刷新WPF命令

Question

有谁知道我怎么可以强制CanExecute调用自定义命令(约什史密斯RelayCommand)？

通常,CanExecute只要在UI上发生交互,就会调用它.如果我点击某些内容,我的命令会更新.

我有一种情况,CanExecute即幕后的计时器打开/关闭条件.因为这不是由用户交互驱动的,CanExecute所以在用户与UI交互之前不会调用.最终结果是我的Button保持启用/禁用,直到用户点击它.点击后,它会正确更新.有时Button显示已启用,但是当用户单击它时,更改为禁用而不是触发.

当计时器更改影响的属性时,如何强制更新代码CanExecute？我对影响的属性尝试了kick PropertyChanged(INotifyPropertyChanged)CanExecute,但这没有帮助.

示例XAML:

<Button Content="Button" Command="{Binding Cmd}"/>

后面的示例代码:

private ICommand m_cmd;
public ICommand Cmd
{
    if (m_cmd == null)
        m_cmd = new RelayCommand(
            (param) => Process(),
            (param) => EnableButton);

    return m_cmd;
}

// Gets updated from a timer (not direct user interaction)
public bool EnableButton { get; set; }

Answer 1

CommandManager.InvalidateRequerySuggested()

Answer 2

很久以前我就知道了CommandManager.InvalidateRequerySuggested()并且使用过它,但它有时对我不起作用.我终于弄清楚为什么会这样!即使它不像其他一些动作那样抛出,你也必须在主线程上调用它.

在后台线程上调用它似乎可以工作,但有时会禁用UI.我真的希望这可以帮助别人,并节省他浪费的时间.

Answer 3

解决方法是绑定IsEnabled到属性:

<Button Content="Button" Command="{Binding Cmd}" IsEnabled="{Binding Path=IsCommandEnabled}"/>

然后在ViewModel中实现此属性.这也使得UnitTesting更容易使用属性而不是命令来查看命令是否可以在某个时间点执行.

我个人觉得它更方便.

Answer 4

这个变种可能适合你:

 public interface IRelayCommand : ICommand
{
    void UpdateCanExecuteState();
}

执行:

 public class RelayCommand : IRelayCommand
{
    public event EventHandler CanExecuteChanged;


    readonly Predicate<Object> _canExecute = null;
    readonly Action<Object> _executeAction = null;

   public RelayCommand( Action<object> executeAction,Predicate<Object> canExecute = null)
    {
        _canExecute = canExecute;
        _executeAction = executeAction;
    }


    public bool CanExecute(object parameter)
    {
       if (_canExecute != null)
            return _canExecute(parameter);
        return true;
    }

    public void UpdateCanExecuteState()
    {
        if (CanExecuteChanged != null)
            CanExecuteChanged(this, new EventArgs());
    }



    public void Execute(object parameter)
    {
        if (_executeAction != null)
            _executeAction(parameter);
        UpdateCanExecuteState();
    }
}

使用简单:

public IRelayCommand EditCommand { get; protected set; }
...
EditCommand = new RelayCommand(EditCommandExecuted, CanEditCommandExecuted);

 protected override bool CanEditCommandExecuted(object obj)
    {
        return SelectedItem != null ;
    }

    protected override void EditCommandExecuted(object obj)
    {
        // Do something
    }

   ...

    public TEntity SelectedItem
    {
        get { return _selectedItem; }
        set
        {
            _selectedItem = value;

            //Refresh can execute
            EditCommand.UpdateCanExecuteState();

            RaisePropertyChanged(() => SelectedItem);
        }
    }

XAML:

<Button Content="Edit" Command="{Binding EditCommand}"/>