c# - 满足条件时 CanExecute() 不启用按钮

Question

我有一个非常简单的应用程序，其中包含一个 TextBox 和一个 Button。当输入到 TextBox 中的文本长度超过 5 个字符时，该按钮将被启用。这是我的 ViewModel 的代码:

private string _text { get; set; }
public string Text
{
    get { return _text; }
    set
    {
        _text = value;
        OnPropertyChanged("Text");
    }
}

private ICommand _buttonCommand;
public ICommand ButtonCommand
{
    get
    {
        if (_buttonCommand == null)
        {
            _buttonCommand = new RelayCommand(
                param => this.ButtonCommandExecute(), 
                param => this.ButtonCommandCanExecute()
            );
        }
        return _buttonCommand;
    }
}

private bool ButtonCommandCanExecute()
{
    if (this.Text.Length < 5)
    {
        return false;
    }
    else
    {
        return true;
    }
}

private void ButtonCommandExecute()
{
    this.Text = "Text changed";
}

public MainWindowViewModel()
{
    //
}

TextBox 和 Button 使用此 XAML 绑定(bind):

<Button Content="Button" HorizontalAlignment="Left" 
                Margin="185,132,0,0" VerticalAlignment="Top" Width="120"
                Command="{Binding Path=ButtonCommand}" />

<TextBox HorizontalAlignment="Left" Height="23" 
         Margin="185,109,0,0" TextWrapping="Wrap" 
         Text="{Binding Path=Text, Mode=TwoWay}" VerticalAlignment="Top" Width="120"/>

DataContext 似乎设置正确，但这只是因为我是 WPF 初学者:

private MainWindowViewModel view_model;

public MainWindow()
{
    InitializeComponent();

    view_model = new MainWindowViewModel();

    this.DataContext = view_model;
}

当我输入 TextBox 时，Button 永远不会启用。

Answer 1

ICommand 接口(interface)的一些实现有特殊的方法来通知“CanExecute”是否已经改变。 RelayCommand 类(MVVM Light)有这样的方法。

private string _text;
public string Text
{
    get { return _text; }
    set
    {
        _text = value;
        OnPropertyChanged("Text");

        // There is a special RelayCommand method to notify "CanExecute" changed.
        // After this call, the "CanExecute" state is "re-evaluated" automatically by binding using CanExecute Func passed into RelayCommand constructor.
        _buttonCommand.RaiseCanExecuteChanged();
    }
}

private RelayCommand _buttonCommand;
public ICommand ButtonCommand
{
    get
    {
        if (_buttonCommand == null)
        {
            _buttonCommand = new RelayCommand(
                param => this.ButtonCommandExecute(), 
                param => this.ButtonCommandCanExecute()
            );
        }
        return _buttonCommand;
    }
}

这个问题很有用:What is CanExecuteChanged for?