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[ASP.net教程]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