MVVM异步调用viewmodel：我怎样才能连接天空数据库与MVVM

Question

我正在开发使用mvvm概念（数据，组，模型）win win phone 8应用程序，我完成了我的应用程序设计使用这个概念。现在我正尝试将我的应用程序连接到Azure DB，我还通过以下代码连接了使用MVVM概念的Azure DB，并且它的工作成功。

var js = new JObject { { "institutionid", obj.institutionid }, { "userid", obj.userid } }; 
var result = await App.MobileService.InvokeApiAsync("school_365_create_dynamic_tile", js, System.Net.Http.HttpMethod.Post, null); 

在创建一个ViewModel我必须做的，使用Azure的移动服务SDK从Azure的移动服务读取数据的服务呼叫。

sdk apis使用async/await来完成这项工作，而且我无法在ViewModel中进行异步调用。

的代码是这样的模型类：

public class ModelMail : INotifyPropertyChanged 
{ 

    //newly added 
    public Group Mail { get; set; } 
    public Group OutBox { get; set; } 
    public Group Draft { get; set; } 
    public Group SendItems { get; set; } 
    public bool IsDataLoaded { get; set; } 

    public event PropertyChangedEventHandler PropertyChanged; 
    private void NotifyPropertyChanged(string property) 
    { 
     if (PropertyChanged != null) 
     { 
      PropertyChanged(this, new PropertyChangedEventArgs(property)); 
     } 
    } 

    public void LoadData() 
    { 
     //Newly created 
     Mail = CreateMail(); 
     OutBox = CreateOutBox(); 
     Draft = CreateDraft(); 
     SendItems = CreateSendItems(); 
     IsDataLoaded = true; 
    } 


    private Group CreateDraft() 
    { 
     Group data = new Group(); 

     data.Title = "all"; 
     string[] gataFromDB = new string[] { "sample ", "sample", "sample", "sample" }; 

     data.Items.Add(new Data { Name_1 = "Nisar Mohamed VM", Subject = "Subject Inbox sample mail subject Inbox sample mail subject Inbox sample mail subject Inbox sample mail subject Inbox sample mail subject 456", Message = "Message This is a sample mail message from nisar This is a sample mail message from nisar This is a sample mail message from nisar This is a sample mail message from nisar This is a sample mail message from nisarThis is a sample mail message from nisar", time = "11:00 AM", IsChecked = false, foreground = "Black", to = "gowthamrajs@hotmail.com", mailFullDateTime = "Fri 9/12 9:25 PM", from = "kkkk@knowledgeq.com" }); 

     foreach (string dataa in gataFromDB) 
     { 

      data.Items.Add(new Data { Name_1 = "Nisar Mohamed VM", Subject = "Subject Inbox sample mail subject Inbox sample mail subject Inbox sample mail subject Inbox sample mail subject Inbox sample mail subject 456", Message = "Message This is a sample mail message from nisar This is a sample mail message from nisar This is a sample mail message from nisar This is a sample mail message from nisar This is a sample mail message from nisarThis is a sample mail message from nisar", time = "11:10 AM", IsChecked = false, foreground = "Black", to = "nisr199@hotmail.com", cc = "nisr19@gmail.com", mailFullDateTime = "Fri 9/12 9:25 PM", from = "kkkk@knowledgeq.com" }); 
     } 


     return data; 
    } 

    private Group CreateOutBox() 
    { 
     Group data = new Group(); 

     data.Title = "unread"; 

     return data; 
    } 

    private Group CreateMail() 
    { 
     Group data = new Group(); 

     data.Title = "all"; 

     return data; 
    } 

    private Group CreateSendItems() 
    { 
     Group data = new Group(); 

     data.Title = "all"; 

     return data; 
    } 

} 

我怎样才能

Answer 1

你会发现我的MSDN article on async data binding很有帮助。总之，如果你是数据绑定，那么当数据下载完成时你需要提升PropertyChanged。 Task没有执行INotifyPropertyChanged，所以需要一些帮助。在我的文章中，我有一个类型NotifyTaskCompletion<T>，它可以用作异步操作的数据绑定包装器。

你会在你的构造函数启动异步操作，并创建一个NotifyTaskCompletion<T>包含（数据绑定）结果：

public class MyViewModel 
{ 
    public MyViewModel() 
    { 
    Mail = new NotifyTaskCompletion<Group>(CreateMailAsync()); 
    } 

    public NotifyTaskCompletion<Group> Mail { get; private set; } 

    private async Task<Group> CreateMailAsync() 
    { 
    // Azure calls go here 
    } 
} 

然后，数据绑定代码可以在NotifyTaskCompletion<T>使用属性更新UI：

<Grid> 
    <!-- Busy indicator --> 
    <Label Content="Loading..." Visibility="{Binding Mail.IsNotCompleted, Converter={StaticResource BooleanToVisibilityConverter}}"/> 

    <!-- Results --> 
    <Label Content="{Binding Mail.Result.Title}" Visibility="{Binding Mail.IsSuccessfullyCompleted, Converter={StaticResource BooleanToVisibilityConverter}}"/> 

    <!-- Error details --> 
    <Label Content="{Binding Mail.ErrorMessage}" Background="Red" Visibility="{Binding Mail.IsFaulted, Converter={StaticResource BooleanToVisibilityConverter}}"/> 
</Grid> 

请注意，您在应用程序UI中需要一些额外的状态。具体而言，操作正在进行时的“加载”状态;以及操作异步失败时的“错误”状态。