在 WPF 中使用 DelegateCommand 我面临以下问题：
如果在 UI 线程上执行命令，则在后台工作人员中执行 Command1 会阻止窗口正确刷新按钮的 CanExecute，一切都按预期工作。在按钮外部单击 CanExecute 会正确刷新。有什么提示吗？
（下面是 DelegateCommand 的代码，如果它太大，抱歉。据我所知，这是 DelegateCommand 的常见实现）
XAML：
<Window x:Class="BackgroundWorkTest.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Title="MainWindow" Height="350" Width="525">
<StackPanel>
<Button Command="{Binding Command1}" Content="1" />
<Button Command="{Binding Command2}" Content="2" />
</StackPanel>
</Window>
C＃：
using System.ComponentModel;
using System.Windows;
using System.Windows.Input;
using Utils.Wpf;
namespace BackgroundWorkTest
{
public partial class MainWindow : Window
{
private bool _canCommand1 = true;
private bool _canCommand2 = true;
private readonly BackgroundWorker _backgroundWorker;
public ICommand Command1 { get; private set; }
public ICommand Command2 { get; private set; }
public MainWindow()
{
this.Command1 = new DelegateCommand(this.ExecuteCommand1, this.CanCommand1);
this.Command2 = new DelegateCommand(this.ExecuteCommand2, this.CanCommand2);
_backgroundWorker = new BackgroundWorker();
_backgroundWorker.DoWork += _backgroundWorker_DoWork;
_backgroundWorker.RunWorkerCompleted += _backgroundWorker_RunWorkerCompleted;
InitializeComponent();
this.DataContext = this;
}
private void _backgroundWorker_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)
{
System.Threading.Thread.Sleep(2000);
}
private void _backgroundWorker_DoWork(object sender, DoWorkEventArgs e)
{
System.Threading.Thread.Sleep(2000);
_canCommand2 = true;
_canCommand1 = false;
}
private bool CanCommand2()
{
return _canCommand2;
}
private void ExecuteCommand2()
{
_canCommand2 = false;
_canCommand1 = true;
}
private bool CanCommand1()
{
return _canCommand1;
}
private void ExecuteCommand1()
{
_backgroundWorker.RunWorkerAsync();
}
}
}
委托命令 C#：
public class DelegateCommand : ICommand
{
#region Constructors
/// <summary>
/// Constructor
/// </summary>
public DelegateCommand(Action executeMethod)
: this(executeMethod, null, false)
{
}
/// <summary>
/// Constructor
/// </summary>
public DelegateCommand(Action executeMethod, Func<bool> canExecuteMethod)
: this(executeMethod, canExecuteMethod, false)
{
}
/// <summary>
/// Constructor
/// </summary>
public DelegateCommand(Action executeMethod, Func<bool> canExecuteMethod, bool isAutomaticRequeryDisabled)
{
if (executeMethod == null)
{
throw new ArgumentNullException("executeMethod");
}
this._executeMethod = executeMethod;
this._canExecuteMethod = canExecuteMethod;
this._isAutomaticRequeryDisabled = isAutomaticRequeryDisabled;
}
#endregion
#region Public Methods
/// <summary>
/// Method to determine if the command can be executed
/// </summary>
public bool CanExecute()
{
if (this._canExecuteMethod != null)
{
return this._canExecuteMethod();
}
return true;
}
/// <summary>
/// Execution of the command
/// </summary>
public void Execute()
{
if (this._executeMethod != null)
{
this._executeMethod();
}
}
/// <summary>
/// Property to enable or disable CommandManager's automatic requery on this command
/// </summary>
public bool IsAutomaticRequeryDisabled
{
get
{
return this._isAutomaticRequeryDisabled;
}
set
{
if (this._isAutomaticRequeryDisabled != value)
{
if (value)
{
CommandManagerHelper.RemoveHandlersFromRequerySuggested(this._canExecuteChangedHandlers);
}
else
{
CommandManagerHelper.AddHandlersToRequerySuggested(this._canExecuteChangedHandlers);
}
this._isAutomaticRequeryDisabled = value;
}
}
}
/// <summary>
/// Raises the CanExecuteChaged event
/// </summary>
public void RaiseCanExecuteChanged()
{
this.OnCanExecuteChanged();
}
/// <summary>
/// Protected virtual method to raise CanExecuteChanged event
/// </summary>
protected virtual void OnCanExecuteChanged()
{
CommandManagerHelper.CallWeakReferenceHandlers(this._canExecuteChangedHandlers);
}
#endregion
#region ICommand Members
/// <summary>
/// ICommand.CanExecuteChanged implementation
/// </summary>
public event EventHandler CanExecuteChanged
{
add
{
if (!this._isAutomaticRequeryDisabled)
{
CommandManager.RequerySuggested += value;
}
CommandManagerHelper.AddWeakReferenceHandler(ref this._canExecuteChangedHandlers, value, 2);
}
remove
{
if (!this._isAutomaticRequeryDisabled)
{
CommandManager.RequerySuggested -= value;
}
CommandManagerHelper.RemoveWeakReferenceHandler(this._canExecuteChangedHandlers, value);
}
}
bool ICommand.CanExecute(object parameter)
{
return this.CanExecute();
}
void ICommand.Execute(object parameter)
{
this.Execute();
}
#endregion
#region Data
private readonly Action _executeMethod;
private readonly Func<bool> _canExecuteMethod;
private bool _isAutomaticRequeryDisabled;
private List<WeakReference> _canExecuteChangedHandlers;
#endregion
}
和 CommandManager：
internal class CommandManagerHelper
{
internal static void CallWeakReferenceHandlers(List<WeakReference> handlers)
{
if (handlers != null)
{
// Take a snapshot of the handlers before we call out to them since the handlers
// could cause the array to me modified while we are reading it.
EventHandler[] callees = new EventHandler[handlers.Count];
int count = 0;
for (int i = handlers.Count - 1; i >= 0; i--)
{
WeakReference reference = handlers[i];
EventHandler handler = reference.Target as EventHandler;
if (handler == null)
{
// Clean up old handlers that have been collected
handlers.RemoveAt(i);
}
else
{
callees[count] = handler;
count++;
}
}
// Call the handlers that we snapshotted
for (int i = 0; i < count; i++)
{
EventHandler handler = callees[i];
handler(null, EventArgs.Empty);
}
}
}
internal static void AddHandlersToRequerySuggested(List<WeakReference> handlers)
{
if (handlers != null)
{
foreach (WeakReference handlerRef in handlers)
{
EventHandler handler = handlerRef.Target as EventHandler;
if (handler != null)
{
CommandManager.RequerySuggested += handler;
}
}
}
}
internal static void RemoveHandlersFromRequerySuggested(List<WeakReference> handlers)
{
if (handlers != null)
{
foreach (WeakReference handlerRef in handlers)
{
EventHandler handler = handlerRef.Target as EventHandler;
if (handler != null)
{
CommandManager.RequerySuggested -= handler;
}
}
}
}
internal static void AddWeakReferenceHandler(ref List<WeakReference> handlers, EventHandler handler)
{
AddWeakReferenceHandler(ref handlers, handler, -1);
}
internal static void AddWeakReferenceHandler(ref List<WeakReference> handlers, EventHandler handler, int defaultListSize)
{
if (handlers == null)
{
handlers = (defaultListSize > 0 ? new List<WeakReference>(defaultListSize) : new List<WeakReference>());
}
handlers.Add(new WeakReference(handler));
}
internal static void RemoveWeakReferenceHandler(List<WeakReference> handlers, EventHandler handler)
{
if (handlers != null)
{
for (int i = handlers.Count - 1; i >= 0; i--)
{
WeakReference reference = handlers[i];
EventHandler existingHandler = reference.Target as EventHandler;
if ((existingHandler == null) || (existingHandler == handler))
{
// Clean up old handlers that have been collected
// in addition to the handler that is to be removed.
handlers.RemoveAt(i);
}
}
}
}
}