public class DeviceThreadDispatcher : IThreadDispatcher
{
    private static int _uiThreadId;

    public bool IsOnUiThread => Environment.CurrentManagedThreadId == _uiThreadId;

    public static void Initialize(int uiThreadId) => _uiThreadId = uiThreadId;

    public T RequestMainThreadAsyncOperation<T>(Func<Task<T>> operation)
    {
        var result = default(T);
        var taskCompletionSource = new TaskCompletionSource<bool>();

        Device.BeginInvokeOnMainThread(async () =>
        {
            result = await operation();
            taskCompletionSource.SetResult(true);
        });

        if (taskCompletionSource.Task.Result)
        {
            return result;
        }

        return default (T);
    }
}