Question

我有一个从数据库获取数据的异步Web服务。我能够很好地返回数据，但似乎问题是应用程序正在向适配器发送列表（此时为空），然后返回null异常。

    protected override void OnCreate(Bundle savedInstanceState)
    {

        // Create your application here
        base.OnCreate(savedInstanceState);
        SetContentView(Resource.Layout.HerdDetailsLayout);

        this.GetData();

        List = FindViewById<ListView>(Resource.Id.Details);

        this.updateAdapter();

        List.ItemClick += (object sender, AdapterView.ItemClickEventArgs e) =>
        {
            var listView = sender as ListView;
            Data item = data.List[e.Position];
        };

    }

    private async void GetData(){

        HerdsRESTFulService herdService = new HerdsRESTFulService();

        herds = await herdService.GetAllAsync();

        Console.WriteLine("Hello");
    }

    public void updateAdapter()
    {
        adapter = new HerdListAdapter(this, herds);
        RunOnUiThread(() => adapter.NotifyDataSetChanged());
        herdList.Adapter = adapter;
    }

....


public async Task<Data> GetAllAsync()
{
    dynamic responseString = await "http://foobar"

            .WithHeader("Authorization", "Bearer numbers")
            .WithHeader("Accept", "application/json")
            .GetAsync().ReceiveString();
}

如何在将异步放入适配器之前等待异步完成？

出现错误的地方：

 public ListAdapter(Activity context, Data data) : base()
 {
    this.context = context;
    this.data = data; // Null here because the async didn't finish on time before the adapter was set.
 }

Answer 1

你无法等待无效的方法，所以GetData()就是＆＃34;一劳永逸的＆＃34;意味着执行将开始，然后代码立即移动到下一条指令。这就是updateAdapter()在GetData()完成之前发生的原因。

将其更改为返回任务，然后等待通话：

private async Task GetData()
{    
    HerdsRESTFulService herdService = new HerdsRESTFulService();
    herds = await herdService.GetAllAsync();
    Console.WriteLine("Hello");
}

然后：

....
await this.updateAdapter();
...

或许更好的是在等待时做一些工作：

Task dataTask = this.GetData();
List = FindViewById<ListView>(Resource.Id.Details);
await dataTask;
this.updateAdapter();

等待异步Web服务在发送到适配器

1 个答案: