How do I make my function correctly use a callback after async functions are completed?

Question

I have this function that loads a number of assets via async and I want to call the onComplete action when all the assets have been loaded but currently given the nature of async I am not sure how to make my function do that as I am relatively new to programming in async.

public void Load(Action onComplete)
{
    foreach (var kvp in _db)
    {
        var key = kvp.Key;
        var assetRef = kvp.Value;
        assetRef.LoadAssetAsync<TAsset>().Completed += handle =>
        {
            if (handle.Status == AsyncOperationStatus.Succeeded)
                _loadedAssets.Add(key, handle.Result);

            //check if all assets have been loaded and then call onComplete
        };
    }
}

Answer 1

You should use Microsoft's Reactive Framework (aka Rx) - NuGet System.Reactive and add using System.Reactive.Linq; - then you can do this:

public void Load2(Action onComplete)
{
    (
        from kvp in _db.ToObservable()
        let key = kvp.Key
        let assetRef = kvp.Value
        let assetLoader = assetRef.LoadAssetAsync<TAsset>()
        from handle in
            Observable
                .FromEvent<Action<AssetHandle>, AssetHandle>(
                    h => assetLoader.Completed += h,
                    h => assetLoader.Completed -= h)
                .Take(1)
        where handle.Status == AsyncOperationStatus.Succeeded
        select new { key, handle.Result }
    )
        .Subscribe(
            x => _loadedAssets.Add(x.key, x.Result),
            () => onComplete());
}

Now, that should work. I had to reverse engineer some types, but I think I worked it out. The race condition still exists in your code.