app: Implement canvas support for touchpad gesture zoom by pinch

app/display/gimpcanvas.h

@@ -28,6 +28,7 @@
                                 GDK_BUTTON_RELEASE_MASK      | \
                                 GDK_SCROLL_MASK              | \
                                 GDK_SMOOTH_SCROLL_MASK       | \
+                                GDK_TOUCHPAD_GESTURE_MASK    | \
                                 GDK_STRUCTURE_MASK           | \
                                 GDK_ENTER_NOTIFY_MASK        | \
                                 GDK_LEAVE_NOTIFY_MASK        | \

app/display/gimpdisplayshell-scale.c

@@ -516,6 +516,9 @@ gimp_display_shell_scale (GimpDisplayShell *shell,
   if (zoom_type == GIMP_ZOOM_SMOOTH)
     delta = -new_scale;
 
+  if (zoom_type == GIMP_ZOOM_PINCH)
+    delta = new_scale;
+
   if (zoom_type != GIMP_ZOOM_TO)
     new_scale = gimp_zoom_model_zoom_step (zoom_type, current_scale, delta);
 

app/display/gimpdisplayshell-tool-events.c

@@ -1247,6 +1247,29 @@ gimp_display_shell_canvas_grab_notify (GtkWidget        *canvas,
     }
 }
 
+void
+gimp_display_shell_zoom_gesture_begin (GtkGestureZoom   *gesture,
+                                       GdkEventSequence *sequence,
+                                       GimpDisplayShell *shell)
+{
+  shell->last_zoom_scale = gtk_gesture_zoom_get_scale_delta (gesture);
+}
+
+void
+gimp_display_shell_zoom_gesture_update (GtkGestureZoom   *gesture,
+                                        GdkEventSequence *sequence,
+                                        GimpDisplayShell *shell)
+{
+  gdouble current_scale = gtk_gesture_zoom_get_scale_delta (gesture);
+  gdouble delta = (current_scale - shell->last_zoom_scale) / shell->last_zoom_scale;
+  shell->last_zoom_scale = current_scale;
+
+  gimp_display_shell_scale (shell,
+                            GIMP_ZOOM_PINCH,
+                            delta,
+                            GIMP_ZOOM_FOCUS_POINTER);
+}
+
 void
 gimp_display_shell_buffer_stroke (GimpMotionBuffer *buffer,
                                   const GimpCoords *coords,

app/display/gimpdisplayshell-tool-events.h

@@ -30,6 +30,13 @@ void       gimp_display_shell_canvas_grab_notify      (GtkWidget        *widget,
                                                        gboolean          was_grabbed,
                                                        GimpDisplayShell *shell);
 
+void       gimp_display_shell_zoom_gesture_begin      (GtkGestureZoom   *gesture,
+                                                       GdkEventSequence *sequence,
+                                                       GimpDisplayShell *shell);
+void       gimp_display_shell_zoom_gesture_update     (GtkGestureZoom   *gesture,
+                                                       GdkEventSequence *sequence,
+                                                       GimpDisplayShell *shell);
+
 void       gimp_display_shell_buffer_stroke           (GimpMotionBuffer *buffer,
                                                        const GimpCoords *coords,
                                                        guint32           time,

app/display/gimpdisplayshell.c

@@ -521,6 +521,10 @@ gimp_display_shell_constructed (GObject *object)
   gimp_display_shell_dnd_init (shell);
   gimp_display_shell_selection_init (shell);
 
+  shell->zoom_gesture = gtk_gesture_zoom_new (GTK_WIDGET (shell->canvas));
+  gtk_event_controller_set_propagation_phase (GTK_EVENT_CONTROLLER (shell->zoom_gesture),
+                                              GTK_PHASE_CAPTURE);
+
   /*  the horizontal ruler  */
   shell->hrule = gimp_ruler_new (GTK_ORIENTATION_HORIZONTAL);
   gtk_widget_set_events (GTK_WIDGET (shell->hrule),
@@ -607,6 +611,12 @@ gimp_display_shell_constructed (GObject *object)
   g_signal_connect (shell->canvas, "key-release-event",
                     G_CALLBACK (gimp_display_shell_canvas_tool_events),
                     shell);
+  g_signal_connect (shell->zoom_gesture, "begin",
+                    G_CALLBACK (gimp_display_shell_zoom_gesture_begin),
+                    shell);
+  g_signal_connect (shell->zoom_gesture, "update",
+                    G_CALLBACK (gimp_display_shell_zoom_gesture_update),
+                    shell);
 
   /*  the zoom button  */
   shell->zoom_button = g_object_new (GTK_TYPE_CHECK_BUTTON,
@@ -757,6 +767,8 @@ gimp_display_shell_dispose (GObject *object)
       shell->filter_idle_id = 0;
     }
 
+  g_clear_object (&shell->zoom_gesture);
+
   g_clear_pointer (&shell->render_cache,       cairo_surface_destroy);
   g_clear_pointer (&shell->render_cache_valid, cairo_region_destroy);
 

app/display/gimpdisplayshell.h

@@ -99,6 +99,7 @@ struct _GimpDisplayShell
   GList             *children;
 
   GtkWidget         *canvas;           /*  GimpCanvas widget                  */
+  GtkGesture        *zoom_gesture;     /*  Zoom gesture handler for the canvas*/
 
   GtkAdjustment     *hsbdata;          /*  adjustments                        */
   GtkAdjustment     *vsbdata;
@@ -203,6 +204,9 @@ struct _GimpDisplayShell
   GdkDevice         *grab_keyboard_source;
   guint32            grab_keyboard_time;
 
+  /*  the state of gimp_display_shell_zoom_gesture_*() */
+  gdouble            last_zoom_scale;
+
   /* Two states are possible when the shell is grabbed: it can be
    * grabbed with space (or space+button1 which is the same state),
    * then if space is released but button1 was still pressed, we wait

app/widgets/gimppaletteeditor.c

@@ -610,7 +610,8 @@ gimp_palette_editor_zoom (GimpPaletteEditor  *editor,
       }
       break;
 
-    case GIMP_ZOOM_SMOOTH: /* can't happen */
+    case GIMP_ZOOM_SMOOTH:
+    case GIMP_ZOOM_PINCH: /* can't happen */
       g_return_if_reached ();
     }