Windows 8 XAML Tips - Async Search Suggestions

There are a lot of examples on the web how you can add Search Suggestions to the Search Charm of a Windows 8 Metro application. But all the examples I found only demonstrate how to implement a synchronous version. In this blog I will show you how you can do this also asynchronous. I use this technique to fill my Search Suggestion by calling a web service asynchronous. Something which is very common because your data will problably be stored in the Web/Cloud.

Implement Search Charm

To demonstrate the solution I created a new project in Visual Studio 2012 using the 'Grid App (XAML)' project template. Next I added the 'Search Contract' item template to the project. This adds an SearchResultsPage to the project and adds the Search Declaration to the Package.appxmanifest.

In my App.cs file I added the InitSearch() method which I call from the OnLaunched() method. In this InitSearch() method I subscribe the app on SuggestionsRequested event of the current SearchPane view.

sealed partial class App : Application
{
    /// <summary>
    /// Initializes the singleton Application object.  This is the first line of authored code
    /// executed, and as such is the logical equivalent of main() or WinMain().
    /// </summary>
    public App()
    {
        this.InitializeComponent();
        this.Suspending += OnSuspending;
    }

    /// <summary>
    /// Invoked when the application is launched normally by the end user.  Other entry points
    /// will be used when the application is launched to open a specific file, to display
    /// search results, and so forth.
    /// </summary>
    /// <param name="args">Details about the launch request and process.</param>
    protected override async void OnLaunched(LaunchActivatedEventArgs args)
    {
        // Do not repeat app initialization when already running, just ensure that
        // the window is active
        if (args.PreviousExecutionState == ApplicationExecutionState.Running)
        {
            Window.Current.Activate();
            return;
        }

        // Create a Frame to act as the navigation context and associate it with
        // a SuspensionManager key
        var rootFrame = new Frame();
        SuspensionManager.RegisterFrame(rootFrame, "AppFrame");

        if (args.PreviousExecutionState == ApplicationExecutionState.Terminated)
        {
            // Restore the saved session state only when appropriate
            await SuspensionManager.RestoreAsync();
        }

        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
            if (!rootFrame.Navigate(typeof(GroupedItemsPage), "AllGroups"))
            {
                throw new Exception("Failed to create initial page");
            }
        }

        // Place the frame in the current Window and ensure that it is active
        Window.Current.Content = rootFrame;
        Window.Current.Activate();

        InitSearch();
    }

    private void InitSearch() {
        var view = SearchPane.GetForCurrentView();
        view.SuggestionsRequested += view_SuggestionsRequested;

        //view.QueryChanged += view__QueryChanged;
        //view.QuerySubmitted += view_QuerySubmitted;
        //view.ResultSuggestionChosen += view_ResultSuggestionChosen;
        //view.VisibilityChanged += view_VisibilityChanged;
    }

    private void view_SuggestionsRequested(SearchPane sender, 
                                    SearchPaneSuggestionsRequestedEventArgs args) {
        try {
            var list = Enumerable.Range(1, 4).Select(n => args.QueryText + " " + n);
            args.Request.SearchSuggestionCollection.AppendQuerySuggestions(list);
        } catch (Exception ex) {
            Debug.WriteLine(ex.Message);
        }
    }

In this example the SuggestionsRequested() method creates 4 query suggestions which are the querytext with the suffix 1 to 4. This will look like this if you run the app and start a search for the word 'demo'.

Problem

To simulate an async network call I added an awaited call to the Task.Delay() method in my view_SuggestionsRequested() method (line 5). I also added the async keyword to make the compiler happy. It is required if you use the await keyword.

private async void view_SuggestionsRequested(SearchPane sender, 
                                             SearchPaneSuggestionsRequestedEventArgs args) {
    try {
        // Simulate an async network call
        await Task.Delay(100);
        var list = Enumerable.Range(1, 4).Select(n => args.QueryText + " " + n);

        args.Request.SearchSuggestionCollection.AppendQuerySuggestions(list);
    } catch (Exception ex) {
        Debug.WriteLine(ex.Message);
        // A method was called at an unexpected time. (Exception from HRESULT: 0x8000000E)
    }
}

If I run the app the Search Suggestions don't work anymore. The catch blocks writes this message of the exception to the output window: 'A method was called at an unexpected time. (Exception from HRESULT: 0x8000000E)'.

Solution

The solution is quite simple but badly documented in the MSDN documentation of the SearchPane.SuggestionsRequested event. It states in a note that if you want to respond to this event asynchronously, you must use SearchPaneSuggestionsRequestedEventArgs.Request.GetDeferral. So in my code I retrieved the Deferral (line 5). And I called the Complete() method on it after the list is appended to the SearchSuggestionCollection (line 14).

private async void view_SuggestionsRequested(SearchPane sender,
                                             SearchPaneSuggestionsRequestedEventArgs args) {
    try {
        // Get the deferral
        var deferral = args.Request.GetDeferral();

        // Simulate an async network call
        await Task.Delay(100);
        var list = Enumerable.Range(1, 4).Select(n => args.QueryText + " " + n);

        args.Request.SearchSuggestionCollection.AppendQuerySuggestions(list);

        // Mark the deferral as Complete
        deferral.Complete();
    } catch (Exception ex) {
        Debug.WriteLine(ex.Message);
    }
}

This fixed the problem and my app runs like a charm ;-)

Closure and download

I hope you like my solution. You can download my code here.

Cheers,

Fons

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