看見很多人在求滾動條滑倒底部自動加載數據的解決方案，各種各樣的方案很多，但令人滿意的確沒幾個。在這裡我分享一個我的自認為滿意的解決方案。
首先說下大致原理：監視滾動條坐標的變化，在達到底部時觸發自己的處理事件。
原理很簡單，但實現起來可沒這麼容易，先上代碼，邊看邊說。
[csharp]
public class ScrollViewerTrigger:TriggerBase<DependencyObject>
{
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()
{
base.OnAttached();
if (this.AssociatedObject != null && this.AssociatedObject is FrameworkElement)
{
(this.AssociatedObject as FrameworkElement).SizeChanged += control_SizeChanged;
}
}
void control_SizeChanged(object sender, SizeChangedEventArgs e)
{
if (this.AssociatedObject == null || !(this.AssociatedObject is FrameworkElement))
return;
ScrollViewer Scroll = this.AssociatedObject.GetFirstDescendantOfType<ScrollViewer>();
if (Scroll != null)
{
AttachedScroll(Scroll);
(this.AssociatedObject as FrameworkElement).SizeChanged -= control_SizeChanged;
}
}
void AttachedScroll(ScrollViewer Scroll)
{
if (Scroll == null)
return;
ScrollView = Scroll;
Binding binding = new Binding();
binding.Source = Scroll;
binding.Path = new PropertyPath("VerticalOffset");
BindingOperations.SetBinding(this, ScrollViewerTrigger.VerticalOffsetProperty, binding);
}
void OnVerticalOffsetChanged()
{
ScrollViewer Scroll = ScrollView;
if (Scroll == null)
return;
switch (DirectionType)
{
case DirectionType.Top:
{
if (Scroll.ScrollableHeight < double.Epsilon || Scroll.VerticalOffset > double.Epsilon)
return;
}
break;
case DirectionType.Bottom:
{
if (Scroll.ScrollableHeight < double.Epsilon || Scroll.VerticalOffset + Scroll.ExtentHeight - Scroll.ScrollableHeight < Scroll.ScrollableHeight)
return;
}
break;
case DirectionType.Left:
{
if (Scroll.ScrollableWidth < double.Epsilon || Scroll.HorizontalOffset > double.Epsilon)
return;
}
break;
case DirectionType.Right:
{
if (Scroll.ScrollableWidth < double.Epsilon || Scroll.HorizontalOffset < Scroll.ScrollableWidth)
return;
}
break;
default: break;
}
if (ScrollTrigger != null)
{
ScrollTrigger(this.AssociatedObject, new EventArgs());
}
base.InvokeActions(null);
}
protected override void OnDetaching()
{
base.OnDetaching();
}
}
public class ScrollViewerTrigger:TriggerBase<DependencyObject>
{
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()
{
base.OnAttached();
if (this.AssociatedObject != null && this.AssociatedObject is FrameworkElement)
{
(this.AssociatedObject as FrameworkElement).SizeChanged += control_SizeChanged;
}
}
void control_SizeChanged(object sender, SizeChangedEventArgs e)
{
if (this.AssociatedObject == null || !(this.AssociatedObject is FrameworkElement))
return;
ScrollViewer Scroll = this.AssociatedObject.GetFirstDescendantOfType<ScrollViewer>();
if (Scroll != null)
{
AttachedScroll(Scroll);
(this.AssociatedObject as FrameworkElement).SizeChanged -= control_SizeChanged;
}
}
void AttachedScroll(ScrollViewer Scroll)
{
if (Scroll == null)
return;
ScrollView = Scroll;
Binding binding = new Binding();
binding.Source = Scroll;
binding.Path = new PropertyPath("VerticalOffset");
BindingOperations.SetBinding(this, ScrollViewerTrigger.VerticalOffsetProperty, binding);
}
void OnVerticalOffsetChanged()
{
ScrollViewer Scroll = ScrollView;
if (Scroll == null)
return;
switch (DirectionType)
{
case DirectionType.Top:
{
if (Scroll.ScrollableHeight < double.Epsilon || Scroll.VerticalOffset > double.Epsilon)
return;
}
break;
case DirectionType.Bottom:
{
if (Scroll.ScrollableHeight < double.Epsilon || Scroll.VerticalOffset + Scroll.ExtentHeight - Scroll.ScrollableHeight < Scroll.ScrollableHeight)
return;
}
break;
case DirectionType.Left:
{
if (Scroll.ScrollableWidth < double.Epsilon || Scroll.HorizontalOffset > double.Epsilon)
return;
}
break;
case DirectionType.Right:
{
if (Scroll.ScrollableWidth < double.Epsilon || Scroll.HorizontalOffset < Scroll.ScrollableWidth)
return;
}
break;
default: break;
}
if (ScrollTrigger != null)
{
ScrollTrigger(this.AssociatedObject, new EventArgs());
}
base.InvokeActions(null);
}
protected override void OnDetaching()
{
base.OnDetaching();
}
} 整個實現是Trigger的擴展，這樣靈活性和通用性非常高，不管是什麼控件。因為Trigger本身是附加元素，對於任何控件都可以附加上去，不要要改動原有代碼就可以實現功能擴展。Trigger自帶的Actions則有很強的靈活性，動作觸發後可以執行多個Action.
DirectionType是觸發方向，分上下左右4個方向，默認是在底部觸發，想做下拉刷新功能時把DirectionType改成Top就可以了了。
VerticalOffset則是實現滾動條監視的關鍵，什麼作用稍後說明。
OnAttached()是整個Trigger的入口，在這裡面我監控了一個通用事件SizeChanged，主要是通過這個事件查找控件內的ScrollViewer控件。這裡不能使用loaded事件，因為有些帶滾動條的控件是在加載元素時才建立ScrollViewer控件，loaded事件觸發時內部元素不一定會被加載，而 SizeChanged觸發時ScrollViewer已經建立好了。
control_SizeChanged中關鍵語句在於 this.AssociatedObject.GetFirstDescendantOfType<ScrollViewer>();這語句是從當前控件的子控件中查找ScrollViewer控件，查找到後就會調用AttachedScroll對ScrollViewer的滾動進行監視。
下面就是關鍵了，AttachedScroll函數要對ScrollViewer進行監控，那麼如何監控呢？
[csharp]
void AttachedScroll(ScrollViewer Scroll)
{
if (Scroll == null)
return;
ScrollView = Scroll;
Binding binding = new Binding();
binding.Source = Scroll;
binding.Path = new PropertyPath("VerticalOffset");
BindingOperations.SetBinding(this, ScrollViewerTrigger.VerticalOffsetProperty, binding);
}
void AttachedScroll(ScrollViewer Scroll)
{
if (Scroll == null)
return;
ScrollView = Scroll;
Binding binding = new Binding();
binding.Source = Scroll;
binding.Path = new PropertyPath("VerticalOffset");
BindingOperations.SetBinding(this, ScrollViewerTrigger.VerticalOffsetProperty, binding);
} 還記得我之前提到的VerticalOffset屬性嗎，我通過綁定將ScrollViewer中的VerticalOffset屬性與這個Trigger中的VerticalOffset綁定在一起，這樣ScrollViewer種的VerticalOffset一旦有變化我這邊可以實時指導。ScrollViewer中是沒有VerticalOffset變化通知的事件的，但通過綁定則可以解決這個問題。若做橫向監控功能， binding.Path = new PropertyPath("VerticalOffset");換成binding.Path = new PropertyPath("HorizontalOffset");就可以了。
關鍵一步解決了剩下的就好辦了，VerticalOffset一旦有變化則會調用到VerticalOffsetPropertyChanged函數，然後再調用OnVerticalOffsetChanged對坐標進行處理。
OnVerticalOffsetChanged中會對滾動條的當前位置進行篩選，若滾動條滑倒了底部就會觸發Trigger自帶的Actions，我這裡提供一個事件ScrollTrigger，在達到底部時也會觸發，方便外部使用。
使用起來很簡單，假如我們的viewmodel中有一個加載更多數據的命令，使用如下
[html]
<ListBox ItemsSource="{Binding List}">
<i:Interaction.Triggers>
<local:ScrollBarTrigger>
<i:InvokeCommandAction Command="{Binding MoreItemCommand}"/>
</local:ScrollBarTrigger>
</i:Interaction.Triggers>
</ListBox>
<ListBox ItemsSource="{Binding List}">
<i:Interaction.Triggers>
<local:ScrollBarTrigger>
<i:InvokeCommandAction Command="{Binding MoreItemCommand}"/>
</local:ScrollBarTrigger>
</i:Interaction.Triggers>
</ListBox> 有些控件中沒有ScrollViewer控件，但是有ScrollBar控件，我這裡提供一個ScrollBarTrigger，原理與ScrollViewerTrigger稍有不同，哪裡不同自己研究。另外ScrollBarTrigger比ScrollViewerTrigger更具通用性，因為ScrollViewer中包含ScrollBar。
下面是ScrollBarTrigger代碼：
[csharp]
public enum DirectionType { Top, Bottom, Left, Right }
public class ScrollBarTrigger : TriggerBase<DependencyObject>
{
ScrollBar ScrollView;
public static readonly DependencyProperty DirectionTypeProperty = DependencyProperty.Register("DirectionType", typeof(DirectionType), typeof(ScrollBarTrigger), new PropertyMetadata(DirectionType.Bottom));
public DirectionType DirectionType
{
get
{
return (DirectionType)base.GetValue(ScrollBarTrigger.DirectionTypeProperty);
}
set
{
base.SetValue(ScrollBarTrigger.DirectionTypeProperty, value);
}
}
public event EventHandler ScrollTrigger;
protected override void OnAttached()
{
base.OnAttached();
if (this.AssociatedObject != null && this.AssociatedObject is FrameworkElement)
{
(this.AssociatedObject as FrameworkElement).SizeChanged += control_SizeChanged;
}
}
protected override void OnDetaching()
{
base.OnDetaching();
if (ScrollView != null)
ScrollView.ValueChanged -= ScrollView_ValueChanged;
if (this.AssociatedObject != null && this.AssociatedObject is FrameworkElement)
{
(this.AssociatedObject as FrameworkElement).SizeChanged -= control_SizeChanged;
}
}
void control_SizeChanged(object sender, SizeChangedEventArgs e)
{
if (this.AssociatedObject == null || !(this.AssociatedObject is FrameworkElement))
return;
ScrollBar Scroll = this.AssociatedObject.GetFirstDescendantOfType<ScrollBar>();
if (Scroll != null)
{
AttachedScroll(Scroll);
(this.AssociatedObject as FrameworkElement).SizeChanged -= control_SizeChanged;
}
}
void AttachedScroll(ScrollBar Scroll)
{
if (Scroll != null)
{
ScrollView = Scroll;
ScrollView.ValueChanged += ScrollView_ValueChanged;
}
}
void ScrollView_ValueChanged(object sender, RoutedPropertyChangedEventArgs<double> e)
{
OnOffsetChanged();
}
void OnOffsetChanged()
{
if (ScrollView == null)
return;
ScrollBar Scroll = ScrollView;
switch (DirectionType)
{
case DirectionType.Top:
case DirectionType.Left:
{
if (Scroll.Maximum < double.Epsilon || Scroll.Value - Scroll.ViewportSize > 0)
return;
}
break;
case DirectionType.Bottom:
case DirectionType.Right:
{
if (Scroll.Maximum < double.Epsilon || Scroll.Value + Scroll.ViewportSize < Scroll.Maximum)
return;
}
break;
default: break;
}
if (ScrollTrigger != null)
{
ScrollTrigger(this.AssociatedObject,new EventArgs());
}
base.InvokeActions(null);
}
}
public enum DirectionType { Top, Bottom, Left, Right }
public class ScrollBarTrigger : TriggerBase<DependencyObject>
{
ScrollBar ScrollView;
public static readonly DependencyProperty DirectionTypeProperty = DependencyProperty.Register("DirectionType", typeof(DirectionType), typeof(ScrollBarTrigger), new PropertyMetadata(DirectionType.Bottom));
public DirectionType DirectionType
{
get
{
return (DirectionType)base.GetValue(ScrollBarTrigger.DirectionTypeProperty);
}
set
{
base.SetValue(ScrollBarTrigger.DirectionTypeProperty, value);
}
}
public event EventHandler ScrollTrigger;
protected override void OnAttached()
{
base.OnAttached();
if (this.AssociatedObject != null && this.AssociatedObject is FrameworkElement)
{
(this.AssociatedObject as FrameworkElement).SizeChanged += control_SizeChanged;
}
}
protected override void OnDetaching()
{
base.OnDetaching();
if (ScrollView != null)
ScrollView.ValueChanged -= ScrollView_ValueChanged;
if (this.AssociatedObject != null && this.AssociatedObject is FrameworkElement)
{
(this.AssociatedObject as FrameworkElement).SizeChanged -= control_SizeChanged;
}
}
void control_SizeChanged(object sender, SizeChangedEventArgs e)
{
if (this.AssociatedObject == null || !(this.AssociatedObject is FrameworkElement))
return;
ScrollBar Scroll = this.AssociatedObject.GetFirstDescendantOfType<ScrollBar>();
if (Scroll != null)
{
AttachedScroll(Scroll);
(this.AssociatedObject as FrameworkElement).SizeChanged -= control_SizeChanged;
}
}
void AttachedScroll(ScrollBar Scroll)
{
if (Scroll != null)
{
ScrollView = Scroll;
ScrollView.ValueChanged += ScrollView_ValueChanged;
}
}
void ScrollView_ValueChanged(object sender, RoutedPropertyChangedEventArgs<double> e)
{
OnOffsetChanged();
}
void OnOffsetChanged()
{
if (ScrollView == null)
return;
ScrollBar Scroll = ScrollView;
switch (DirectionType)
{
case DirectionType.Top:
case DirectionType.Left:
{
if (Scroll.Maximum < double.Epsilon || Scroll.Value - Scroll.ViewportSize > 0)
return;
}
break;
case DirectionType.Bottom:
case DirectionType.Right:
{
if (Scroll.Maximum < double.Epsilon || Scroll.Value + Scroll.ViewportSize < Scroll.Maximum)
return;
}
break;
default: break;
}
if (ScrollTrigger != null)
{
ScrollTrigger(this.AssociatedObject,new EventArgs());
}
base.InvokeActions(null);
}
} 如果對Action,Trigger,Behavior有不了解的朋友可以看我之前的博客，會有詳細解釋。
示例裡的工程名譽我將的有寫不一樣，因為我復用了之前的示例，但不影響我博客裡的內容
