滚动条滑到底完美解决方案（适合任何带滚动条或ScrollBar控件 (一)

看见很多人在求滚动条滑倒底部自动加载数据的解决方案，各种各样的方案很多，但令人满意的确没几个。在这里我分享一个我的自认为满意的解决方案。
首先说下大致原理：监视滚动条坐标的变化，在达到底部时触发自己的处理事件。
原理很简单，但实现起来可没这么容易，先上代码，边看边说。
[csharp]
public class ScrollViewerTrigger:TriggerBase
{
ScrollViewer ScrollView;
public static readonly DependencyProperty DirectionTypeProperty = DependencyProperty.Register("DirectionType", typeof(DirectionType), typeof(ScrollViewerTrigger), new PropertyMetadata(DirectionType.Bottom));

public DirectionType DirectionType
{
get
{
return (DirectionType)base.GetValue(ScrollViewerTrigger.DirectionTypeProperty);
}
set
{
base.SetValue(ScrollViewerTrigger.DirectionTypeProperty, value);
}
}

public event EventHandler ScrollTrigger;

public static readonly DependencyProperty VerticalOffsetProperty = DependencyProperty.Register("VerticalOffset", typeof(double), typeof(ScrollViewerTrigger), new PropertyMetadata(0.0, new PropertyChangedCallback(VerticalOffsetPropertyChanged)));

public double VerticalOffset
{
get
{
return (double)base.GetValue(ScrollViewerTrigger.VerticalOffsetProperty);
}
set
{
base.SetValue(ScrollViewerTrigger.VerticalOffsetProperty, value);
}

}

public static void VerticalOffsetPropertyChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
{
var behavior = d as ScrollViewerTrigger;
if (behavior != null)
behavior.OnVerticalOffsetChanged();
}

protected override void OnAttached()
{