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Loading resources outside of CreateResources

This document discusses how apps using Win2D's XAML controls, CanvasControl, CanvasVirtualControl and CanvasAnimatedControl, can load resources from outside the CreateResources handler.

Resource loading and CanvasControl / CanvasVirtualControl

Normally, apps are expected to use the CreateResources handler for creating controls' resources, so that device dependent resources are re-recreated as necessary if the device is lost. This includes resources that are loaded asynchronously. For asynchronous resource loading, controls are encouraged to use TrackAsyncAction(IAsyncAction) with CreateResources to ensure correct behavior.

All of this works well for cases where all resources are loaded at startup only.

But, what about apps which need to load some resources at startup, and some other resources later? For example, consider a game with different levels, and the levels need different graphical assets. Win2D doesn't have something built-in with CreateResources to enable this- an app cannot manually tell a control, "Re-issue CreateResources now, so that I can load different assets from before". However, the building blocks are there to make this work, and allow very good flexibility for how and when the resources are loaded, and be robust with respect to lost device.

Really, what an app wants to do in this case, is have a custom LoadResourcesForLevelAsync method, a "custom" CreateResources-launched task, like this:

async Task LoadResourcesForLevelAsync(CanvasControl resourceCreator, int level)
{
    levelBackground = await CanvasBitmap.LoadAsync(resourceCreator, ...);
    levelThingie = await CanvasBitmap.LoadAsync(resourceCreator, ...);
    // etc.
}

The app needs to load some of its resources after CreateResources is completed. In particular, the app will issue the level load after CreateResources has completed - e.g., from its Draw handler. In the code below, the app's Draw handler controls the progress of its level-loading Task.

To make CreateResources work in this situation, and be robust against lost devices, an app needs to do four things:

  1. Track when LoadResourcesForLevelAsync is in progress.
  2. Allow Win2D to handle any exceptions (in particular, device lost) that the app doesn't know how to handle.
  3. If Win2D raises the CreateResources event to recover from a lost device while LoadResourcesForLevelAsync is in progress, your CreateResources handler should cancel that task.
  4. If Win2D raises CreateResources to recover from a lost device after you have finished loading data using LoadResourcesForLevelAsync, your CreateResources handler must reload that custom data as well as its usual global resources.

Using a CreateResources handler called CanvasControl_CreateResources, and the LoadResourcesForLevelAsync method shown above, here is a complete implementation that handles all four requirements:

 int? currentLevel, wantedLevel;

 // This implements requirement #1.
 Task levelLoadTask;


 public void LoadNewLevel(int newLevel)
 {
     Debug.Assert(levelLoadTask == null);
     wantedLevel = newLevel;
     levelLoadTask = LoadResourcesForLevelAsync(canvasControl, newLevel);
 }

 void CanvasControl_CreateResources(CanvasControl sender,
                                    CanvasCreateResourcesEventArgs args)
 {
     // Synchronous resource creation, for globally-required resources goes here:
     x = new CanvasRenderTarget(sender, ...);
     y = new CanvasRadialGradientBrush(sender, ...);
     // etc.

     args.TrackAsyncAction(CreateResourcesAsync(sender).AsAsyncAction());
 }  

 async Task CreateResourcesAsync(CanvasControl sender)
 {
     // If there is a previous load in progress, stop it, and
     // swallow any stale errors. This implements requirement #3.
     if (levelLoadTask != null)
     {
         levelLoadTask.AsAsyncAction().Cancel();
         try { await levelLoadTask; } catch { }
         levelLoadTask = null;
     }

     // Unload resources used by the previous level here.

     // Asynchronous resource loading, for globally-required resources goes here:
     baz = await CanvasBitmap.LoadAsync(sender, ...);
     qux = await CanvasBitmap.LoadAsync(sender, ...);
     // etc.

     // If we are already in a level, reload its per-level resources.
     // This implements requirement #4.
     if (wantedLevel.HasValue)
     {
        LoadNewLevel(wantedLevel.Value);
     }
 }

// Because of how this is designed to throw an exception, this must only 
// ever be called from a Win2D event handler.
bool IsLoadInProgress()
{
    // No loading task?
    if (levelLoadTask == null)
        return false;

    // Loading task is still running?
    if (!levelLoadTask.IsCompleted)
        return true;

     // Query the load task results and re-throw any exceptions
     // so Win2D can see them. This implements requirement #2.
     try
     {
         levelLoadTask.Wait();
     }
     catch (AggregateException aggregateException)
     {
         // .NET async tasks wrap all errors in an AggregateException.
         // We unpack this so Win2D can directly see any lost device errors.
         aggregateException.Handle(exception => { throw exception; });
     }
     finally
     {
         levelLoadTask = null;
     }

     currentLevel = wantedLevel;
     return false;
 }


 void CanvasControl_Draw(CanvasControl sender, CanvasDrawEventArgs args)
 {
     if (IsLoadInProgress())
     {
         DrawLoadingScreen();
     }
     else
     {
         DrawCurrentLevel(currentLevel);
     }
 }

Resource loading and CanvasAnimatedControl

Much of the above information for CanvasControl generalizes to CanvasAnimatedControl.

However, CanvasAnimatedControl has a game loop thread, which should be taken into consideration while deciding how an app should load its resources.

In particular:

  • Apps should avoid blocking or waiting from within the Draw or Update handlers.
  • Apps may need to synchronize data which is accessed between multiple threads.

We can adjust the above implementation to be compatible with CanvasAnimatedControl. In particular, we should ensure that the code checking "is the level loaded" will poll instead of wait, and no CanvasAnimatedControl event handlers should be marked as async.

async Task LoadResourcesForLevelAsync(ICanvasAnimatedControl resourceCreator, int level)
{
    return GameLoopSynchronizationContext.RunOnGameLoopThreadAsync(resourceCreator, async() =>
    {
        levelBackground = await CanvasBitmap.LoadAsync(resourceCreator, ...);
        levelThingie = await CanvasBitmap.LoadAsync(resourceCreator, ...);
        // etc.
    }
}


// Shared state between all threads, and a lock to control access to it.
bool needToLoad;
int? currentLevel, wantedLevel;
Task levelLoadTask; // This implements requirement #1.

Object lockable = new Object();

void LoadNewLevel(int level)
{            
    lock(lockable)
    {
        wantedLevel = level;
        needToLoad = true;
    }
}

void canvasAnimatedControl_CreateResources(ICanvasAnimatedControl sender,
                                   CanvasCreateResourcesEventArgs args)
{
     // Synchronous resource creation, for globally-required resources goes here:
     x = new CanvasRenderTarget(sender, ...);
     y = new CanvasRadialGradientBrush(sender, ...);
     // etc.

     args.TrackAsyncAction(CreateResourcesAsync(sender).AsAsyncAction());
}

async Task CreateResourcesAsync(CanvasAnimatedControl sender)
{
    // If there is a previous load in progress, stop it, and
    // swallow any stale errors. This implements requirement #3.
    lock(lockable)
    {
        if (levelLoadTask != null)
        {
            levelLoadTask.AsAsyncAction().Cancel();
            try { await levelLoadTask; } catch { }
            levelLoadTask = null;
        }
    }

    // Unload resources used by the previous level here.

    // Asynchronous resource loading, for globally-required resources goes here:
    baz = await CanvasBitmap.LoadAsync(sender, ...);
    qux = await CanvasBitmap.LoadAsync(sender, ...);
    // etc.

    // If we are already in a level, reload its per-level resources.
    // This implements requirement #4.
    int? levelThatNeedsReloading;
    lock(lockable)
    {
        levelThatNeedsReloading = wantedLevel;
    }            
    if (levelThatNeedsReloading.HasValue)
    {
        LoadNewLevel(wantedLevel.Value);
    }
}

void canvasAnimatedControl_Update(ICanvasAnimatedControl sender, CanvasAnimatedUpdateEventArgs args)
{
    lock(lockable)
    {
        // Check if there is already an outstanding level-loading Task.
        // If so, don't try to spin up a new one.
        bool beginLoad = levelLoadTask == null && needToLoad;
        needToLoad = false;

        if (beginLoad)
        {
            levelLoadTask = LoadResourcesForLevelAsync(sender, wantedLevel);
        }

        // Indicates the loading task was run and just finished.
        if(levelLoadTask != null && levelLoadTask.IsCompleted)
        {
            AggregateException levelLoadException = levelLoadTask.Exception;
            levelLoadTask = null;

            // Query the load task results and re-throw any exceptions
            // so Win2D can see them. This implements requirement #2.
            if(levelLoadException != null)
            {
                // .NET async tasks wrap all errors in an AggregateException.
                // We unpack this so Win2D can directly see any lost device errors.
                levelLoadException.Handle(exception => { throw exception; });
            }

            currentLevel = wantedLevel;
        }
    }
}

bool IsLoadInProgress()
{
    lock(lockable)
    {
        return levelLoadTask != null;
    }
}


void canvasAnimatedControl_Draw(ICanvasAnimatedControl sender, CanvasAnimatedDrawEventArgs args)
{
    if (IsLoadInProgress())
    {
        DrawLoadingScreen(args.DrawingSession);
    }
    else
    {
        DrawCurrentLevel(args.DrawingSession, currentLevel);
    }
}

It's worth pointing out that there are special implications of using await from within application code which runs on the game loop thread. For example, when a task is run using RunOnGameLoopThreadAsync and contains an await, the first await may result in RunOnGameLoopThreadAsync finishing. The remainder of the task is scheduled according a continuation handler. And by default, this continuation handler is chosen from the application's thread pool, according to the default .NET SynchronizationContext. The remainder of the task might not run on the game loop thread at all.

To remedy this, and schedule work using RunOnGameLoopThreadAsync which contains await where the work must all be run on the game loop thread, see the GameLoopSynchronizationContext sample.