Add API to programmatically initiate a drag

For some use cases we have other behavior on mouse press and want
to manually control when a drag starts.  Split out the drag initiation
code into startDrag.

js/ui/dnd.js

@@ -8,15 +8,17 @@ const Tweener = imports.ui.tweener;
 
 const SNAP_BACK_ANIMATION_TIME = 0.25;
 
-function _Draggable(actor) {
+function _Draggable(actor, manualMode) {
-    this._init(actor);
+    this._init(actor, manualMode);
 }
 
 _Draggable.prototype = {
-    _init : function(actor) {
+    _init : function(actor, manualMode) {
         this.actor = actor;
+        if (!manualMode)
             this.actor.connect('button-press-event',
                                Lang.bind(this, this._onButtonPress));
+        this._haveSourceGrab = false;
     },
 
     _onButtonPress : function (actor, event) {
@@ -25,6 +27,7 @@ _Draggable.prototype = {
         if (Tweener.getTweenCount(actor))
             return false;
 
+        this._haveSourceGrab = true;
         this._grabActor(actor);
 
         let [stageX, stageY] = event.get_coords();
@@ -66,16 +69,22 @@ _Draggable.prototype = {
             return false;
     },
 
-    _onMotion : function (actor, event) {
+    /**
-        let [stageX, stageY] = event.get_coords();
+     * startDrag:
+     * @actor: Origin actor for drag and drop
+     * @stageX: X coordinate of event
+     * @stageY: Y coordinate of event
+     * @time: Event timestamp
+     *
+     * Directly initiate a drag and drop operation from the given actor.
+     * This function is useful to call if you've specified manualMode
+     * for the draggable.
+     */
+    startDrag: function (actor, stageX, stageY, time) {
+        this.emit('drag-begin', time);
 
-        // If we haven't begun a drag, see if the user has moved the
+        this._dragStartX = stageX;
-        // mouse enough to trigger a drag
+        this._dragStartY = stageY;
-        let threshold = Gtk.Settings.get_default().gtk_dnd_drag_threshold;
-        if (!this._dragActor &&
-            (Math.abs(stageX - this._dragStartX) > threshold ||
-             Math.abs(stageY - this._dragStartY) > threshold)) {
-            this.emit('drag-begin', event.get_time());
 
         if (this.actor._delegate && this.actor._delegate.getDragActor) {
             this._dragActor = this.actor._delegate.getDragActor(this._dragStartX, this._dragStartY);
@@ -89,16 +98,24 @@ _Draggable.prototype = {
                 // around the pointer
                 let [sourceX, sourceY] = this._dragActorSource.get_transformed_position();
                 let [sourceWidth, sourceHeight] = this._dragActorSource.get_transformed_size();
+                let x, y;
                 if (stageX > sourceX && stageX <= sourceX + sourceWidth &&
-                        stageY > sourceY && stageY <= sourceY + sourceHeight)
+                    stageY > sourceY && stageY <= sourceY + sourceHeight) {
-                        this._dragActor.set_position(sourceX, sourceY);
+                    x = sourceX;
-                    else
+                    y = sourceY;
-                        this._dragActor.set_position(stageX - this._dragActor.width / 2, stageY - this._dragActor.height / 2);
+                } else {
+                    x = stageX - this._dragActor.width / 2;
+                    y = stageY - this._dragActor.height / 2;
+                }
+                this._dragActor.set_position(x, y);
             } else {
                 this._dragActorSource = this.actor;
             }
             this._dragOrigParent = undefined;
+            if (this._haveSourceGrab) {
+                this._haveSourceGrab = false;
                 this._ungrabActor(actor);
+            }
             this._grabActor(this._dragActor);
 
             this._dragOffsetX = this._dragActor.x - this._dragStartX;
@@ -124,6 +141,18 @@ _Draggable.prototype = {
 
         this._dragActor.reparent(actor.get_stage());
         this._dragActor.raise_top();
+    },
+
+    _onMotion : function (actor, event) {
+        let [stageX, stageY] = event.get_coords();
+
+        // If we haven't begun a drag, see if the user has moved the
+        // mouse enough to trigger a drag
+        let threshold = Gtk.Settings.get_default().gtk_dnd_drag_threshold;
+        if (!this._dragActor &&
+            (Math.abs(stageX - this._dragStartX) > threshold ||
+             Math.abs(stageY - this._dragStartY) > threshold)) {
+                this.startDrag(actor, stageX, stageY, event.get_time());
         }
 
         // If we are dragging, update the position
@@ -225,6 +254,13 @@ _Draggable.prototype = {
 
 Signals.addSignalMethods(_Draggable.prototype);
 
-function makeDraggable(actor) {
+/**
-    return new _Draggable(actor);
+ * makeDraggable:
+ * @actor: Source actor
+ * @manualMode: If given, do not automatically start drag and drop on click
+ *
+ * Create an object which controls drag and drop for the given actor.
+ */
+function makeDraggable(actor, manualMode) {
+    return new _Draggable(actor, manualMode);
 }