Question

我无法将异步方法的结果传播到UI。

XAML

<Window x:Class="COVMin.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    xmlns:local="clr-namespace:COVMin"        
    mc:Ignorable="d"
    Title="MainWindow" Height="350" Width="525">
    <Window.DataContext>
        <local:MainViewModel/>
    </Window.DataContext>
    <Grid Margin="0,0,0,0">
        <Border BorderThickness="1" BorderBrush="Black" Background="White" Margin="4" VerticalAlignment="Top" Height="170">
        <Polygon Points="{Binding Points}" Stretch="Fill" Fill="Black" Opacity="0.8" />
    </Border>
    <Button x:Name="button" Content="Button" HorizontalAlignment="Left" Height="24" Marin="0,0,0,10" VerticalAlignment="Bottom" Width="75" Command="{Binding DrawPointsCommand}"/>
    </Grid>

视图模型

class MainViewModel : ViewModelBase
{        
    private PointCollection points { get; set; }

    public PointCollection Points
    {
        get { return this.points; }
        set
        {                
            this.points = value;
            OnPropertyChanged("Points");
        }
    }

    public ICommand DrawPointsCommand { get; private set; }

    /// <summary>
    /// Simplified, in real it´s long time operation causing UI to freeze.
    /// </summary>        
    private Task<PointCollection> ConvertToPointCollection()
    {
        return Task.Run<PointCollection>(() =>
        {
            PointCollection points = new PointCollection();
            points.Add(new System.Windows.Point(0, 6236832));                
            points.Add(new System.Windows.Point(255, 6236832));

            return points;
        });
    }

    /// <summary>
    /// 
    /// </summary>
    private async Task<PointCollection> Process()
    {            
        this.Points = await ConvertToPointCollection();
        return this.Points;
    }

    /// <summary>
    /// Method calling long-time operation bound to button as a Command.
    /// </summary>
    private async void GetValues()
    {
        this.Points = await Process();            
    }

    /// <summary>
    /// Constructor.
    /// </summary>
    public MainViewModel()
    {
        this.DrawPointsCommand = new DelegateCommand(GetValues);
    }
}

ViewModelBase

    /// <summary>
    /// Base class for PropertyChanged event handling.
    /// </summary>        
    class ViewModelBase : INotifyPropertyChanged
    {  
        public event PropertyChangedEventHandler PropertyChanged;

        public void OnPropertyChanged(string name)
        {
            if (PropertyChanged != null)
            {
                PropertyChanged(this, new PropertyChangedEventArgs(name));
            }
        }
    }

DelegateCommand类

public class DelegateCommand : ICommand
{        
    private readonly Action _action;

    public DelegateCommand(Action action)
    {
        _action = action;
    }

    public void Execute(object parameter)
    {
        _action();
    }

    public bool CanExecute(object parameter)
    {
        return true;
    }

    public event EventHandler CanExecuteChanged;        
}

问题出在OnPropertyChange上，导致System.ArgumentException告诉我必须在与DependencyObject相同的线程上创建DependencySource。我花了相当多的时间尝试使用Dispatchers，但事实并非如此。

Answer 1

主要问题是PointCollection是DependencyObject，因此由创建它的线程拥有。通常，您不能在其他线程中使用此类对象。使用Dispatcher（显式或隐式地使用await作为您的代码示例）在这里没有用，因为它不是拥有该对象的UI线程。实际上，它正试图在导致问题的UI线程上使用该对象。

对于DependencyObjects同样是Freezable对象，“{不是跨线程共享”规则有一个重要的例外，因为PointCollection是ConvertToPointCollection()对象。如果在任何其他线程尝试访问它之前冻结拥有线程中的对象，那么在其他线程中使用它是安全的。

因此，您可以将private Task<PointCollection> ConvertToPointCollection() { return Task.Run<PointCollection>(() => { PointCollection points = new PointCollection(); points.Add(new System.Windows.Point(0, 6236832)); points.Add(new System.Windows.Point(255, 6236832)); points.Freeze(); return points; }); }方法更改为：

PointCollection

当然，这也会阻止以后修改对象。如果您需要能够修改集合，那么您将不得不采取不同的方法。例如，在UI线程中创建List<Point>，然后使用中间类型（如PointCollection）将新点从后台线程传递到UI线程，然后UI线程可以复制那些指向beforeInsert hook。

将异步PointCollection更改传播到UI

1 个答案: