UWP开发入门（二）——RelativePanel

　　RelativePanel也是Win10 UWP新增的控件，和上篇提到的SplitView一样在UWP的UI布局起到非常重要的作用。说句实在话，这货其实就是为了UWP的Adaptive UI而特意增加的，由于他的功能和DockPanel有相当的重叠，以至于DockPanel被从Win10 SDK中被撸掉了……太惨了……

　　为什么说RelativePanel可以替代DockPanel，我们可以先从几个比较重要的属性看起：AlignLeftWithPanel,AlignRightWithPanel,AlignTopWithPanel,AlignBottomWithPanel。这几个属性如果是True的话，看上去的效果分明就是原先的DockPanel.Left,Right,Top,Bottom。我们先来看原先可以用DockPanel实现的下图，采用RelativePanel是如何编写的：

    <RelativePanel Background="{ThemeResource ApplicationPageBackgroundThemeBrush}" >
        <Button x:Name="ButtonHamburger" FontFamily="{ThemeResource SymbolThemeFontFamily}" Content="" RelativePanel.AlignLeftWithPanel="True"></Button>
        <TextBlock Text="类别" RelativePanel.RightOf="ButtonHamburger" RelativePanel.AlignVerticalCenterWith="ButtonHamburger" Margin="10,0,0,0"></TextBlock>
        <Button FontFamily="{ThemeResource SymbolThemeFontFamily}" Content="" RelativePanel.LeftOf="ButtonAdd"/>
        <Button x:Name="ButtonAdd" FontFamily="{ThemeResource SymbolThemeFontFamily}" Content="" RelativePanel.AlignRightWithPanel="True"/>
    </RelativePanel>

　　RelativePanel中共有三个Button，一个TextBlock。分别靠左右对齐，用到了RelativePanel的几个附加属性：AlignLeftWithPanel，RightOf，LeftOf，AlignRightWithPanel。这里还有一点要注意一下，TextBlock为了实现纵向的居中对齐，使用了AlignVerticalCenterWith，有兴趣的同学可以试一下，在RelativePanel里VerticalAlignment优先级较低，仅在空间不足以显示控件时才起到居中对齐的作用。

　　有的童鞋会说以上的效果即使用Grid也是可以实现的，话是没有错啦，但在UWP开发中，RelativePanel一般都是要配合AdaptiveTrigger来实现自适应布局的，比如下面两张图对比：

　　在平板或者桌面模式，采用左右的菜单布局，而在手机则变成上下菜单布局，在UWP中实现这种动态变化的效果，完全可以通过纯XAML来实现：

    <RelativePanel Background="{ThemeResource ApplicationPageBackgroundThemeBrush}">
        <VisualStateManager.VisualStateGroups>
            <VisualStateGroup>
                <VisualState >
                    <VisualState.StateTriggers>
                        <AdaptiveTrigger MinWindowWidth="401" />
                    </VisualState.StateTriggers>
                </VisualState>
                <VisualState >
                    <VisualState.StateTriggers>
                        <AdaptiveTrigger MinWindowWidth="0" />
                    </VisualState.StateTriggers>
                    <VisualState.Setters>
                        <Setter Target="RelativeNavigation.(RelativePanel.AlignTopWithPanel)" Value="False"></Setter>
                        <Setter Target="RelativeNavigation.(RelativePanel.AlignRightWithPanel)" Value="True"></Setter>
                        <Setter Target="ButtonHome.(RelativePanel.AlignTopWithPanel)" Value="False"></Setter>
                        <Setter Target="ButtonHome.(RelativePanel.AlignLeftWithPanel)" Value="True"></Setter>
                        <Setter Target="ButtonMessage.(RelativePanel.Below)" Value=""></Setter>
                        <Setter Target="ButtonMessage.(RelativePanel.RightOf)" Value="ButtonHome"></Setter>
                        <Setter Target="ButtonAdd.(RelativePanel.Below)" Value=""></Setter>
                        <Setter Target="ButtonAdd.(RelativePanel.RightOf)" Value="ButtonMessage"></Setter>
                        <Setter Target="ButtonFind.(RelativePanel.Below)" Value=""></Setter>
                        <Setter Target="ButtonFind.(RelativePanel.RightOf)" Value="ButtonAdd"></Setter>
                        <Setter Target="ButtonMe.(RelativePanel.Below)" Value=""></Setter>
                        <Setter Target="ButtonMe.(RelativePanel.RightOf)" Value="ButtonFind"></Setter>
                        <Setter Target="GridContent.(RelativePanel.AlignBottomWithPanel)" Value="False"></Setter>
                        <Setter Target="GridContent.(RelativePanel.AlignLeftWithPanel)" Value="True"></Setter>
                        <Setter Target="GridContent.(RelativePanel.AlignBottomWith)" Value="RelativeNavigation"></Setter>
                    </VisualState.Setters>
                </VisualState>
            </VisualStateGroup>
        </VisualStateManager.VisualStateGroups>
        <RelativePanel x:Name="RelativeNavigation" RelativePanel.AlignLeftWithPanel="True" RelativePanel.AlignBottomWithPanel="True" RelativePanel.AlignTopWithPanel="True" Background="LightGray">
            <Button x:Name="ButtonHome" FontFamily="{ThemeResource SymbolThemeFontFamily}" Content="" RelativePanel.AlignTopWithPanel="True"/>
            <Button x:Name="ButtonMessage" FontFamily="{ThemeResource SymbolThemeFontFamily}" Content="" RelativePanel.Below="ButtonHome"/>
            <Button x:Name="ButtonFind" FontFamily="{ThemeResource SymbolThemeFontFamily}" Content="" RelativePanel.Below="ButtonMessage"/>
            <Button x:Name="ButtonMe" FontFamily="{ThemeResource SymbolThemeFontFamily}" Content="" RelativePanel.Below="ButtonFind"/>
            <Button x:Name="ButtonAdd" FontFamily="{ThemeResource SymbolThemeFontFamily}" Content="" Background="Orange" RelativePanel.Below="ButtonMe"/>
        </RelativePanel>
        <Grid x:Name="GridContent" RelativePanel.AlignRightWithPanel="True" RelativePanel.AlignBottomWithPanel="True" RelativePanel.AlignTopWithPanel="True" RelativePanel.RightOf="RelativeNavigation" >
            <TextBlock Text="我是一个水印" Foreground="LightGray" VerticalAlignment="Center" HorizontalAlignment="Center"></TextBlock>
        </Grid>
    </RelativePanel>

　　看上去啰里啰唆写了一大堆，主要还是为了展示RelativePanel的用法，并不是最优的写法，如果能提供各位一丝丝的灵感，那俺就很满意了。