异步执行的期待的Windows Phone 8 [英] async await execution windows phone 8

问题描述

我是新来的异步和等待的编程风格。我该如何解决以下问题：

我先调用低于code。这里的问题是，在第一线正在等待它需要填充 categoriesvm.Categorieslist ，它没有，但在第二行被调用。 （我认为是的await的默认行为）

我怎样才能确保第二行被称为只有当 categoriesvm.Categorieslist 在第一线填充？

 保护覆盖无效的OnNavigatedTo（System.Windows.Navigation.NavigationEventArgs E）
{
    categoriesvm.GetCategories（）;
    BookCategories.DataContext =从虚拟机中categoriesvm.Categorieslist选择虚拟机;
}
 

在code，当我执行它低于第一线之上，其中 Categorieslist 是我上面访问列表。

 公共异步无效GetCategories（）
{
    Categorieslist =新的ObservableCollection＆LT;类别＆GT;（等待PhoneClient.GetDefaultCategories（））;
}
 

的 phoneclient 小于

 公共类PhoneClient
{
    私人静态只读HttpClient的客户端;    公共静态乌里ServerBaseUri
    {
        {返回新的URI（http://169.254.80.80:30134/api/）; }
    }    静态PhoneClient（）
    {
       客户端=新的HttpClient（）;
       client.MaxResponseContentBufferSize = 256000;
       client.DefaultRequestHeaders.Add（用户代理，Mozilla的/ 5.0（兼容; MSIE 10.0; Windows NT的6.2; WOW64;三叉戟/ 6.0））;
    }    公共异步静态任务＆LT;名单，LT;分类＆GT;＆GT; GetDefaultCategories（）
    {
        HTT presponseMessage的GetResponse =等待client.GetAsync（ServerBaseUri +Categoryss）;
        JSON字符串等待= getresponse.Content.ReadAsStringAsync（）;
        JSON = json.Replace（＆LT; BR＆gt;中，Environment.NewLine）;
        VAR类别= JsonConvert.DeserializeObject＆LT;名单，LT;分类＆GT;＆GT;（JSON）;
        返回categories.ToList（）;
    }
}
 

解决方案

您应该避免异步无效。我在 MSDN文章解释这一指导方针。

一旦你改变你的异步无效方法异步任务：

 公共异步任务GetCategoriesAsync（）
{
  Categorieslist =新的ObservableCollection＆LT;类别＆GT;（等待PhoneClient.GetDefaultCategories（））;
}
 

然后你可以等待它是这样：

 保护覆盖异步无效的OnNavigatedTo（System.Windows.Navigation.NavigationEventArgs E）
{
  等待categoriesvm.GetCategoriesAsync（）;
  BookCategories.DataContext =从虚拟机中categoriesvm.Categorieslist选择虚拟机;
}
 

不过，我建议你做的所有虚拟机初始化的UI事件之外 - 这会让你的code更容易测试。看看我的 异步构造博客文章的想法。

I am new to async and await style of programming. How can I solve the following problem:

I am calling the below code first. The problem here is that the first line is awaiting which need to populate the categoriesvm.Categorieslist, which it does not, but the second line is called. (which I think is the default behaviour of await)

How can I make sure that the second line is called only when the categoriesvm.Categorieslist is populated in the first line?

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

In the code above when I execute the first line it goes below where Categorieslist is the list I am accessing above.

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

The phoneclient is below

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();
    }
}

解决方案

You should avoid async void. I explain this guideline in an MSDN article.

Once you change your async void method to async Task:

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

Then you can await it as such:

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

However, I recommend doing all your VM initialization outside of your UI events - this will make your code easier to test. Take a look at my async constructors blog post for ideas.

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