在后台操作过程中显示模态UI并继续

我有一个运行后台任务的WPF应用程序，它使用async/await 。 任务是在进展时更新应用程序的状态UI。 在此过程中，如果满足某个条件，我需要显示一个模态窗口，让用户知道这样的事件，然后继续处理，现在也更新该模态窗口的状态UI。

这是我想要实现的草图版本：

 async Task AsyncWork(int n, CancellationToken token) { // prepare the modal UI window var modalUI = new Window(); modalUI.Width = 300; modalUI.Height = 200; modalUI.Content = new TextBox(); using (var client = new HttpClient()) { // main loop for (var i = 0; i < n; i++) { token.ThrowIfCancellationRequested(); // do the next step of async process var data = await client.GetStringAsync("http://www.bing.com/search?q=item" + i); // update the main window status var info = "#" + i + ", size: " + data.Length + Environment.NewLine; ((TextBox)this.Content).AppendText(info); // show the modal UI if the data size is more than 42000 bytes (for example) if (data.Length < 42000) { if (!modalUI.IsVisible) { // show the modal UI window modalUI.ShowDialog(); // I want to continue while the modal UI is still visible } } // update modal window status, if visible if (modalUI.IsVisible) ((TextBox)modalUI.Content).AppendText(info); } } } 

modalUI.ShowDialog()的问题在于它是一个阻塞调用，因此处理将停止，直到对话框关闭。 如果窗口是无模式的，那将不是问题，但它必须是模态的，如项目要求所规定的那样。

有没有办法用async/await解决这个问题？

Answer 1

这可以通过异步执行modalUI.ShowDialog()来实现（在UI线程的消息循环的未来迭代中）。 以下ShowDialogAsync实现通过使用TaskCompletionSource （ EAP任务模式 ）和SynchronizationContext.Post 。

这样的执行工作流程可能有点难以理解，因为您的异步任务现在分布在两个单独的WPF消息循环中：主线程的一个和新的嵌套一个（由ShowDialog启动）。 IMO，这很好，我们只是利用C＃编译器提供的async/await状态机。

虽然，当您的任务在模态窗口仍处于打开状态时结束时，您可能希望等待用户关闭它。 这就是下面的CloseDialogAsync所做的事情。 此外，您可能应该考虑用户在任务中间关闭对话框的情况（AFAIK，WPF窗口不能重复用于多个ShowDialog调用）。

以下代码适用于我：

 using System; using System.Net.Http; using System.Threading; using System.Threading.Tasks; using System.Windows; using System.Windows.Controls; namespace WpfAsyncApp { public partial class MainWindow : Window { public MainWindow() { InitializeComponent(); this.Content = new TextBox(); this.Loaded += MainWindow_Loaded; } // AsyncWork async Task AsyncWork(int n, CancellationToken token) { // prepare the modal UI window var modalUI = new Window(); modalUI.Width = 300; modalUI.Height = 200; modalUI.Content = new TextBox(); try { using (var client = new HttpClient()) { // main loop for (var i = 0; i < n; i++) { token.ThrowIfCancellationRequested(); // do the next step of async process var data = await client.GetStringAsync("http://www.bing.com/search?q=item" + i); // update the main window status var info = "#" + i + ", size: " + data.Length + Environment.NewLine; ((TextBox)this.Content).AppendText(info); // show the modal UI if the data size is more than 42000 bytes (for example) if (data.Length < 42000) { if (!modalUI.IsVisible) { // show the modal UI window asynchronously await ShowDialogAsync(modalUI, token); // continue while the modal UI is still visible } } // update modal window status, if visible if (modalUI.IsVisible) ((TextBox)modalUI.Content).AppendText(info); } } // wait for the user to close the dialog (if open) if (modalUI.IsVisible) await CloseDialogAsync(modalUI, token); } finally { // always close the window modalUI.Close(); } } // show a modal dialog asynchronously static async Task ShowDialogAsync(Window window, CancellationToken token) { var tcs = new TaskCompletionSource(); using (token.Register(() => tcs.TrySetCanceled(), useSynchronizationContext: true)) { RoutedEventHandler loadedHandler = (s, e) => tcs.TrySetResult(true); window.Loaded += loadedHandler; try { // show the dialog asynchronously // (presumably on the next iteration of the message loop) SynchronizationContext.Current.Post((_) => window.ShowDialog(), null); await tcs.Task; Debug.Print("after await tcs.Task"); } finally { window.Loaded -= loadedHandler; } } } // async wait for a dialog to get closed static async Task CloseDialogAsync(Window window, CancellationToken token) { var tcs = new TaskCompletionSource(); using (token.Register(() => tcs.TrySetCanceled(), useSynchronizationContext: true)) { EventHandler closedHandler = (s, e) => tcs.TrySetResult(true); window.Closed += closedHandler; try { await tcs.Task; } finally { window.Closed -= closedHandler; } } } // main window load event handler async void MainWindow_Loaded(object sender, RoutedEventArgs e) { var cts = new CancellationTokenSource(30000); try { // test AsyncWork await AsyncWork(10, cts.Token); MessageBox.Show("Success!"); } catch (Exception ex) { MessageBox.Show(ex.ToString()); } } } } 

[EDITED]下面是一个稍微不同的方法，它使用Task.Factory.StartNew异步调用modalUI.ShowDialog() 。 可以稍后等待返回的Task以确保用户已关闭modal dialog。

 async Task AsyncWork(int n, CancellationToken token) { // prepare the modal UI window var modalUI = new Window(); modalUI.Width = 300; modalUI.Height = 200; modalUI.Content = new TextBox(); Task modalUITask = null; try { using (var client = new HttpClient()) { // main loop for (var i = 0; i < n; i++) { token.ThrowIfCancellationRequested(); // do the next step of async process var data = await client.GetStringAsync("http://www.bing.com/search?q=item" + i); // update the main window status var info = "#" + i + ", size: " + data.Length + Environment.NewLine; ((TextBox)this.Content).AppendText(info); // show the modal UI if the data size is more than 42000 bytes (for example) if (data.Length < 42000) { if (modalUITask == null) { // invoke modalUI.ShowDialog() asynchronously modalUITask = Task.Factory.StartNew( () => modalUI.ShowDialog(), token, TaskCreationOptions.None, TaskScheduler.FromCurrentSynchronizationContext()); // continue after modalUI.Loaded event var modalUIReadyTcs = new TaskCompletionSource(); using (token.Register(() => modalUIReadyTcs.TrySetCanceled(), useSynchronizationContext: true)) { modalUI.Loaded += (s, e) => modalUIReadyTcs.TrySetResult(true); await modalUIReadyTcs.Task; } } } // update modal window status, if visible if (modalUI.IsVisible) ((TextBox)modalUI.Content).AppendText(info); } } // wait for the user to close the dialog (if open) if (modalUITask != null) await modalUITask; } finally { // always close the window modalUI.Close(); } } 

Answer 2

作为一种完全不同的方法，请看看David Wheelers概念应用程序的有趣变化。 http://coloringinguy.com/2012/11/07/model-view-viewmodel-sample/

基本上他有一个半透明的覆盖层，他移动到控制器的前面需要很长时间才能更新。 我认为这是一个很酷的用户体验，值得回顾，因为它创建了模态体验而不会阻止UI更新。