Automatically scroll container to make the focused child visible

AutoScrollManager.as

package {
    
    import flash.display.InteractiveObject;
    import flash.events.EventDispatcher;
    import flash.events.FocusEvent;
    import flash.events.TimerEvent;
    import flash.geom.Point;
    import flash.utils.Timer;
    
    import mx.collections.ArrayCollection;
    import mx.core.Application;
    import mx.core.Container;
    import mx.core.EdgeMetrics;
    import mx.core.IFlexDisplayObject;
    import mx.core.IMXMLObject;
    import mx.core.UIComponent;
    import mx.core.UITextField;

    public class AutoScrollManager extends EventDispatcher implements IMXMLObject {
        
        protected static const NEXT_FRAME_TIME : Number = 1;

        public function AutoScrollManager(target : IFlexDisplayObject = null) {
            super();

            _timer = new Timer(NEXT_FRAME_TIME, 1);
            _timer.addEventListener(TimerEvent.TIMER_COMPLETE, onTimerComplete);
        }
        
        public function initialized(document : Object, id : String) : void {
            _document = document ? IFlexDisplayObject(document) : IFlexDisplayObject(Application.application);
        }
        
        public function addInstance(container : Container) : void {
            if (!_containers.contains(container)) {
                container.addEventListener(FocusEvent.FOCUS_IN, onContainerFocusIn);
                _containers.addItem(container);
            }
        }
        
        public function removeInstance(container : Container) : void {
            if (_containers.contains(container)) {
                container.removeEventListener(FocusEvent.FOCUS_IN, onContainerFocusIn);
                _containers.removeItemAt(_containers.getItemIndex(container));
            }
        }
        
        protected function onContainerFocusIn(event : FocusEvent) : void {
            var target : InteractiveObject = InteractiveObject(event.target);
            _container = Container(event.currentTarget);

            if (_container.contains(target)) {
                _focused = (target is UITextField ? target.parent : target);
                _focusedChanged = true;
                if (!_timer.running) {
                    _timer.start();
                }
            }
        }
        
        protected function onTimerComplete(event : TimerEvent) : void {
            if (_focusedChanged) {
                var focusThickness : Number =
                    (_focused is UIComponent ?
                        UIComponent(_focused).getStyle("focusThickness") : 0);
    
                var viewMetricsAndPadding : EdgeMetrics = _container.viewMetricsAndPadding;
    
                var localPosition : Point = new Point(_focused.x, _focused.y);
                var globalPosition : Point = _focused.parent.localToGlobal(localPosition);
                var containerPosition : Point = _container.globalToLocal(globalPosition);

                var focusedBottom : Number = containerPosition.y + _focused.height + focusThickness;
                var containerBottom : Number = _container.height - viewMetricsAndPadding.bottom;
                if (focusedBottom > containerBottom) {
                    _container.verticalScrollPosition += (focusedBottom - containerBottom);
                }
                
                var focusedTop : Number = containerPosition.y - focusThickness;
                var containerTop : Number = viewMetricsAndPadding.top;
                if (focusedTop < containerTop) {
                    _container.verticalScrollPosition -= (containerTop - focusedTop);
                }
                
                var focusedRight : Number = containerPosition.x + _focused.width + focusThickness;
                var containerRight : Number = _container.width - viewMetricsAndPadding.right;
                if (focusedRight > containerRight) {
                    _container.horizontalScrollPosition += (focusedRight - containerRight);
                }
                
                var focusedLeft : Number = containerPosition.x - focusThickness;
                var containerLeft : Number = viewMetricsAndPadding.left;
                if (focusedLeft < containerLeft) {
                    _container.horizontalScrollPosition -= (containerLeft - focusedLeft);
                }
                
                _focusedChanged = false;
            }
        }
        
        private var _document : IFlexDisplayObject;
        
        private var _containers : ArrayCollection /* of containers */ = new ArrayCollection();
        
        private var _timer : Timer;
        
        private var _container : Container;
        
        private var _focused : InteractiveObject;
        
        private var _focusedChanged : Boolean;
    }
}