为什么在 AsyncTaskManager PropertyChanged 中始终为空？

Question

我尝试实现类似于本文中介绍的模式： http://msdn.microsoft.com/en-us/magazine/dn605875.aspx

以下是我的问题描述：

在我看来，我已经设置：

<Button Content="LoadData" Command="{Binding LoadDataCommand}" />
<ListBox Grid.Row="1" x:Name="listBox" ItemsSource="{Binding DataSource.Result}"

然后在codeBehind中：

this.DataContext = new ProductViewModel();

然后在 ProductViewModel 中：

public AsyncTaskManager<ObservableCollection<Product>> DataSource { get; private set; }

与：

public ProductViewModel()
    {
        _loadDataCommand = new DelegateCommand(LoadDataAction);

    }
private void LoadDataAction(object p)
    {
        ProductRepository pRepository = new ProductRepository();
        DataSource = new AsyncTaskManager<ObservableCollection<Product>>(pRepository.LoadProducts());      
    }

并且在 AsyncTaskManager PropertyChanged 中始终为 null :( 为什么会这样？绑定有什么问题？

但是当我删除“加载数据”按钮和 _loadDataCommand 并简单地设置时

ProductRepository pRepository = new ProductRepository();
        DataSource = new AsyncTaskManager<ObservableCollection<Product>>(pRepository.LoadProducts());   

在ProductViewModel 构造函数中，它的工作方式与示例中一样，但我希望用户能够使用构造函数中未启动的按钮调用加载数据：/

下面是 AsyncTaskManager 代码：

using System;
using System.ComponentModel;
using System.Threading.Tasks;

namespace PhoneClientApp.Models
{
    public sealed class AsyncTaskManager<TResult> : INotifyPropertyChanged
    {
    public AsyncTaskManager(Task<TResult> task)
    {
        Task = task;
        if (!task.IsCompleted)
        {
            var _ = WatchTaskAsync(task);
        }
    }

    private async Task WatchTaskAsync(Task task)
    {
        try
        {
            await task;
        }
        catch
        {
        }
        var propertyChanged = PropertyChanged;
        if (propertyChanged == null)
            return;
        propertyChanged(this, new PropertyChangedEventArgs("Status"));
        propertyChanged(this, new PropertyChangedEventArgs("IsCompleted"));
        propertyChanged(this, new PropertyChangedEventArgs("IsNotCompleted"));
        if (task.IsCanceled)
        {
            propertyChanged(this, new PropertyChangedEventArgs("IsCanceled"));
        }
        else if (task.IsFaulted)
        {
            propertyChanged(this, new PropertyChangedEventArgs("IsFaulted"));
            propertyChanged(this, new PropertyChangedEventArgs("Exception"));
            propertyChanged(this,
                new PropertyChangedEventArgs("InnerException"));
            propertyChanged(this, new PropertyChangedEventArgs("ErrorMessage"));
        }
        else
        {
            propertyChanged(this,
                new PropertyChangedEventArgs("IsSuccessfullyCompleted"));
            propertyChanged(this, new PropertyChangedEventArgs("Result"));
        }
    }

    public Task<TResult> Task { get; private set; }

    public TResult Result
    {
        get
        {
            return (Task.Status == TaskStatus.RanToCompletion)
                ? Task.Result
                : default(TResult);
        }
    }

    public TaskStatus Status
    {
        get { return Task.Status; }
    }

    public bool IsCompleted
    {
        get { return Task.IsCompleted; }
    }

    public bool IsNotCompleted
    {
        get { return !Task.IsCompleted; }
    }

    public bool IsSuccessfullyCompleted
    {
        get
        {
            return Task.Status ==
                   TaskStatus.RanToCompletion;
        }
    }

    public bool IsCanceled
    {
        get { return Task.IsCanceled; }
    }

    public bool IsFaulted
    {
        get { return Task.IsFaulted; }
    }

    public AggregateException Exception
    {
        get { return Task.Exception; }
    }

    public Exception InnerException
    {
        get
        {
            return (Exception == null)
                ? null
                : Exception.InnerException;
        }
    }

    public string ErrorMessage
    {
        get
        {
            return (InnerException == null)
                ? null
                : InnerException.Message;
        }
    }

    public event PropertyChangedEventHandler PropertyChanged;
}

}

Answer 1

请尝试创建最小的可重现代码集，并观察调试器中的输出窗口是否存在数据绑定错误。

以下代码对我来说很好用（在 LINQPad 中）：

void Main()
{
    var context = new ParserContext();
    context.XmlnsDictionary.Add("", "http://schemas.microsoft.com/winfx/2006/xaml/presentation");
    var xaml = @"<Grid><ListBox ItemsSource=""{Binding DataSource.Result}"" /></Grid>";
    var element = (FrameworkElement)XamlReader.Parse(xaml, context);
    element.DataContext = new ProductViewModel();

    PanelManager.StackWpfElement(element);
}

class ProductViewModel
{
    public ProductViewModel()
    {
        DataSource = new AsyncTaskManager<ObservableCollection<string>>(LoadProductsAsync());
    }

    private async Task<ObservableCollection<string>> LoadProductsAsync()
    {
        await Task.Delay(10000);
        return new ObservableCollection<string> { "first", "second", "third" };
    }

    public AsyncTaskManager<ObservableCollection<string>> DataSource { get; private set; }
}

列表框首先显示为空，然后在延迟后填充值。