UWP开发入门（十）——通过继承来扩展ListView

原文: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