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&＃xff0c;各种各样的方案很多&＃xff0c;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&＃xff1a;监视滚动条坐标的变化&＃xff0c;在达到底部时触发自己的处理事件。
原理很简单&＃xff0c;但实现起来可没这么容易&＃xff0c;先上代码&＃xff0c;边看边说。
public class ScrollViewerTrigger:TriggerBase{ScrollViewer ScrollView;public static readonly DependencyProperty DirectionTypeProperty &＃61; 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 &＃61; 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 &＃61; d as ScrollViewerTrigger;if (behavior !&＃61; null)behavior.OnVerticalOffsetChanged();}protected override void OnAttached(){base.OnAttached();if (this.AssociatedObject !&＃61; null && this.AssociatedObject is FrameworkElement){(this.AssociatedObject as FrameworkElement).SizeChanged &＃43;&＃61; control_SizeChanged;}}void control_SizeChanged(object sender, SizeChangedEventArgs e){if (this.AssociatedObject &＃61;&＃61; null || !(this.AssociatedObject is FrameworkElement))return;ScrollViewer Scroll &＃61; this.AssociatedObject.GetFirstDescendantOfType();if (Scroll !&＃61; null){AttachedScroll(Scroll);(this.AssociatedObject as FrameworkElement).SizeChanged -&＃61; control_SizeChanged;}}void AttachedScroll(ScrollViewer Scroll){if (Scroll &＃61;&＃61; null)return;ScrollView &＃61; Scroll;Binding binding &＃61; new Binding();binding.Source &＃61; Scroll;binding.Path &＃61; new PropertyPath("VerticalOffset");BindingOperations.SetBinding(this, ScrollViewerTrigger.VerticalOffsetProperty, binding);}void OnVerticalOffsetChanged(){ScrollViewer Scroll &＃61; ScrollView;if (Scroll &＃61;&＃61; null)return;switch (DirectionType){case DirectionType.Top:{if (Scroll.ScrollableHeight double.Epsilon)return;}break;case DirectionType.Bottom:{if (Scroll.ScrollableHeight double.Epsilon)return;}break;case DirectionType.Right:{if (Scroll.ScrollableWidth
整个实现是Trigger的扩展&＃xff0c;这样灵活性和通用性非常高&＃xff0c;不管是什么控件。因为Trigger本身是附加元素&＃xff0c;对于任何控件都可以附加上去&＃xff0c;不要要改动原有代码就可以实现功能扩展。Trigger自带的Actions则有很强的灵活性&＃xff0c;动作触发后可以执行多个Action.
DirectionType是触发方向&＃xff0c;分上下左右4个方向&＃xff0c;默认是在底部触发&＃xff0c;想做下拉刷新功能时把DirectionType改成Top就可以了了。
VerticalOffset则是实现滚动条监视的关键&＃xff0c;什么作用稍后说明。
OnAttached()是整个Trigger的入口&＃xff0c;在这里面我监控了一个通用事件SizeChanged&＃xff0c;主要是通过这个事件查找控件内的ScrollViewer控件。这里不能使用loaded事件&＃xff0c;因为有些带滚动条的控件是在加载元素时才建立ScrollViewer控件&＃xff0c; loaded事件触发时内部元素不一定会被加载&＃xff0c;而 SizeChanged触发时 ScrollViewer已经建立好了。
control_SizeChanged中关键语句在于 this.AssociatedObject.GetFirstDescendantOfType();这语句是从当前控件的子控件中查找ScrollViewer控件&＃xff0c;查找到后就会调用AttachedScroll对 ScrollViewer的滚动进行监视。
下面就是关键了&＃xff0c;AttachedScroll函数要对 ScrollViewer进行监控&＃xff0c;那么如何监控呢&＃xff1f;
void AttachedScroll(ScrollViewer Scroll){if (Scroll &＃61;&＃61; null)return;ScrollView &＃61; Scroll;Binding binding &＃61; new Binding();binding.Source &＃61; Scroll;binding.Path &＃61; new PropertyPath("VerticalOffset");BindingOperations.SetBinding(this, ScrollViewerTrigger.VerticalOffsetProperty, binding);}
还记得我之前提到的VerticalOffset属性吗&＃xff0c;我通过绑定将ScrollViewer中的 VerticalOffset属性与这个 Trigger中的VerticalOffset绑定在一起&＃xff0c;这样ScrollViewer种的 VerticalOffset一旦有变化我这边可以实时指导。ScrollViewer中是没有 VerticalOffset变化通知的事件的&＃xff0c;但通过绑定则可以解决这个问题。若做横向监控功能&＃xff0c; binding.Path &＃61; new PropertyPath("VerticalOffset");换成 binding.Path &＃61; new PropertyPath("HorizontalOffset");就可以了。
关键一步解决了剩下的就好办了&＃xff0c; VerticalOffset一旦有变化则会调用到VerticalOffsetPropertyChanged函数&＃xff0c;然后再调用OnVerticalOffsetChanged对坐标进行处理。 OnVerticalOffsetChanged中会对滚动条的当前位置进行筛选&＃xff0c;若滚动条滑倒了底部就会触发Trigger自带的Actions&＃xff0c;我这里提供一个事件ScrollTrigger&＃xff0c;在达到底部时也会触发&＃xff0c;方便外部使用。
使用起来很简单&＃xff0c;假如我们的viewmodel中有一个加载更多数据的命令&＃xff0c;使用如下
有些控件中没有ScrollViewer控件&＃xff0c;但是有ScrollBar控件&＃xff0c;我这里提供一个ScrollBarTrigger&＃xff0c;原理与ScrollViewerTrigger稍有不同&＃xff0c;哪里不同自己研究。另外ScrollBarTrigger比ScrollViewerTrigger更具通用性&＃xff0c;因为ScrollViewer中包含ScrollBar。
下面是 ScrollBarTrigger代码&＃xff1a;
public enum DirectionType { Top, Bottom, Left, Right }public class ScrollBarTrigger : TriggerBase{ScrollBar ScrollView;public static readonly DependencyProperty DirectionTypeProperty &＃61; 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 !&＃61; null && this.AssociatedObject is FrameworkElement){(this.AssociatedObject as FrameworkElement).SizeChanged &＃43;&＃61; control_SizeChanged;}}protected override void OnDetaching(){base.OnDetaching();if (ScrollView !&＃61; null)ScrollView.ValueChanged -&＃61; ScrollView_ValueChanged;if (this.AssociatedObject !&＃61; null && this.AssociatedObject is FrameworkElement){(this.AssociatedObject as FrameworkElement).SizeChanged -&＃61; control_SizeChanged;}}void control_SizeChanged(object sender, SizeChangedEventArgs e){if (this.AssociatedObject &＃61;&＃61; null || !(this.AssociatedObject is FrameworkElement))return;ScrollBar Scroll &＃61; this.AssociatedObject.GetFirstDescendantOfType();if (Scroll !&＃61; null){AttachedScroll(Scroll);(this.AssociatedObject as FrameworkElement).SizeChanged -&＃61; control_SizeChanged;}}void AttachedScroll(ScrollBar Scroll){if (Scroll !&＃61; null){ScrollView &＃61; Scroll;ScrollView.ValueChanged &＃43;&＃61; ScrollView_ValueChanged;}}void ScrollView_ValueChanged(object sender, RoutedPropertyChangedEventArgs e){OnOffsetChanged();}void OnOffsetChanged(){if (ScrollView &＃61;&＃61; null)return;ScrollBar Scroll &＃61; ScrollView;switch (DirectionType){case DirectionType.Top:case DirectionType.Left:{if (Scroll.Maximum 0)return;}break;case DirectionType.Bottom:case DirectionType.Right:{if (Scroll.Maximum
如果对Action,Trigger,Behavior有不了解的朋友可以看我之前的博客&＃xff0c;会有详细解释。
示例里的工程名誉我将的有写不一样&＃xff0c;因为我复用了之前的示例&＃xff0c;但不影响我博客里的内容
示例&＃xff1a; Test.zip
京公网安备 11010802041100号