UWP开发入门（十）——通过继承来扩展ListView

　　本篇之所以起这样一个名字，是因为重点并非如何自定义控件，不涉及创建CustomControl和UserControl使用的Template和XAML概念。而是通过继承的方法来扩展一个现有的类，在继承的子类中增加属性和扩展行为。

　　我们在《UWP开发入门（七）——下拉刷新》中提到过嵌套ScrollViewer的实现思路，本篇我们对ListView的第一个扩展行为，即是摒弃嵌套的做法，而是通过访问ListView内部的ScrollViewer控件，来监听ViewChanged事件。

　　访问ListView内部的ScrollViewer，必定离不开VisualTreeHelper类中的以下两个方法：　　

public static DependencyObject GetChild(DependencyObject reference, System.Int32 childIndex);

public static System.Int32 GetChildrenCount(DependencyObject reference);

　　可以将这两个方法进一步组合得到：

        static T FindFirstChild<T>(FrameworkElement element) where T : FrameworkElement
        {
            int childrenCount = VisualTreeHelper.GetChildrenCount(element);
            var children = new FrameworkElement[childrenCount];

            for (int i = 0; i < childrenCount; i++)
            {
                var child = VisualTreeHelper.GetChild(element, i) as FrameworkElement;
                children[i] = child;
                if (child is T)
                    return (T)child;
            }

            for (int i = 0; i < childrenCount; i++)
                if (children[i] != null)
                {
                    var subChild = FindFirstChild<T>(children[i]);
                    if (subChild != null)
                        return subChild;
                }

            return null;
        }

　　该方法通过递归来遍历FrameworkElement内部的元素，并返回第一个符合类型的元素。ListViewEx的第一个扩展如下：

    public class ListViewEx : ListView, INotifyPropertyChanged
    {
        private ScrollViewer _scrollViewer;

        public event EventHandler LoadHistoryEvent;
        public event PropertyChangedEventHandler PropertyChanged;

        protected void OnProperyChanged([CallerMemberName] string name = null)
        {
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(name));
        }

        public ListViewEx()
        {
            this.Loaded += ListViewEx_Loaded;
            this.Unloaded += ListViewEx_Unloaded;
        }

        private void ListViewEx_Unloaded(object sender, RoutedEventArgs e)
        {
            this.Unloaded -= ListViewEx_Unloaded;
            if (_scrollViewer != null)
            {
                _scrollViewer.ViewChanged -= Sv_ViewChanged;
            }
        }

        private void ListViewEx_Loaded(object sender, Windows.UI.Xaml.RoutedEventArgs e)
        {
            this.Loaded -= ListViewEx_Loaded;
            _scrollViewer = FindFirstChild<ScrollViewer>(this);
            if (_scrollViewer != null)
            {
                _scrollViewer.ViewChanged += Sv_ViewChanged;
            }
        }

        private async void Sv_ViewChanged(object sender, ScrollViewerViewChangedEventArgs e)
        {
            if (e.IsIntermediate == false && _scrollViewer.VerticalOffset < 1)
            {
                _scrollViewer.ChangeView(null, 50, null);
                await Task.Delay(10);
                LoadHistoryEvent?.Invoke(this, EventArgs.Empty);
            }
        }

　　嗯嗯，可以看到优雅的 -= event和恰到好处的null check，啊啊！忍不住想点个赞！在ViewChanged事件监测到滚动条到达顶部后，果断触发ListViewEx内定义的LoadHistoryEvent来通知更新数据。

　　本篇如果仅增加一个LoadHistoryEvent，又会被人非议是在补完前篇，一篇拆成两篇写……那好吧，我们再给ListViewEx增加第二个扩展GoBottomVisiblity属性，顾名思义即是ListViewEx向上滚动几屏以后，显示一个GoBottom的按钮。

　　在ListViewEx的类中，首先定义属性GoBottomVisibility，然后同样是在ViewChanged事件中，计算是否显示GoBottom按钮。

        public Visibility GoBottomVisiblity
        {
            get { return _goBottomVisiblity; }
            set
            {
                _goBottomVisiblity = value;
                this.OnProperyChanged();
            }
        }
        private void Sv_ViewChanged2(object sender, ScrollViewerViewChangedEventArgs e)
        {
            if (e.IsIntermediate == false)
            {
                CheckGoBottomVisibility();
            }
        }

        private void CheckGoBottomVisibility()
        {
            if (_scrollViewer.VerticalOffset + _scrollViewer.ViewportHeight < _scrollViewer.ScrollableHeight)
            {
                GoBottomVisiblity = Visibility.Visible;
            }
            else
            {
                GoBottomVisiblity = Visibility.Collapsed;
            }
        }

　　代码没法全部贴上来，一个是太长了显得啰嗦，二是会被管理员说没内涵从首页删掉……

　　大体上本篇就是给ListView扩展了LoadHistoryEvent事件和GoBottomVisibility属性。最后说说怎么用，XAML里使用ListViewEx代替默认的ListView，会发现多出一个LoadHistoryEvent，挂上加载数据的事件就OK了。然后在列表的下部画一个三角箭头，Visibility绑定到ListViewEx的GoBottomVisibility属性上就收工了。

　　

    <Grid>
        <local:ListViewEx x:Name="listViewEx" ItemsSource="{x:Bind Items}" LoadHistoryEvent="ListViewEx_LoadHistoryEvent"></local:ListViewEx>
        <Grid Margin="10" VerticalAlignment="Bottom" HorizontalAlignment="Center" 
              Tapped="Grid_Tapped" Visibility="{x:Bind listViewEx.GoBottomVisiblity,Mode=OneWay}">
            <Ellipse Fill="LightGray" Width="30" Height="30"></Ellipse>
            <Polyline Stroke="White" Points="10,10 15,20 20,10"></Polyline>
        </Grid>
    </Grid>

　　完整代码放在GayHub上，地址：https://github.com/manupstairs/UWPSamples

　　非常感谢各位捧场，能够点开页面看到这里，拜谢了！Orz