ScrollViewer滚动到底来触发加载数据的Behavior

最近项目中遇到加载数据的性能问题， 原因是.net4.0的虚拟化支持不够完成，有好多bug在4.5才修复。 我们只能利用大家通用的做法来延迟加载数据：

每次加载固定少量的数据，当拖动到底后，继续加载后续相同数量的数据。

思路：

监听ScrollViewer的VerticalOffsetProperty，如果值达到允许滚动的高度ScrollableHeight，则发出event通知外部处理加载逻辑。

使用方法：

1.对于ItemsControl编辑控件模板，在其中的ScrollViewer中加入behavior。

<ScrollViewer x :Name="DG_ScrollViewer" Focusable="false">

                                                                                                                                 <i: Interaction.Behaviors>

                                                                                                                                                 <local: ReachingBottomBehavior ReachingBottomEvent="LazyLoadingBehavior_OnReachingBottomEvent" />

                                                                                                                                 </i: Interaction.Behaviors>

在blend中，比较简单，直接对控件右键，编辑模板/副本，进入ScrollViewer后，把这个behavior拖到ScrollViewer的元素下即可， 生成的代码就是上面的内容。

xmlns:i ="http://schemas.microsoft.com/expression/2010/interactivity"

2.实现上面ReachingBottomEvent的处理逻辑。比如：

  private void LazyLoadingBehavior_OnReachingBottomEvent ()

        {

            var vm = LayoutRoot. DataContext as MainViewModel;

            if (vm != null)

            {

                int count = dtgRoot. Items .Count ;

                for (int i = count ; i < count + 30; i ++)

                {

                    vm .DataItems . Add(

                                     new DataItem ()

                                     {

                                         Item1 = i . ToString(),

                                         Item2 = i . ToString() + i .ToString ()

                                     });

                }

            }

        }

代码如下：

 public class ReachingBottomBehavior : Behavior< ScrollViewer>

    {

        public ReachingBottomBehavior ()

        {

            // Insert code required on object creation below this point.

        }

        protected override void OnAttached()

        {

            base.OnAttached ();

            // Insert code that you would want run when the Behavior is attached to an object.

            var dpd = DependencyPropertyDescriptor. FromProperty(ScrollViewer .VerticalOffsetProperty, AssociatedType);

            dpd.AddValueChanged (

                                AssociatedObject,

                                (sender,

                                 args) =>

                                {

                                    RaiseReachingBottomEvent();

                                });

        }

        protected override void OnDetaching()

        {

            base.OnDetaching ();

            // Insert code that you would want run when the Behavior is removed from an object.

        }

        private void RaiseReachingBottomEvent()

        {

            bool isReachingBottom = AssociatedObject. VerticalOffset >= AssociatedObject .ScrollableHeight;

            if (isReachingBottom )

            {

                if (this .ReachingBottomEvent != null)

                {

                    this.ReachingBottomEvent ();

                }

            }

        }

        public event Action ReachingBottomEvent;

    }