属性更改时未调用按钮命令 CanExecute

Question

我有一个带有文本框和按钮的表单。

当该文本框的值发生变化时，按钮命令不会调用其命令的 CanExecute 方法。

命令参数已设置但似乎没有更改。加载 window 后，按钮保持禁用状态。

<TextBox Text="{Binding Name, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" />
<Button Content="Save" Command="{Binding SaveChangesCommand}" CommandParameter="{Binding Name}" />

我知道绑定正在工作，因为我创建了一个接收目标绑定并在绑定更改时引发 CanExecute 的行为。

对于这种行为，CanExecute 被正常调用。

<Button Content="Save" Command="{Binding SaveChangesCommand}">
    <i:Interaction.Behaviors>
        <behaviors:CallCommandCanExecuteWhenBindingChange Target="{Binding Name}" />
    </i:Interaction.Behaviors>
</Button>

视图模型：

public class EditViewModel : INotifyPropertyChanged
{
    private string _name;

    public EditViewModel()
    {
        SaveChangesCommand = new DelegateCommand(p => SaveChanges(), p => CanSaveChanges());
    }

    public string Name
    {
        get { return _name; }
        set
        {
            if (value == _name) return;
            _name = value;
            OnPropertyChanged();
        }
    }

    public DelegateCommand SaveChangesCommand { get; private set; }

    private void SaveChanges()
    {
    }
    private bool CanSaveChanges()
    {
        return !string.IsNullOrWhiteSpace(Name);
    }
}

委托命令：

public interface IBaseCommand : ICommand
{
    void OnCanExecuteChanged();
}

public class DelegateCommand : IBaseCommand
{
    private readonly Action<object> _execute;
    private readonly Func<object, bool> _canExecute;

    public DelegateCommand(Action<object> execute, Func<object, bool> canExecute)
    {
        _execute = execute;
        _canExecute = canExecute;
    }

    public event EventHandler CanExecuteChanged;

    public bool CanExecute(object parameter)
    {
        return _canExecute(parameter);
    }
    public void Execute(object parameter)
    {
        _execute(parameter);
        OnCanExecuteChanged();
    }

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

CallCommandCanExecuteWhenBindingChange:

public class CallCommandCanExecuteWhenBindingChange : Behavior<FrameworkElement>
{
    public static readonly DependencyProperty<CallCommandCanExecuteWhenBindingChange, object> TargetProperty;
    private ICommandBase _command;

    static CallCommandCanExecuteWhenBindingChange()
    {
        var dependency = new DependencyRegistry<CallCommandCanExecuteWhenBindingChange>();

        TargetProperty = dependency.Register(b => b.Target, s => s.OnTargetChange());
    }

    public object Target
    {
        get { return TargetProperty.Get(this); }
        set { TargetProperty.Set(this, value); }
    }

    private void OnTargetChange()
    {
        if (_command == null && AssociatedObject != null)
        {
            var field = AssociatedObject.GetType().GetProperty("Command");
            _command = (IBaseCommand)field.GetValue(AssociatedObject);
        }

        if (_command != null)
            _command.OnCanExecuteChanged();
    }
}

有谁知道为什么按钮不调用 CanExecute？

Answer 1

在您的 DelegateCommand 实施中 CanExecuteChanged 应该 add/remove 到 CommandManager.RequerySuggested 事件

Occurs when the CommandManager detects conditions that might change the ability of a command to execute.

改为

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

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