Windows 8 系列（十一）：开发小技巧分享

临时记录、分享一些小的开发技巧：

1. 在数据绑定时，如果数据源需要被清空：collection.Clear()； 建议在语句后把集合置为null：collection.Clear(); collection=  null;（开发过程中因为没有置为null而报错，Windows.UI.Xaml.dll的错误）

2. async void 函数与async Task函数不能混为一谈。

　async void FUNCTIONNAME1(){ ...  await ...;  Debug.WriteLine("FUNCTIONNAME1 over")};   

   async Task FUNCTIONNAME2(){ ...  await ...;  Debug.WriteLine("FUNCTIONNAME2 over")};

   section1: FUNCTIONNAME1(); Debug.WriteLine("FUNCTIONNAME1")；

　section2: await FUNCTIONNAME2(); Debug.WriteLine("FUNCTIONNAME2")；

   以上两段代码中section1的代码在执行时会异步执行，有可能先输出FUNCTIONNAME1，然后输出FUNCTIONNAME1 over。第二段代码则能保证在输出FUNCTIONNAME2 over后再输出FUNCTIONNAME2。

3. 你的代码是不是真的实现了虚拟化？不能单单看你的代码是不是写了相关控件，而需要用VirtualizingStackPanel的CleanUpVirtualizedItemEvent事件来确定是不是真正的实现了虚拟化。我们在gridview的style中修改了样式，导致上述事件不触发：

<Style x:Key="PaddingGridViewStyle" TargetType="GridView">
        <Setter Property="Padding" Value="0,0,0,10"/>
        <Setter Property="IsTabStop" Value="False"/>
        <Setter Property="TabNavigation" Value="Once"/>
        <Setter Property="ScrollViewer.HorizontalScrollBarVisibility" Value="Auto"/>
        <Setter Property="ScrollViewer.VerticalScrollBarVisibility" Value="Disabled"/>
        <Setter Property="ScrollViewer.HorizontalScrollMode" Value="Enabled"/>
        <Setter Property="ScrollViewer.IsHorizontalRailEnabled" Value="False"/>
        <Setter Property="ScrollViewer.VerticalScrollMode" Value="Disabled"/>
        <Setter Property="ScrollViewer.IsVerticalRailEnabled" Value="False"/>
        <Setter Property="ScrollViewer.ZoomMode" Value="Disabled"/>
        <Setter Property="IsSwipeEnabled" Value="True"/>
        <Setter Property="ItemContainerTransitions">
            <Setter.Value>
                <TransitionCollection>
                    <AddDeleteThemeTransition/>
                    <ContentThemeTransition/>
                    <ReorderThemeTransition/>
                    <EntranceThemeTransition IsStaggeringEnabled="False"/>
                </TransitionCollection>
            </Setter.Value>
        </Setter>
        <Setter Property="Template">
            <Setter.Value>
                <ControlTemplate TargetType="GridView">
                    <Border BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}" Background="{TemplateBinding Background}" Padding="1">
                        <ScrollViewer x:Name="ScrollViewer" HorizontalScrollMode="{TemplateBinding ScrollViewer.HorizontalScrollMode}" HorizontalScrollBarVisibility="{TemplateBinding ScrollViewer.HorizontalScrollBarVisibility}" 
                                      IsHorizontalRailEnabled="{TemplateBinding ScrollViewer.IsHorizontalRailEnabled}" IsHorizontalScrollChainingEnabled="{TemplateBinding ScrollViewer.IsHorizontalScrollChainingEnabled}" IsVerticalScrollChainingEnabled="{TemplateBinding ScrollViewer.IsVerticalScrollChainingEnabled}" 
                                      IsVerticalRailEnabled="{TemplateBinding ScrollViewer.IsVerticalRailEnabled}" TabNavigation="{TemplateBinding TabNavigation}" VerticalScrollBarVisibility="{TemplateBinding ScrollViewer.VerticalScrollBarVisibility}" VerticalScrollMode="{TemplateBinding ScrollViewer.VerticalScrollMode}" 
                                      ZoomMode="{TemplateBinding ScrollViewer.ZoomMode}">
                            <Grid Margin="{TemplateBinding Padding}">
                                <ItemsPresenter HeaderTemplate="{TemplateBinding HeaderTemplate}" Header="{TemplateBinding Header}" HeaderTransitions="{TemplateBinding HeaderTransitions}" />
                            </Grid>
                        </ScrollViewer>
                    </Border>
                </ControlTemplate>
            </Setter.Value>
        </Setter>
    </Style>

代码中红色的部分导致虚拟化被破坏，修改后

<ScrollViewer x:Name="ScrollViewer" ...>
     <ItemsPresenter HeaderTemplate="{TemplateBinding HeaderTemplate}" Header="{TemplateBinding Header}" Margin="{TemplateBinding Padding}" HeaderTransitions="{TemplateBinding HeaderTransitions}" />
</ScrollViewer>