Xamarin async gems

LocalCache.cs

public static class LocalCache {

	static HashSet<string> tempKeys = new HashSet<string> ();

	public static Task<Stream> GetCachedOrDownload (string key, Uri uri, bool temporary = true)
	{
		return GetCachedOrDownload (key, uri, CreateNativeHttpClient, temporary);
	}

	public static Task<Stream> GetCachedOrDownload (string key, Uri uri, Func<HttpClient> createHttpClient, bool temporary = true)
	{
		return GetCachedOrCreate (
			key: key,
			temporary: temporary,
			create: async () => {
				using (var client = createHttpClient ())
					return await client.GetStreamAsync (uri);
			});
	}

	public static async Task<Stream> GetCachedOrCreate (string key, Func<Task<Stream>> create, bool temporary = true)
	{
		using (var isoStore = IsolatedStorageFile.GetUserStoreForApplication ()) {
			await AddToCacheIfMissing (isoStore, key, create, temporary);
			return isoStore.OpenFile (key, FileMode.Open, FileAccess.Read, FileShare.Read);
		}
	}

	public static async Task AddToCacheIfMissing (string key, Func<Task<Stream>> create, bool temporary = true)
	{
		using (var isoStore = IsolatedStorageFile.GetUserStoreForApplication ())
			await AddToCacheIfMissing (isoStore, key, create, temporary);
	}

	static async Task AddToCacheIfMissing (IsolatedStorageFile isoStore, string key, Func<Task<Stream>> create, bool temporary = true)
	{
		if (!isoStore.FileExists (key)) {
			using (var outStream = isoStore.CreateFile (key)) {
				var inStream = new BufferedStream (await create ());
				await inStream.CopyToAsync (outStream);
			}
			if (temporary)
				tempKeys.Add (key);
		}
	}

	public static void CleanUpTemporaryItems ()
	{
		foreach (var key in tempKeys)
			Invalidate (key);
	}

	public static void Invalidate (string key)
	{
		using (var isoStore = IsolatedStorageFile.GetUserStoreForApplication ()) {
			if (isoStore.FileExists (key))
				isoStore.DeleteFile (key);
		}
	}

	static HttpClient CreateNativeHttpClient ()
	{
		#if __IOS__
		return new HttpClient (new CFNetworkHandler ());
		#else
		return new HttpClient ();
		#endif
	}
}

ShowAlert.cs

// Displays a UIAlertView and returns the index of the button pressed.
public static Task<int> ShowAlert (string title, string message, params string [] buttons)
{
	var tcs = new TaskCompletionSource<int> ();
	var alert = new UIAlertView {
		Title = title,
		Message = message
	};
	foreach (var button in buttons)
		alert.AddButton (button);
	alert.Clicked += (s, e) => tcs.TrySetResult (e.ButtonIndex);
	alert.Show ();
	return tcs.Task;
}