可以执行DelegateCommand的逻辑 [英] CanExecute Logic for DelegateCommand

问题描述

我遇到了一个问题， CanExecute 为 DelegateCommand 。在我调用 RaiseCanExecuteChanged 之前，它不会更新，这是否是所需的行为？

我上传了一个简单的示例项目，在此重现此问题： http://dl.dropbox.com/u/39657172/DelegateCommandProblem.zip

问题是这样，我有两个按钮这样。一个是将命令绑定到 RelayCommand 实现，另一个绑定到的Prism实现/ code code
$ b $ pre $ < Button Command ={Binding DelegateSaveCommand}/>
< Button Command ={Binding RelaySaveCommand}/>


ViewModel ICommands

  DelegateSaveCommand = new DelegateCommand（Save，CanSaveDelegate）; 
 RelaySaveCommand = new RelayCommand（param => Save（），param => CanSaveRelay）; 
  

和 CanExecute 方法/谓词





  public bool CanSaveDelegate（）
 {
 return HasChanges; 
} 
 public bool CanSaveRelay 
 {
 get {return HasChanges; } 
} 
  

两者都使用属性 HasChanges 。当 HasChanges 更新时，只有 CanSaveRelay 更新。是这样的吗？

解决方案

正如已经提到的，这是 DelagateCommand ，而不是错误。
DelegateCommand 不会自动提高 CanExecuteChanged 事件，您必须通过调用 RaiseCanExecuteChanged 在适当的时候。而$ code> RelayCommand 继承 CommandManager.RequerySuggested 事件。每次用户点击某处或按下按钮时，都会提高此事件。

对于不太方便的情况或没有适当的地方调用 RaiseCanExecuteChanged （如在您的场景中）您必须在模型上订阅 PropertyChanged 事件）我创建了以下简单的包装器，确保 CanExecute CommandManager.RequerySuggested 事件：

  public class AutoCanExecuteCommandWrapper：ICommand 
 {
 public ICommand WrappedCommand {get;私人集合
 
 public AutoCanExecuteCommandWrapper（ICommand wrappedCommand）
 {
 if（wrappedCommand == null）
 {
 throw new ArgumentNullException（wrappedCommand）; 
} 
 
 WrappedCommand = wrappedCommand; 
} 
 
 public void Execute（object parameter）
 {
 WrappedCommand.Execute（parameter）; 
} 
 
 public bool CanExecute（object parameter）
 {
 return WrappedCommand.CanExecute（parameter）; 
} 
 
 public event EventHandler CanExecuteChanged 
 {
 add {CommandManager.RequerySuggested + = value; } 
 remove {CommandManager.RequerySuggested  -  = value; } 
} 
} 
  

你可以这样使用：

  DelegateSaveCommand = new AutoCanExecuteCommandWrapper（new DelegateCommand（Save，CanSaveDelegate））; 
  

Update: The focus became MVVM instead of the actual question so I'm updating it.

I'm having a problem with CanExecute for DelegateCommand. It doesn't update before I call RaiseCanExecuteChanged, is this the desired behavior?

I uploaded a simple sample project reproducing this problem here : http://dl.dropbox.com/u/39657172/DelegateCommandProblem.zip

The problem is this, I have two Buttons like this. One is Binding Command to a RelayCommand implementation and the other is binding to the Prism implementation of DelegateCommand

<Button Command="{Binding DelegateSaveCommand}"/>
<Button Command="{Binding RelaySaveCommand}"/>

The ViewModel ICommands

DelegateSaveCommand = new DelegateCommand(Save, CanSaveDelegate);
RelaySaveCommand = new RelayCommand(param => Save(), param => CanSaveRelay);

and the CanExecute method/predicate

public bool CanSaveDelegate()
{
    return HasChanges;
}
public bool CanSaveRelay
{
    get { return HasChanges; }
}

Both are using the property HasChanges. When HasChanges is updated, only the CanSaveRelay updates. Is this the way it's meant to be?

解决方案

As it already was mentioned, this is intended behavior of DelagateCommand, not a bug. DelegateCommand doesn't raise CanExecuteChanged event automatically, you have to raise that event manually by calling RaiseCanExecuteChanged when appropriate. Whereas RelayCommand relays on CommandManager.RequerySuggested event for that. This event is raised every time the user clicks somewhere or presses a button.

For situations when it is not very convenient or there is no appropriate place for calling RaiseCanExecuteChanged (like in your scenario you have to subscribe to PropertyChanged event on the model, etc) I have created the following simple wrapper that ensures that the CanExecute method of the wrapped command is executed automatically on CommandManager.RequerySuggested event:

public class AutoCanExecuteCommandWrapper : ICommand
{
    public ICommand WrappedCommand { get; private set; }

    public AutoCanExecuteCommandWrapper(ICommand wrappedCommand)
    {
        if (wrappedCommand == null) 
        {
            throw new ArgumentNullException("wrappedCommand");
        }

        WrappedCommand = wrappedCommand;
    }

    public void Execute(object parameter)
    {
        WrappedCommand.Execute(parameter);
    }

    public bool CanExecute(object parameter)
    {
        return WrappedCommand.CanExecute(parameter);
    }

    public event EventHandler CanExecuteChanged
    {
        add { CommandManager.RequerySuggested += value; }
        remove { CommandManager.RequerySuggested -= value; }
    }
}

You can use it like this:

DelegateSaveCommand = new AutoCanExecuteCommandWrapper(new DelegateCommand(Save, CanSaveDelegate));

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