UWP 系统级页面导航

UWP之前，要实现应用内页面后退，Win8.1要自己弄一个后退按钮，WP8.1还可以选择Handle后退按钮事件。

而在UWP中，后退的实现变得更加方便。

在Windows.UI.Core命名空间里，有SystemNavigationManager这么个东西，

只需handle它的BackRequested事件，就可以响应各种系统级的后退事件了，

包括：UWP应用标题栏的后退按钮，Win10平板模式下任务栏上的后退按钮，Win10m的硬件后退按钮，快捷键Win+Backspace, 以及用Cortana触发的后退事件。

SystemNavigationManager.GetForCurrentView().BackRequested += 
    App_BackRequested;

private void App_BackRequested(object sender, BackRequestedEventArgs e)
{
    Frame rootFrame = Window.Current.Content as Frame;
    if (rootFrame == null)
        return;

    // Navigate back if possible, and if the event has not 
    // already been handled .
    if (rootFrame.CanGoBack && e.Handled == false)
    {
        e.Handled = true;
        rootFrame.GoBack();
    }
}

另外，标题栏的后退按钮可以通过SystemNavigationManager来控制是否显示。

  if (rootFrame.CanGoBack)
            {
                // If we have pages in our in-app backstack and have opted in to showing back, do so
                SystemNavigationManager.GetForCurrentView().AppViewBackButtonVisibility = AppViewBackButtonVisibility.Visible;
            }
            else
            {
                // Remove the UI from the title bar if there are no pages in our in-app back stack
                SystemNavigationManager.GetForCurrentView().AppViewBackButtonVisibility = AppViewBackButtonVisibility.Collapsed;
            }

话说这东西我翻文档翻了半天才找到...

参考：https://msdn.microsoft.com/en-us/library/windows/apps/mt465734.aspx