按钮保持禁用状态 - DelegateCommand 未重新评估 CanExecute 处理程序

Question

问题：按钮永远无法启用。

<Button Name="btnCompareAxises"Command="{Binding CompareCommand}"
          Content="{Binding VM.CompareAxisButtonLabel}" 
        IsEnabled="{Binding VM.IsCompareButtonEnabled}">
</Button>

ViewModel 构造函数：

 this.CompareCommand = new DelegateCommand(CompareCommand, ValidateCompareCommand);

问题似乎与按钮的注册Command的CanExecute事件处理程序有关。 CanExecute 处理程序在应用程序加载时返回 false。 这很好，因为最初没有满足条件。

canExecute 处理程序仅在应用程序启动或单击按钮时运行。您不能单击禁用的按钮，因此如果从 CanExecute 处理程序返回的初始值为 false，则该按钮将永远保持禁用状态！

问题：
我是否必须再次启用该按钮，仅使用绑定到它的命令。 像，嘿命令，请重新评估此按钮的条件是否满足？

为什么 IsEnabled 属性位于 Coercion 部分而不是 local 部分？

命令：

public class DelegateCommand : ICommand
{
    private readonly Func<object, bool> canExecute;
    private readonly Action<object> execute;

    public DelegateCommand(Action<object> execute, Func<object, bool> canExecute = null)
    {
        this.execute = execute;
        this.canExecute = canExecute;
    }

    public event EventHandler CanExecuteChanged;

    public bool CanExecute(object parameter)
    {
        return this.canExecute == null || this.canExecute(parameter);
    }

    public void Execute(object parameter)
    {
        this.execute(parameter);
    }

    public void RaiseCanExecuteChanged()
    {
        this.OnCanExecuteChanged();
    }

    protected virtual void OnCanExecuteChanged()
    {
        var handler = this.CanExecuteChanged;
        if (handler != null)
        {
            handler(this, EventArgs.Empty);
        }
    }
}

Answer 1

已解决：

我必须调整 DelegateCommand 类以使其工作：

我已将CommandManager.RequerySuggested 添加到公共CanExecuteChanged 事件属性中。

现在它会在 UI 发生变化时自动重新评估命令的 CanExecute 方法！

public class DelegateCommand : ICommand
{
    private readonly Func<object, bool> canExecute;
    private readonly Action<object> execute;

    public DelegateCommand(Action<object> execute, Func<object, bool> canExecute = null)
    {
        this.execute = execute;
        this.canExecute = canExecute;
    }

    /// CommandManager
    /// Go to the "References" part of your class library and select "Add Reference". 
    /// Look for an assembly called "PresentationCore" and add it.
    public event EventHandler CanExecuteChanged
    {
        add
        {
            _internalCanExecuteChanged += value;
          CommandManager.RequerySuggested += value;

        }
        remove
        {
            _internalCanExecuteChanged -= value;
            CommandManager.RequerySuggested -= value;
        }
    }

    event EventHandler _internalCanExecuteChanged;

    public bool CanExecute(object parameter)
    {
        return this.canExecute == null || this.canExecute(parameter);
    }

    public void Execute(object parameter)
    {
        this.execute(parameter);
    }

    public void RaiseCanExecuteChanged()
    {
        this.OnCanExecuteChanged();
    }

    protected virtual void OnCanExecuteChanged()
    {
        var handler = this._internalCanExecuteChanged;
        if (handler != null)
        {
            handler(this, EventArgs.Empty);
        }
    }
}

从按钮中删除了这个：

 IsEnabled="{Binding VM.IsCompareButtonEnabled}"

这里的绑定不是必需的，因为 CanExecute 处理程序会处理按钮的启用/禁用状态！