    /**
     * A DragTracker listens for drag events on an Element and fires events at the start and end of the drag,
     * as well as during the drag. This is useful for components such as {@link Ext.slider.Multi}, where there is
     * an element that can be dragged around to change the Slider's value.
     *
     * DragTracker provides a series of template methods that should be overridden to provide functionality
     * in response to detected drag operations. These are onBeforeStart, onStart, onDrag and onEnd.
     * See {@link Ext.slider.Multi}'s initEvents function for an example implementation.
     */
    Ext.define('Ext.dd.DragTracker', {

        uses: ['Ext.util.Region'],

        mixins: {
            observable: 'Ext.util.Observable'
        },

        /**
         * @property {Boolean} active
         * Indicates whether the user is currently dragging this tracker.
         * @readonly
         */
        active: false,

        /**
         * @property {HTMLElement} dragTarget
         * The element being dragged.
         *
         * Only valid during drag operations.
         *
         * If the {@link #delegate} option is used, this will be the delegate element which was mousedowned.
         * @readonly
         */

        /**
         * @cfg {Boolean} trackOver
         * Set to true to fire mouseover and mouseout events when the mouse enters or leaves the target element.
         *
         * This is implicitly set when an {@link #overCls} is specified.
         *
         * If the {@link #delegate} option is used, these events fire only when a delegate element is entered of left.
         */
        trackOver: false,

        /**
         * @cfg {String} overCls
         * A CSS class to add to the DragTracker's target element when the element (or, if the {@link #delegate}
         * option is used, when a delegate element) is mouseovered.
         *
         * If the {@link #delegate} option is used, these events fire only when a delegate element is entered of left.
         */

        /**
         * @cfg {Ext.util.Region/Ext.Element} constrainTo
         * A {@link Ext.util.Region Region} (Or an element from which a Region measurement will be read)
         * which is used to constrain the result of the {@link #getOffset} call.
         *
         * This may be set any time during the DragTracker's lifecycle to set a dynamic constraining region.
         */

        /**
         * @cfg {Number} tolerance
         * Number of pixels the drag target must be moved before dragging is
         * considered to have started.
         */
        tolerance: 5,

        /**
         * @cfg {Boolean/Number} autoStart
         * Specify `true` to defer trigger start by 1000 ms.
         * Specify a Number for the number of milliseconds to defer trigger start.
         */
        autoStart: false,

        /**
         * @cfg {String} delegate
         * A {@link Ext.DomQuery DomQuery} selector which identifies child elements within the DragTracker's encapsulating
         * Element which are the tracked elements. This limits tracking to only begin when the matching elements are mousedowned.
         *
         * This may also be a specific child element within the DragTracker's encapsulating element to use as the tracked element.
         */

        /**
         * @cfg {Boolean} [preventDefault=true]
         * Specify `false` to enable default actions on onMouseDown events.
         */

        /**
         * @cfg {Boolean} [stopEvent=false]
         * Specify `true` to stop the `mousedown` event from bubbling to outer listeners from the target element (or its delegates).
         */

        constructor : function(config){
            Ext.apply(this, config);
            this.addEvents(
                /**
                 * @event mouseover
                 * Fires when the mouse enters the DragTracker's target element (or if {@link #delegate} is
                 * used, when the mouse enters a delegate element).
                 *
                 * **Only available when {@link #trackOver} is `true`**
                 *
                 * @param {Object} this
                 * @param {Object} e event object
                 * @param {HTMLElement} target The element mouseovered.
                 */
                'mouseover',

                /**
                 * @event mouseout
                 * Fires when the mouse exits the DragTracker's target element (or if {@link #delegate} is
                 * used, when the mouse exits a delegate element).
                 *
                 * **Only available when {@link #trackOver} is `true`**
                 *
                 * @param {Object} this
                 * @param {Object} e event object
                 */
                'mouseout',

                /**
                 * @event mousedown
                 * Fires when the mouse button is pressed down, but before a drag operation begins. The
                 * drag operation begins after either the mouse has been moved by {@link #tolerance} pixels,
                 * or after the {@link #autoStart} timer fires.
                 *
                 * Return false to veto the drag operation.
                 *
                 * @param {Object} this
                 * @param {Object} e event object
                 */
                'mousedown',

                /**
                 * @event mouseup
                 * @param {Object} this
                 * @param {Object} e event object
                 */
                'mouseup',

                /**
                 * @event mousemove
                 * Fired when the mouse is moved. Returning false cancels the drag operation.
                 * @param {Object} this
                 * @param {Object} e event object
                 */
                'mousemove',

                /**
                 * @event beforestart
                 * @param {Object} this
                 * @param {Object} e event object
                 */
                'beforedragstart',

                /**
                 * @event dragstart
                 * @param {Object} this
                 * @param {Object} e event object
                 */
                'dragstart',

                /**
                 * @event dragend
                 * @param {Object} this
                 * @param {Object} e event object
                 */
                'dragend',

                /**
                 * @event drag
                 * @param {Object} this
                 * @param {Object} e event object
                 */
                'drag'
            );

            this.dragRegion = new Ext.util.Region(0,0,0,0);

            if (this.el) {
                this.initEl(this.el);
            }

            // Dont pass the config so that it is not applied to 'this' again
            this.mixins.observable.constructor.call(this);
            if (this.disabled) {
                this.disable();
            }

        },

        /**
         * Initializes the DragTracker on a given element.
         * @param {Ext.Element/HTMLElement} el The element
         */
        initEl: function(el) {
            this.el = Ext.get(el);

            // The delegate option may also be an element on which to listen
            this.handle = Ext.get(this.delegate);

            // If delegate specified an actual element to listen on, we do not use the delegate listener option
            this.delegate = this.handle ? undefined : this.delegate;

            if (!this.handle) {
                this.handle = this.el;
            }

            // Add a mousedown listener which reacts only on the elements targeted by the delegate config.
            // We process mousedown to begin tracking.
            this.mon(this.handle, {
                mousedown: this.onMouseDown,
                delegate: this.delegate,
                scope: this
            });

            // If configured to do so, track mouse entry and exit into the target (or delegate).
            // The mouseover and mouseout CANNOT be replaced with mouseenter and mouseleave
            // because delegate cannot work with those pseudoevents. Entry/exit checking is done in the handler.
            if (this.trackOver || this.overCls) {
                this.mon(this.handle, {
                    mouseover: this.onMouseOver,
                    mouseout: this.onMouseOut,
                    delegate: this.delegate,
                    scope: this
                });
            }
        },

        disable: function() {
            this.disabled = true;
        },

        enable: function() {
            this.disabled = false;
        },

        destroy : function() {
            this.clearListeners();
            delete this.el;
        },

        // When the pointer enters a tracking element, fire a mouseover if the mouse entered from outside.
        // This is mouseenter functionality, but we cannot use mouseenter because we are using "delegate" to filter mouse targets
        onMouseOver: function(e, target) {
            var me = this;
            if (!me.disabled) {
                if (Ext.EventManager.contains(e) || me.delegate) {
                    me.mouseIsOut = false;
                    if (me.overCls) {
                        me.el.addCls(me.overCls);
                    }
                    me.fireEvent('mouseover', me, e, me.delegate ? e.getTarget(me.delegate, target) : me.handle);
                }
            }
        },

        // When the pointer exits a tracking element, fire a mouseout.
        // This is mouseleave functionality, but we cannot use mouseleave because we are using "delegate" to filter mouse targets
        onMouseOut: function(e) {
            if (this.mouseIsDown) {
                this.mouseIsOut = true;
            } else {
                if (this.overCls) {
                    this.el.removeCls(this.overCls);
                }
                this.fireEvent('mouseout', this, e);
            }
        },

        onMouseDown: function(e, target){
            // If this is disabled, or the mousedown has been processed by an upstream DragTracker, return
            if (this.disabled ||e.dragTracked) {
                return;
            }

            // This information should be available in mousedown listener and onBeforeStart implementations
            this.dragTarget = this.delegate ? target : this.handle.dom;
            this.startXY = this.lastXY = e.getXY();
            this.startRegion = Ext.fly(this.dragTarget).getRegion();

            if (this.fireEvent('mousedown', this, e) === false ||
                this.fireEvent('beforedragstart', this, e) === false ||
                this.onBeforeStart(e) === false) {
                return;
            }

            // Track when the mouse is down so that mouseouts while the mouse is down are not processed.
            // The onMouseOut method will only ever be called after mouseup.
            this.mouseIsDown = true;

            // Flag for downstream DragTracker instances that the mouse is being tracked.
            e.dragTracked = true;

            if (this.preventDefault !== false) {
                e.preventDefault();
            }
            Ext.getDoc().on({
                scope: this,
                mouseup: this.onMouseUp,
                mousemove: this.onMouseMove,
                selectstart: this.stopSelect
            });
            if (this.autoStart) {
                this.timer =  Ext.defer(this.triggerStart, this.autoStart === true ? 1000 : this.autoStart, this, [e]);
            }
        },

        onMouseMove: function(e, target){
            // BrowserBug: IE hack to see if button was released outside of window.
            // Needed in IE6-9 in quirks and strictmode
            if (this.active && Ext.isIE && !e.browserEvent.button) {
                e.preventDefault();
                this.onMouseUp(e);
                return;
            }

            e.preventDefault();
            var xy = e.getXY(),
                s = this.startXY;

            this.lastXY = xy;
            if (!this.active) {
                if (Math.max(Math.abs(s[0]-xy[0]), Math.abs(s[1]-xy[1])) > this.tolerance) {
                    this.triggerStart(e);
                } else {
                    return;
                }
            }

            // Returning false from a mousemove listener deactivates
            if (this.fireEvent('mousemove', this, e) === false) {
                this.onMouseUp(e);
            } else {
                this.onDrag(e);
                this.fireEvent('drag', this, e);
            }
        },

        onMouseUp: function(e) {
            // Clear the flag which ensures onMouseOut fires only after the mouse button
            // is lifted if the mouseout happens *during* a drag.
            this.mouseIsDown = false;

            // If we mouseouted the el *during* the drag, the onMouseOut method will not have fired. Ensure that it gets processed.
            if (this.mouseIsOut) {
                this.mouseIsOut = false;
                this.onMouseOut(e);
            }
            e.preventDefault();
            this.fireEvent('mouseup', this, e);
            this.endDrag(e);
        },

        /**
         * @private
         * Stop the drag operation, and remove active mouse listeners.
         */
        endDrag: function(e) {
            var doc = Ext.getDoc(),
            wasActive = this.active;

            doc.un('mousemove', this.onMouseMove, this);
            doc.un('mouseup', this.onMouseUp, this);
            doc.un('selectstart', this.stopSelect, this);
            this.clearStart();
            this.active = false;
            if (wasActive) {
                this.onEnd(e);
                this.fireEvent('dragend', this, e);
            }
            // Private property calculated when first required and only cached during a drag
            delete this._constrainRegion;

            // Remove flag from event singleton.  Using "Ext.EventObject" here since "endDrag" is called directly in some cases without an "e" param
            delete Ext.EventObject.dragTracked;
        },

        triggerStart: function(e) {
            this.clearStart();
            this.active = true;
            this.onStart(e);
            this.fireEvent('dragstart', this, e);
        },

        clearStart : function() {
            if (this.timer) {
                clearTimeout(this.timer);
                delete this.timer;
            }
        },

        stopSelect : function(e) {
            e.stopEvent();
            return false;
        },

        /**
         * Template method which should be overridden by each DragTracker instance. Called when the user first clicks and
         * holds the mouse button down. Return false to disallow the drag
         * @param {Ext.EventObject} e The event object
         * @template
         */
        onBeforeStart : function(e) {

        },

        /**
         * Template method which should be overridden by each DragTracker instance. Called when a drag operation starts
         * (e.g. the user has moved the tracked element beyond the specified tolerance)
         * @param {Ext.EventObject} e The event object
         * @template
         */
        onStart : function(xy) {

        },

        /**
         * Template method which should be overridden by each DragTracker instance. Called whenever a drag has been detected.
         * @param {Ext.EventObject} e The event object
         * @template
         */
        onDrag : function(e) {

        },

        /**
         * Template method which should be overridden by each DragTracker instance. Called when a drag operation has been completed
         * (e.g. the user clicked and held the mouse down, dragged the element and then released the mouse button)
         * @param {Ext.EventObject} e The event object
         * @template
         */
        onEnd : function(e) {

        },

        /**
         * Returns the drag target. This is usually the DragTracker's encapsulating element.
         *
         * If the {@link #delegate} option is being used, this may be a child element which matches the
         * {@link #delegate} selector.
         *
         * @return {Ext.Element} The element currently being tracked.
         */
        getDragTarget : function(){
            return this.dragTarget;
        },

        /**
         * @private
         * @returns {Ext.Element} The DragTracker's encapsulating element.
         */
        getDragCt : function(){
            return this.el;
        },

        /**
         * @private
         * Return the Region into which the drag operation is constrained.
         * Either the XY pointer itself can be constrained, or the dragTarget element
         * The private property _constrainRegion is cached until onMouseUp
         */
        getConstrainRegion: function() {
            if (this.constrainTo) {
                if (this.constrainTo instanceof Ext.util.Region) {
                    return this.constrainTo;
                }
                if (!this._constrainRegion) {
                    this._constrainRegion = Ext.fly(this.constrainTo).getViewRegion();
                }
            } else {
                if (!this._constrainRegion) {
                    this._constrainRegion = this.getDragCt().getViewRegion();
                }
            }
            return this._constrainRegion;
        },

        getXY : function(constrain){
            return constrain ? this.constrainModes[constrain](this, this.lastXY) : this.lastXY;
        },

        /**
         * Returns the X, Y offset of the current mouse position from the mousedown point.
         *
         * This method may optionally constrain the real offset values, and returns a point coerced in one
         * of two modes:
         *
         *  - `point`
         *    The current mouse position is coerced into the constrainRegion and the resulting position is returned.
         *  - `dragTarget`
         *    The new {@link Ext.util.Region Region} of the {@link #getDragTarget dragTarget} is calculated
         *    based upon the current mouse position, and then coerced into the constrainRegion. The returned
         *    mouse position is then adjusted by the same delta as was used to coerce the region.\
         *
         * @param constrainMode {String} (Optional) If omitted the true mouse position is returned. May be passed
         * as `point` or `dragTarget`. See above.
         * @returns {Number[]} The `X, Y` offset from the mousedown point, optionally constrained.
         */
        getOffset : function(constrain){
            var xy = this.getXY(constrain),
                s = this.startXY;

            return [xy[0]-s[0], xy[1]-s[1]];
        },

        constrainModes: {
            // Constrain the passed point to within the constrain region
            point: function(me, xy) {
                var dr = me.dragRegion,
                    constrainTo = me.getConstrainRegion();

                // No constraint
                if (!constrainTo) {
                    return xy;
                }

                dr.x = dr.left = dr[0] = dr.right = xy[0];
                dr.y = dr.top = dr[1] = dr.bottom = xy[1];
                dr.constrainTo(constrainTo);

                return [dr.left, dr.top];
            },

            // Constrain the dragTarget to within the constrain region. Return the passed xy adjusted by the same delta.
            dragTarget: function(me, xy) {
                var s = me.startXY,
                    dr = me.startRegion.copy(),
                    constrainTo = me.getConstrainRegion(),
                    adjust;

                // No constraint
                if (!constrainTo) {
                    return xy;
                }

                // See where the passed XY would put the dragTarget if translated by the unconstrained offset.
                // If it overflows, we constrain the passed XY to bring the potential
                // region back within the boundary.
                dr.translateBy(xy[0]-s[0], xy[1]-s[1]);

                // Constrain the X coordinate by however much the dragTarget overflows
                if (dr.right > constrainTo.right) {
                    xy[0] += adjust = (constrainTo.right - dr.right);    // overflowed the right
                    dr.left += adjust;
                }
                if (dr.left < constrainTo.left) {
                    xy[0] += (constrainTo.left - dr.left);      // overflowed the left
                }

                // Constrain the Y coordinate by however much the dragTarget overflows
                if (dr.bottom > constrainTo.bottom) {
                    xy[1] += adjust = (constrainTo.bottom - dr.bottom);  // overflowed the bottom
                    dr.top += adjust;
                }
                if (dr.top < constrainTo.top) {
                    xy[1] += (constrainTo.top - dr.top);        // overflowed the top
                }
                return xy;
            }
        }
    });