Add showOverViewAction Clutter.GestureAction

This gesture implements 3-finger pinch, that will be used to show
the overview.

js/js-resources.gresource.xml

@@ -80,6 +80,7 @@
     <file>ui/shellDBus.js</file>
     <file>ui/shellEntry.js</file>
     <file>ui/shellMountOperation.js</file>
+    <file>ui/showOverviewAction.js</file>
     <file>ui/slider.js</file>
     <file>ui/switcherPopup.js</file>
     <file>ui/tweener.js</file>

js/ui/showOverviewAction.js

@@ -0,0 +1,66 @@
+// -*- mode: js; js-indent-level: 4; indent-tabs-mode: nil -*-
+
+const Signals = imports.signals;
+const Lang = imports.lang;
+const Meta = imports.gi.Meta;
+const Clutter = imports.gi.Clutter;
+
+const ShowOverviewAction = new Lang.Class({
+    Name: 'ShowOverviewAction',
+    Extends: Clutter.GestureAction,
+
+    _init : function() {
+        this.parent();
+        this.set_n_touch_points (3);
+        global.display.connect('grab-op-begin', Lang.bind(this, this.cancel));
+        global.display.connect('grab-op-end', Lang.bind(this, this.cancel));
+    },
+
+    vfunc_gesture_prepare : function(action, actor) {
+        return this.get_n_current_points() == this.get_n_touch_points();
+    },
+
+    _getBoundingRect : function(motion) {
+        let minX, minY, maxX, maxY;
+
+        for (let i = 0; i < this.get_n_current_points(); i++) {
+            let x, y;
+
+            if (motion == true) {
+                [x, y] = this.get_motion_coords(i);
+            } else {
+                [x, y] = this.get_press_coords(i);
+            }
+
+            if (i == 0) {
+                minX = maxX = x;
+                minY = maxY = y;
+            } else {
+                minX = Math.min(minX, x);
+                minY = Math.min(minY, y);
+                maxX = Math.max(maxX, x);
+                maxY = Math.max(maxY, y);
+            }
+        }
+
+        return new Meta.Rectangle({ x: minX,
+                                    y: minY,
+                                    width: maxX - minX,
+                                    height: maxY - minY });
+    },
+
+    vfunc_gesture_begin : function(action, actor) {
+        this._initialRect = this._getBoundingRect(false);
+        return true;
+    },
+
+    vfunc_gesture_end : function(action, actor) {
+        let rect = this._getBoundingRect(true);
+        let oldArea = this._initialRect.width * this._initialRect.height;
+        let newArea = rect.width * rect.height;
+        let areaDiff = newArea / oldArea;
+
+        this.emit('activated', areaDiff);
+    }
+});
+Signals.addSignalMethods(ShowOverviewAction.prototype);