从 UI 线程通过异步任务调用 Web 服务导致死锁

Question

我是 Xamarin Forms 的新手，并且有一个异步获取事务的应用程序，但是当我调用 Web 服务时它遇到了死锁。

我有一个 TransactionView：

这是 XAML：

<Label Text="Account:" Grid.Row="0" Grid.Column="0" Style="{StaticResource LabelStyle}"/> 
<ctrls:BindablePicker x:Name="ddlAccountsWinPhone" ItemsSource="{Binding Accounts}" SelectedIndex="{Binding AccountID}"  Grid.Row="0" Grid.Column="1" />
<Label Text="From Date:" Grid.Row="2" Grid.Column="0" Style="{StaticResource LabelStyle}"/> <DatePicker x:Name="dtFromWinPhone" Date="{Binding FromDate}" Grid.Row="2" Grid.Column="1"  /> 
<Label Text="To Date:" Grid.Row="3" Grid.Column="0" Style="{StaticResource LabelStyle}"/> <DatePicker x:Name="dtToWinPhone" Date="{Binding ToDate}"  Grid.Row="3" Grid.Column="1" /> 
<Button x:Name="btnViewWinPhone" Text="View" Grid.Row="5" Grid.Column="0" Grid.ColumnSpan="2"  
Command ="{Binding ShowTransactions}" />

这是 TransactionView 背后的代码，注意我将在名为 TransactionSubPageView 的子页面上显示一个交易网格：

public partial class TransactionView : ContentPage
{
    private TransactionViewModel transactionViewModel;
    private TransactionViewModel ViewModel
    {
        get {
            if (transactionViewModel == null) transactionViewModel = new TransactionViewModel(
             this.Navigation, new TransactionSubPageView()); //Pass sub page to ViewModel ctor

            return transactionViewModel;
        }
    }

    public TransactionView()
    {
        InitializeComponent();
        BindingContext = ViewModel;
    }

这是 TransactionViewModel：

public class TransactionViewModel : ViewModelBase
{
    private int accountID;
    private List<Account> accounts = new List<Account>();
    private Account accountItemSelected;
    private DateTime fromDate = new DateTime(1900,1,1);
    private DateTime toDate = new DateTime(1900, 1, 1);

    private ContentPage transactionGridViewNav;

    public TransactionViewModel(INavigation navigationInterface, ContentPage transactionSubPageView)
    {
        base.navigation = navigationInterface;
        transactionSubPageViewNav = transactionSubPageView; //I SAVE A REFERENCE TO THE SUBPAGE HERE
        accounts.AddRange(Client.Accounts);
    }

    public ICommand ShowTransactions
    {
        get
        {
            return new Command(async () =>
            { 
                //HERE IS WHERE "I THINK" I WANT TO FETCH THE TRANSACTIONS

                //THEN NAVIGATE TO THE THE SUBPAGE WITH THE GRID OF TRANSACTIONS
                await navigation.PushAsync(transactionSubPageViewNav);
            });
        }
    }

    public int AccountID
    {
        get{ return this.accountID; }
        set{
            this.accountID = value;
            this.OnPropertyChanged("AccountID");
            }
    }

    public List<Account> Accounts
    {
        get{
            return this.accounts;}
        set{
            this.accounts = value;
            this.OnPropertyChanged("Accounts");
            }
    }

    public Account AccountItemSelected
    {
        get{return accountItemSelected;}
        set {
                if (accountItemSelected != value)
                {
                    accountItemSelected = value;
                    OnPropertyChanged("AccountItemSelected");
                }
            }
    }

    public DateTime FromDate { get; set; }
    public DateTime ToDate { get; set; }}
    ...

这是 TransactionSubPageView：

public partial class TransactionSubPageView : ContentPage
{
    public TransactionSubPageViewModel transactionSubPageViewModel;

    public TransactionSubPageViewModel ViewModel
    {
        get
        {
            if (transactionSubPageViewModel == null)
                transactionSubPageViewModel = new TransactionSubPageViewModel();
            return transactionGridViewModel;
        }
    }

    private Grid gridTransactions;

    public TransactionSubPageView()
    {
        InitializeComponent();
        BindingContext = ViewModel;
    }

    protected async override void OnAppearing()
    {
        base.OnAppearing();
        //THIS IS A VOID TO POPULATE A GRID AND SET THE PAGE'S CONTENT, IT USES 
        //transactionSubPageViewModel.TransactionsGrid!!
        PopulateGridTransactions();
    }

这是子页面视图模型：

public class TransactionSubPageViewModel : ViewModelBase
{
    public List<Transaction> transactionsGrid = new List<Transaction>();

    public int accountId = 1636;
    public DateTime fromDate = new DateTime(2015, 8, 1);
    public DateTime toDate = new DateTime(2015, 9, 1);

    public TransactionGridViewModel() { }

    public List<Transaction> TransactionsGrid
    {
        get {
            if (transactionsGrid == null) transactionsGrid = MyWebService.GetTransactions(1636, new DateTime(2015, 8, 1), new DateTime(2015, 9, 1)).Result;
            return transactionsGrid;}
    }
}

最后是导致问题的 WebService 调用：

public static async Task<List<Transaction>> GetTransactions(int accountId, DateTime fromDate, DateTime toDate)
{
    var client = new System.Net.Http.HttpClient(new NativeMessageHandler());
    client.BaseAddress = new Uri("http://theWebAddress.com/);
    var response = await client.GetAsync("API.svc/Transactions/" + accountId + "/" + fromDate.ToString("yyyy-MM-dd") + "/" + toDate.ToString("yyyy-MM-dd")); //.ConfigureAwait(false);
    var transactionJson = response.Content.ReadAsStringAsync().Result;
    var transactions = JsonConvert.DeserializeObject<List<Transaction>>(transactionJson);
    return transactions;
}

感谢到目前为止的阅读，问题是 webmethod 调用中的这一行总是挂起：

var response = await client.GetAsync("API.svc/Transactions/" + accountId + "/" + fromDate.ToString("yyyy-MM-dd") + "/" + toDate.ToString("yyyy-MM-dd")); 

如果我从子页面的OnAppearing 事件调用GetTransactions web 服务，它会挂起，如果我从ICommand ShowTransactions 调用它，它也会挂起。我错过了await 还是continue？

我有read 和fair 几个documents 和similar 人和who are confused，我知道我是encountering a deadlock，但我只是不知道如何解决它。

我试过ConfigureAwait(false) 没有运气。如果我可以在页面中的 background thread, show a progress bar and when the operation is complete render the results 上调用 WebService，那就太理想了。

Answer 1

成功！！我知道这将是一个在这里询问的情况，然后我会解决它。有趣的是它是如何工作的！

这就是我让它工作的方式，请随意批评它：

---

我把TransactionSubPageViewModel's TransactionGrid属性变成了一个Async方法，直接把webservice调用放在那里：

public async Task<List<Transaction>> TransactionsGrid()
{
    if (transactionsGrid == null)
    {
        var client = new System.Net.Http.HttpClient(new ModernHttpClient.NativeMessageHandler());
        client.BaseAddress = new Uri("http://theWebAddress.com/);
        var response = await client.GetAsync("API.svc/Transactions/" + accountId + "/" + fromDate.ToString("yyyy-MM-dd") + "/" + toDate.ToString("yyyy-MM-dd"));
        var transactionJson = await response.Content.ReadAsStringAsync(); //DONT USE Result HERE, USE AWAIT AS PER @Noseratio suggested
        var transactions = Newtonsoft.Json.JsonConvert.DeserializeObject<List<Transaction>>(transactionJson);
        transactionsGrid = transactions;
    }
        return transactionsGrid;
}

然后我从我想调用它的地方调用了 Web 服务。

public ICommand ShowTransactions
{
    get
    {
        return new Command(async () =>
        { 
            //THIS IS WHERE I WAS MISSING THE AWAIT!!!
            await ((TransactionGridViewModel)transactionSubPageViewNav.BindingContext).TransactionsGrid();

            await navigation.PushAsync(transactionSubPageViewNav);
        });
    }

然后，当我从 OnAppearing() 调用 PopulateGridTransactions 时，数据已经可用：

public void PopulateGridTransactions()
{
...
  foreach (Models.Transaction transaction in transactionSubPageViewModel.TransactionsGrid().Result)
  {

编辑：

正如@Noseratio 指出的那样，让我解释一下为什么您需要在异步中使用 Try/Catch。

巧合的是，在收到来自 web 服务的响应后，我在反序列化 json web 服务结果时在下一行出现错误。

我会在 Xamarin App.cs 中设置一个全局异常处理程序，以便它能够捕获它：

通过将 Try/Catch 放入异步中，我可以在同一帧上捕获异常（在堆栈被 Application_UnhandledException 捕获之前展开之前 - 这是 very annoying to track down ）：