滾動條滑到底完美解決方案（適合任何帶滾動條或ScrollBar控件

  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。在這裏我分享一個我的自認爲滿意的解決方案。

  首先說下大致原理：監視滾動條座標的變化，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。

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。因爲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，在這裏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。這裏不能使用loaded事件，因爲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？

       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對座標進行處理。

  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，我這裏提供一個事件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

                  <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，我這裏提供一個ScrollBarTrigger，原理與ScrollViewerTrigger稍有不同，哪裏不同自己研究。另外ScrollBarTrigger比ScrollViewerTrigger更具通用性，因爲ScrollViewer中包含ScrollBar。

  下面是ScrollBarTrigger代碼：

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有不瞭解的朋友可以看我之前的博客，會有詳細解釋。

  示例裏的工程名譽我將的有寫不一樣，因爲我複用了之前的示例，但不影響我博客裏的內容

  示例：Test.zip