异步等待执行Windows Phone 8

Question

我是异步的新手,等待编程风格.如何解决以下问题:

我先调用下面的代码.这里的问题是第一行正在等待填充它的需要categoriesvm.Categorieslist,但它没有,但第二行被调用.(我认为这是await的默认行为)

如何确保仅categoriesvm.Categorieslist在第一行中填充第二行时才调用第二行？

protected override void OnNavigatedTo(System.Windows.Navigation.NavigationEventArgs e)
{
    categoriesvm.GetCategories();                 
    BookCategories.DataContext = from vm in categoriesvm.Categorieslist select vm;
}

在上面的代码中,当我执行第一行时,它会在Categorieslist我上面访问的列表的下方.

public async void GetCategories()
{            
    Categorieslist = new ObservableCollection<Categories>(await PhoneClient.GetDefaultCategories());          
}

在phoneclient下面

public class PhoneClient
{   
    private static readonly HttpClient client;

    public static Uri ServerBaseUri
    {
        get { return new Uri("http://169.254.80.80:30134/api/"); }
    }

    static PhoneClient()
    {        
       client =new HttpClient();
       client.MaxResponseContentBufferSize = 256000;
       client.DefaultRequestHeaders.Add("user-agent", "Mozilla/5.0 (compatible; MSIE 10.0; Windows NT 6.2; WOW64; Trident/6.0)");
    }      

    public async static Task<List<Categories>> GetDefaultCategories()
    {      
        HttpResponseMessage getresponse = await client.GetAsync(ServerBaseUri + "Categoryss");                      
        string json = await getresponse.Content.ReadAsStringAsync();         
        json = json.Replace("<br>", Environment.NewLine);
        var categories = JsonConvert.DeserializeObject<List<Categories>>(json);
        return categories.ToList();
    }
}

Answer 1

你应该避免async void.我在MSDN文章中解释了这个指南.

将async void方法更改为async Task:

public async Task GetCategoriesAsync()
{            
  Categorieslist = new ObservableCollection<Categories>(await PhoneClient.GetDefaultCategories());          
}

然后就可以await这样:

protected override async void OnNavigatedTo(System.Windows.Navigation.NavigationEventArgs e)
{
  await categoriesvm.GetCategoriesAsync();                 
  BookCategories.DataContext = from vm in categoriesvm.Categorieslist select vm;
}

但是,我建议在UI事件之外进行所有VM初始化 - 这将使您的代码更容易测试.看看我的async构造函数博客文章的想法.