WP8.1 实现Continuation程序(打开文件，保存文件等）

以保存文件为例

首先，在项目中加入ContinuationManager.cs类，以及SuspensionManager.cs类。

其次，在App.xaml.cs中，完成如下步骤：

1. 添加ContinuationManager类的实例作为属性。

public ContinuationManager ContinuationManager { get; private set; }

2. 加入如下的方法

        // for continuable
        private Frame CreateRootFrame()
        {
            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();

                // Set the default language
                rootFrame.Language = Windows.Globalization.ApplicationLanguages.Languages[0];
                rootFrame.NavigationFailed += OnNavigationFailed;

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

            return rootFrame;
        }

        private async Task RestoreStatusAsync(ApplicationExecutionState previousExecutionState)
        {
            // Do not repeat app initialization when the Window already has content,
            // just ensure that the window is active
            if (previousExecutionState == ApplicationExecutionState.Terminated)
            {
                // Restore the saved session state only when appropriate
                try
                {
                    await SuspensionManager.RestoreAsync();
                }
                catch (SuspensionManagerException)
                {
                    //Something went wrong restoring state.
                    //Assume there is no state and continue
                }
            }
        }

        void OnNavigationFailed(object sender, NavigationFailedEventArgs e)
        {
            throw new Exception("Failed to load Page " + e.SourcePageType.FullName);
        }

        protected async override void OnActivated(IActivatedEventArgs e)
        {
            base.OnActivated(e);
            ContinuationManager = new ContinuationManager();

            Frame rootFrame = CreateRootFrame();

            await RestoreStatusAsync(e.PreviousExecutionState);

            if (rootFrame.Content == null)
            {
                rootFrame.Navigate(typeof(MainPage));
            }

            var continuationEventArgs = e as IContinuationActivatedEventArgs;
            if (continuationEventArgs != null)
            {
                // Call ContinuationManager to handle continuation activation
                ContinuationManager.Continue(continuationEventArgs);

            }
            Window.Current.Activate();
        }

最后， 实现保存文件的操作， 在保存文件的页面，使该页面可以实现 IFileSavePickerContinuable接口。

public sealed partial class MainPage: Page, IFileSavePickerContinuable
{
        string OriginalPictureUrl = string.Empty;
        private void DownloadAppBarButton_Click(object sender, RoutedEventArgs e)
        {
            var item = FlipViewControl.SelectedItem as ViewSource;
            if (null != item)
            {
                FileSavePicker savePicker = new FileSavePicker();
                savePicker.SuggestedStartLocation = PickerLocationId.PicturesLibrary;
                savePicker.FileTypeChoices.Add("JPG File", new ObservableCollection<string>() { ".jpg" });
                savePicker.DefaultFileExtension = ".jpg";
                savePicker.SuggestedFileName = item.ViewImageUrl.Substring(item.ViewImageUrl.LastIndexOf("/") + 1);
                savePicker.PickSaveFileAndContinue();
                OriginalPictureUrl = item.ViewImageUrl;
            }
        }

        public async void ContinueFileSavePicker(FileSavePickerContinuationEventArgs args)
        {
            if (null != args.File)
            {
                StorageFile saveFile = args.File;
                var uri = new Uri(OriginalPictureUrl);
                var fileName = saveFile.DisplayName;
                var thumbnail = RandomAccessStreamReference.CreateFromUri(uri);

                var remoteFile = await StorageFile.CreateStreamedFileFromUriAsync(fileName, uri, thumbnail);
                await remoteFile.CopyAndReplaceAsync(saveFile);

                var toast = new ToastPrompt { Message = "download completed." };
                toast.Show();
            }
        }

}

另外，若要实现打开文件的操作，页面需要实现 IFileOpenPickerContinuable 接口。

public sealed partial class SettingPage : Page, IFileOpenPickerContinuable
{
        private void PickPhoto()
        {
            FileOpenPicker openPicker = new FileOpenPicker();
            openPicker.ViewMode = PickerViewMode.Thumbnail;
            openPicker.SuggestedStartLocation = PickerLocationId.PicturesLibrary;
            openPicker.FileTypeFilter.Add(".jpg");
            openPicker.FileTypeFilter.Add(".jpeg");
            openPicker.FileTypeFilter.Add(".png");
            openPicker.PickSingleFileAndContinue();
        }

        public async void ContinueFileOpenPicker(FileOpenPickerContinuationEventArgs args)
        {
            if (args.Files.Count > 0)
            {
                StorageFile bgFile = null;
                string id = Guid.NewGuid().ToString();
                string bgFileName = string.Format(@"{0}.jpg", id);
                bgFile = await ApplicationData.Current.LocalFolder.CreateFileAsync(bgFileName, CreationCollisionOption.OpenIfExists);
                App.BgImg = bgFileName;
                bool isSaved = await new data().SetBackground(App.BgImg);

                App.isBGChanged = true;
                await args.Files[0].CopyAndReplaceAsync(bgFile);

                // display the selected image
                ShowImage();
            }
        }

}