[置顶] 滚动条滑到底完美解决方案(适合任何带滚动条或ScrollBar控件
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。
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(); } }
整个实现是Trigger的扩展,这样灵活性和通用性非常高,不管是什么控件。因为Trigger本身是附加元素,对于任何控件都可以附加上去,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,动作触发后可以执行多个Action.
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); }
还记得我之前提到的VerticalOffset属性吗,我通过绑定将ScrollViewer中的 VerticalOffset属性与这个 Trigger中的VerticalOffset绑定在一起,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,但通过绑定则可以解决这个问题。若做横向监控功能, binding.Path = new PropertyPath("VerticalOffset");换成 binding.Path = new PropertyPath("HorizontalOffset");就可以了。
<ListBox ItemsSource="{Binding List}"> <i:Interaction.Triggers> <local:ScrollBarTrigger> <i:InvokeCommandAction Command="{Binding MoreItemCommand}"/> </local:ScrollBarTrigger> </i:Interaction.Triggers> </ListBox>
有些控件中没有ScrollViewer控件,但是有ScrollBar控件,我这里提供一个ScrollBarTrigger,原理与ScrollViewerTrigger稍有不同,哪里不同自己研究。另外ScrollBarTrigger比ScrollViewerTrigger更具通用性,因为ScrollViewer中包含ScrollBar。
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); } }
如果对Action,Trigger,Behavior有不了解的朋友可以看我之前的博客,会有详细解释。