Launching a File

Any app can request that the OS launch a file. Launching a file is achieved using the Windows.System.Launcher class. The following excerpt from the MainPageViewModel retrieves a file stored in the app’s private storage area and launches it using the Launcher’s LaunchFileAsync method:

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async Task LaunchFile()
{
    StorageFolder folder = ApplicationData.Current.LocalFolder;
    StorageFile file = await folder.GetFileAsync("Sample.unl");
    Launcher.LaunchFileAsync(file);
}

The path to the file must exist. If the file does not exist, a FileNotFoundException is raised.

The MainPageViewModel in the WPUnleashed.AppThatLaunchesACustomFileType project saves a sample file with the .unl extension (see Listing 30.4). The storage API is used to create a new file. A message is constructed, converted to bytes, and then written to the file using the Stream WriteAsync method.

LISTING 30.4. MainPageViewModel.SaveSampleFile Method

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async void SaveSampleFile()
{
    string assemblyName = Assembly.GetExecutingAssembly().FullName;
    assemblyName = assemblyName.Substring(
        0, assemblyName.IndexOf(",", StringComparison.Ordinal));

    string fileContent = string.Format(
        "This text was saved to a file at {0} from the assembly: {1}.",
        DateTime.Now, assemblyName);

    IStorageFolder folder = ApplicationData.Current.LocalFolder;
    IStorageFile file = await folder.CreateFileAsync(
                                fileName, CreationCollisionOption.ReplaceExisting);

    using (Stream stream = await file.OpenStreamForWriteAsync())
    {
        byte[] content = Encoding.UTF8.GetBytes(fileContent);
        await stream.WriteAsync(content, 0, content.Length);
    }
}

An ICommand named launchFileCommand calls the LaunchFile method, like so:

launchFileCommand = new DelegateCommand(obj => LaunchFile());

A button in the view is bound to the LaunchFileCommand property, as shown:

<Button Content="Launch File" Command="{Binding LaunchFileCommand}" />

Tapping the button causes the file to be loaded by the LaunchFile method, and the app is deactivated. The OS identifies the app that is registered to handle the .unl file extension and launches the WPUnleashed.FileAndProtocolAssociations app, which then navigates to the FileView.xaml page to display the file contents (see Figure 30.1).

FIGURE 30.1 The file content is displayed on the FileView.xaml page.

TABLE 30.2. File Types Reserved for Built-In Apps

TABLE 30.3. File Types Reserved for the Windows Phone OS