[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。

  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。

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.

  DirectionType是触发方向，分上下左右4个方向，默认是在底部触发，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，什么作用稍后说明。

  OnAttached()是整个Trigger的入口，在这里面我监控了一个通用事件SizeChanged，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，因为有些带滚动条的控件是在加载元素时才建立ScrollViewer控件， loaded事件触发时内部元素不一定会被加载，而   SizeChanged触发时 ScrollViewer已经建立好了。

  control_SizeChanged中关键语句在于 this.AssociatedObject.GetFirstDescendantOfType<ScrollViewer>();这语句是从当前控件的子控件中查找ScrollViewer控件，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，AttachedScroll函数要对 ScrollViewer进行监控，那么如何监控呢？

       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。

  关键一步解决了剩下的就好办了， VerticalOffset一旦有变化则会调用到VerticalOffsetPropertyChanged函数，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，若滚动条滑倒了底部就会触发Trigger自带的Actions，我这里提供一个事件ScrollTrigger，在达到底部时也会触发，方便外部使用。

  使用起来很简单，假如我们的viewmodel中有一个加载更多数据的命令，使用如下

                  <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。

  下面是 ScrollBarTrigger代码：

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。

  示例里的工程名誉我将的有写不一样，因为我复用了之前的示例，但不影响我博客里的内容

  示例： Test.zip