Xamarin + MvvmCross + Azure 移动应用 table 请求无效

Question

我一直在构建一个简单的 Xamarin 应用程序，使用 MvvmCross 框架和 Azure 移动应用程序服务来存储和检索来自云的数据。 UI 项目是特定于平台的。

我的应用程序的主视图显示了一个 post 的列表，这个列表应该是从 Azure 云中检索的。发生的情况是，在启动画面之后，应用程序只显示黑屏。似乎正确触发了检索数据的调用，但从未收到响应。

这是我的 post 视图模型：

public class PostsViewModel : MvxViewModel
    {
        readonly IPostService _postService;
        readonly IMvxNavigationService _navigationService;
        readonly MvxSubscriptionToken token;

        public PostsViewModel(IPostService postService, IMvxMessenger messenger, IMvxNavigationService navigationService)
        {
            _postService = postService;
            _navigationService = navigationService;

            token = messenger.SubscribeOnMainThread<PostsChangedMessage>
                (async mex => await LoadPosts());

            Posts = new ObservableCollection<PostViewModel>();
            AddNewPostCommand = new MvxAsyncCommand(AddNewPost);
            ShowPostDetailsCommand = new MvxAsyncCommand<PostViewModel>(ShowPostDetails);
        }

        #region Properties
        public ObservableCollection<PostViewModel> Posts { get; set; }

        private bool isLoading = true;
        public bool IsLoading
        {
            get => isLoading;
            set => SetProperty(ref isLoading, value);
        }
        #endregion

        #region Commands
        public IMvxAsyncCommand AddNewPostCommand { get; }
        private async Task AddNewPost()
        {
            await _navigationService.Navigate(typeof(PostViewModel), new Post());
        }

        public IMvxAsyncCommand<PostViewModel> ShowPostDetailsCommand { get; }
        private async Task ShowPostDetails(PostViewModel postViewModel)
        {
            await _navigationService.Navigate(typeof(PostDetailsViewModel), postViewModel);
        }
        #endregion

        #region Functions
        public override async Task Initialize()
        {
            await LoadPosts();
        }

        private async Task LoadPosts()
        {
            // Remove all counters before reloading
            Posts.Clear();

            var posts = await _postService.GetAllPosts();
            foreach(var post in posts)
            {
                var viewModel = new PostViewModel(_postService, _navigationService);
                viewModel.Prepare(post);
                Posts.Add(viewModel);
            }

            IsLoading = false;
        }
        #endregion
    }

这是我的 post 服务：

    public class PostService : IPostService
    {
        readonly IPostRepository _repository;
        readonly IMvxMessenger _messenger;

    public PostService(IPostRepository repository, IMvxMessenger messenger)
    {
        _repository = repository;
        _messenger = messenger;
    }

    public async Task<Post> AddNewPost(Post post)
    {
        // Just for testing purposes
        post.Date = DateTimeOffset.Now;
        // Temporary hard-coded author; this will be get from the user object 
        post.Author = "John Smith";

        await _repository.Save(post).ConfigureAwait(false);
        _messenger.Publish(new PostsChangedMessage(this));

        return post;
    }

    public async Task DeletePost(Post post)
    {
        await _repository.Delete(post).ConfigureAwait(false);
        _messenger.Publish(new PostsChangedMessage(this));
    }

    public Task<List<Post>> GetAllPosts()
    {
        return _repository.GetAll();
    }

这是我的存储库：

public class PostRepository : IPostRepository
{
    MobileServiceClient MobileService
        = new MobileServiceClient(@"https://vntestapp.azurewebsites.net", new LoggingHandler(true));

    IMobileServiceTable<Post> postTable;

public PostRepository()
{
    postTable = MobileService.GetTable<Post>();
}

public Task Delete(Post post)
{
    return postTable.DeleteAsync(post);
}

public async Task<List<Post>> GetAll()
{
    return await postTable.ToListAsync();
}

public async Task<Post> Save(Post post)
{
    await postTable.InsertAsync(post);
    return post;
}

}

Answer 1

如果运行平台是 android，则对 API 调用使用 Task.Run() 构造。

根据我使用 Mvvmcross 的实际经验，在 android 中，有时异步方法内部的等待调用会阻塞该线程，并且它永远不会 returns、

在这种情况下，如果您在 Task.Run() 中调用它，它就可以正常工作。而 iOS 没有这个问题。

我会寻找更深入的解释来提供它的原因。

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