DelegateCommand的CanExecute逻辑 [英] CanExecute Logic for DelegateCommand

问题描述

更新：焦点变成了MVVM，而不是实际的问题，所以我更新它。

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

CanExecute 代表 DelegateCommand 。它在我调用 RaiseCanExecuteChanged 之前不更新，这是否是所需的行为？

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

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

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

问题是这样，我有两个 Buttons 一个是绑定命令到 RelayCommand 实现，另一个绑定到Prism实现 DelegateCommand

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}"/>

ViewModel ICommands

The ViewModel ICommands

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

和 CanExecute p>

and the CanExecute method/predicate

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。

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

推荐答案

正如已经提到的，这是 DelagateCommand ，而不是一个错误。
DelegateCommand 不会自动引发 CanExecuteChanged 事件，您必须通过调用 RaiseCanExecuteChanged 。而 RelayCommand 继电器的 CommandManager.RequerySuggested 事件。每次用户单击某处或按下按钮时，都会引发此事件。

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.

如果情况不是很方便，或没有适当的地方调用 RaiseCanExecuteChanged 你必须订阅 PropertyChanged 模型上的事件等）我创建了以下简单的包装器，确保 CanExecute CommandManager.RequerySuggested 事件自动执行包装命令的方法

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; }
    }
}

您可以这样使用：

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

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