14

gtk_gesture_handles_sequence ()

gboolean
gtk_gesture_handles_sequence (GtkGesture *gesture,
                              GdkEventSequence *sequence);

Returns TRUE if gesture is currently handling events corresponding to sequence .

Since: 3.14

gtk_gesture_get_last_updated_sequence ()

GdkEventSequence *
gtk_gesture_get_last_updated_sequence (GtkGesture *gesture);

Returns the GdkEventSequence that was last updated on gesture .

Since: 3.14

gtk_gesture_get_last_event ()

const GdkEvent *
gtk_gesture_get_last_event (GtkGesture *gesture,
                            GdkEventSequence *sequence);

Returns the last event that was processed for sequence .

Note that the returned pointer is only valid as long as the sequence is still interpreted by the gesture . If in doubt, you should make a copy of the event.

gtk_gesture_get_point ()

gboolean
gtk_gesture_get_point (GtkGesture *gesture,
                       GdkEventSequence *sequence,
                       gdouble *x,
                       gdouble *y);

If sequence is currently being interpreted by gesture , this function returns TRUE and fills in x and y with the last coordinates stored for that event sequence. The coordinates are always relative to the widget allocation.

Since: 3.14

gtk_gesture_get_bounding_box ()

gboolean
gtk_gesture_get_bounding_box (GtkGesture *gesture,
                              GdkRectangle *rect);

If there are touch sequences being currently handled by gesture , this function returns TRUE and fills in rect with the bounding box containing all active touches. Otherwise, FALSE will be returned.

Note: This function will yield unexpected results on touchpad gestures. Since there is no correlation between physical and pixel distances, these will look as if constrained in an infinitely small area, rect width and height will thus be 0 regardless of the number of touchpoints.

Since: 3.14

gtk_gesture_get_bounding_box_center ()

gboolean
gtk_gesture_get_bounding_box_center (GtkGesture *gesture,
                                     gdouble *x,
                                     gdouble *y);

If there are touch sequences being currently handled by gesture , this function returns TRUE and fills in x and y with the center of the bounding box containing all active touches. Otherwise, FALSE will be returned.

Since: 3.14

gtk_gesture_group ()

void
gtk_gesture_group (GtkGesture *group_gesture,
                   GtkGesture *gesture);

Adds gesture to the same group than group_gesture . Gestures are by default isolated in their own groups.

When gestures are grouped, the state of GdkEventSequences is kept in sync for all of those, so calling gtk_gesture_set_sequence_state(), on one will transfer the same value to the others.

Groups also perform an "implicit grabbing" of sequences, if a GdkEventSequence state is set to GTK_EVENT_SEQUENCE_CLAIMED on one group, every other gesture group attached to the same GtkWidget will switch the state for that sequence to GTK_EVENT_SEQUENCE_DENIED.

Since: 3.14

gtk_gesture_ungroup ()

void
gtk_gesture_ungroup (GtkGesture *gesture);

Separates gesture into an isolated group.

Since: 3.14

gtk_gesture_get_group ()

GList *
gtk_gesture_get_group (GtkGesture *gesture);

Returns all gestures in the group of gesture

Since: 3.14

gtk_gesture_is_grouped_with ()

gboolean
gtk_gesture_is_grouped_with (GtkGesture *gesture,
                             GtkGesture *other);

Returns TRUE if both gestures pertain to the same group.

Since: 3.14

GtkGesture

typedef struct _GtkGesture GtkGesture;

enum GtkEventSequenceState

Describes the state of a GdkEventSequence in a GtkGesture.

Since: 3.14

The “n-points” property

  “n-points”                 guint

The number of touch points that trigger recognition on this gesture,

Owner: GtkGesture

Flags: Read / Write / Construct Only

Allowed values: >= 1

Default value: 1

Since: 3.14

The “window” property

  “window”                   GdkWindow *

If non-NULL, the gesture will only listen for events that happen on this GdkWindow, or a child of it.

Owner: GtkGesture

Flags: Read / Write

Since: 3.14

The “begin” signal

void
user_function (GtkGesture       *gesture,
               GdkEventSequence *sequence,
               gpointer          user_data)

This signal is emitted when the gesture is recognized. This means the number of touch sequences matches “n-points”, and the “check” handler(s) returned TRUE.

Note: These conditions may also happen when an extra touch (eg. a third touch on a 2-touches gesture) is lifted, in that situation sequence won't pertain to the current set of active touches, so don't rely on this being true.

Flags: Run Last

Since: 3.14

The “cancel” signal

void
user_function (GtkGesture       *gesture,
               GdkEventSequence *sequence,
               gpointer          user_data)

This signal is emitted whenever a sequence is cancelled. This usually happens on active touches when gtk_event_controller_reset() is called on gesture (manually, due to grabs...), or the individual sequence was claimed by parent widgets' controllers (see gtk_gesture_set_sequence_state()).

gesture must forget everything about sequence as a reaction to this signal.

Flags: Run Last

Since: 3.14

The “end” signal

void
user_function (GtkGesture       *gesture,
               GdkEventSequence *sequence,
               gpointer          user_data)

This signal is emitted when gesture either stopped recognizing the event sequences as something to be handled (the “check” handler returned FALSE), or the number of touch sequences became higher or lower than “n-points”.

Note: sequence might not pertain to the group of sequences that were previously triggering recognition on gesture (ie. a just pressed touch sequence that exceeds “n-points”). This situation may be detected by checking through gtk_gesture_handles_sequence().

Flags: Run Last

Since: 3.14

The “sequence-state-changed” signal

void
user_function (GtkGesture           *gesture,
               GdkEventSequence     *sequence,
               GtkEventSequenceState state,
               gpointer              user_data)

This signal is emitted whenever a sequence state changes. See gtk_gesture_set_sequence_state() to know more about the expectable sequence lifetimes.

Flags: Run Last

Since: 3.14

The “update” signal

void
user_function (GtkGesture       *gesture,
               GdkEventSequence *sequence,
               gpointer          user_data)

This signal is emitted whenever an event is handled while the gesture is recognized. sequence is guaranteed to pertain to the set of active touches.

Flags: Run Last

Since: 3.14