win-universal-app - UWP 页面状态管理

Question

我想学习如何管理导航之间的页面状态。例如，导航到第 1 页，然后导航到第 2 页，但是当我导航回第 1 页时，UI 元素必须已经存在并且具有与以前相同的数据，并且不得重新初始化它们或不得再次绑定(bind)数据编译器。另外我可以做些什么来管理整个应用程序的状态，这样我就终止了应用程序，然后当我下次启动它时，与上次相同的状态已经存在。我可以将它应用于整个应用程序吗？或者如果我只想在几页上应用它怎么办？任何帮助将不胜感激。

Answer 1

or example a navigate onto page1 and then i navigate to page2, but when i navigate back to page1, the UI elements must already be there with the same data as before and they must not be re-initialized or data must not be binded again by the compiler.

对于这个问题，你可以使用UIElement.CacheMode property和 Frame.CacheSize property . CacheSize 属性设置导航历史中可以为框架缓存的页面数，CacheMode 属性设置一个值，指示呈现的内容应缓存为复合内容尽可能使用位图。

我们知道，UWP 应用程序默认使用 rootFrame 来显示多个页面，我们只需使用 Navigation 方法来更改框架中的内容。您可以在空白 UWP 应用的 OnLaunched(LaunchActivatedEventArgs e) 方法中看到这一点。但是如何实现缓存功能呢？例如，您的应用有两个页面和一个根框架。您可以在 OnLaunched(LaunchActivatedEventArgs e) 方法中定义 CacheSize 属性，例如:

protected override void OnLaunched(LaunchActivatedEventArgs e)
{
    ...
    // Ensure the current window is active

    rootFrame.CacheSize = 2;
    Window.Current.Activate();
}

然后在您的两个页面的构造函数中启用 CacheMode 属性，例如:

public MainPage()
{
    this.InitializeComponent();
    this.NavigationCacheMode = NavigationCacheMode.Enabled;
}

Also what I can do to manage state of whole application such that, I terminate the app and then when i launch it next time, the same state is already there as last time. can i apply it on whole application?

对于这个问题，您需要使用 Frame.GetNavigationState method 在 OnSuspending(object sender, SuspendingEventArgs e) 方法中保存页面状态，您可以将此状态保存到应用程序的本地设置中。例如:

private void OnSuspending(object sender, SuspendingEventArgs e)
{
    var deferral = e.SuspendingOperation.GetDeferral();
    Frame rootFrame = Window.Current.Content as Frame;
    string navstate = rootFrame.GetNavigationState();
    var localSettings = ApplicationData.Current.LocalSettings;
    localSettings.Values["nav"] = navstate;
    deferral.Complete();
}

如何检索这些信息？您可以重写您的 OnLaunched(LaunchActivatedEventArgs e) 方法，首先您需要判断您的应用上次是如何关闭的，由用户关闭，还是由系统使用 ApplicationExecutionState enumeration 关闭。 ，例如像这样:

protected override void OnLaunched(LaunchActivatedEventArgs e)
{
    //#if DEBUG
    //            if (System.Diagnostics.Debugger.IsAttached)
    //            {
    //                this.DebugSettings.EnableFrameRateCounter = true;
    //            }
    //#endif

    Frame rootFrame = Window.Current.Content as Frame;

    // Do not repeat app initialization when the Window already has content,
    // just ensure that the window is active
    if (rootFrame == null)
    {
        // Create a Frame to act as the navigation context and navigate to the first page
        rootFrame = new Frame();

        rootFrame.NavigationFailed += OnNavigationFailed;

        if (e.PreviousExecutionState == ApplicationExecutionState.Terminated)
        {
            //TODO: Load state from previously suspended application
        }

        // Place the frame in the current Window
        Window.Current.Content = rootFrame;
    }

    if (rootFrame.Content == null)
    {
        // When the navigation stack isn't restored navigate to the first page,
        // configuring the new page by passing required information as a navigation
        // parameter
        //rootFrame.Navigate(typeof(MainPage), e.Arguments);
        if (e.PreviousExecutionState == ApplicationExecutionState.Terminated || 
            e.PreviousExecutionState == ApplicationExecutionState.ClosedByUser)
        {
            object value;
            var localSettings = ApplicationData.Current.LocalSettings;
            if (localSettings.Values.TryGetValue("nav", out value))
            {
                rootFrame.SetNavigationState(value as string);
            }
            else
            {
                rootFrame.Navigate(typeof(MainPage), e.Arguments);
            }
        }
        else
        {
            rootFrame.Navigate(typeof(MainPage), e.Arguments);
        }
    }
    // Ensure the current window is active
    rootFrame.CacheSize = 2;
    Window.Current.Activate();
}

但是需要注意的是，当一个应用关闭后，下次启动这个应用时，UI元素会重新初始化，这个函数只能导航到上次关闭应用时的页面，但是里面的数据该页面将丢失。但您也可以将数据保存到本地设置，当您导航到页面时，将值设置为那些 UI 元素。