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与a TextBox和a Button.当输入的文本TextBox长度超过5个字符时,将启用该按钮.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()
{
    //
}

该TextBox和Button使用此XAML约束:

<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"/>

在DataContext似乎是正确设置,但在这里它仅仅是因为我是一个WPF初学者:

private MainWindowViewModel view_model;

public MainWindow()
{
    InitializeComponent();

    view_model = new MainWindowViewModel();

    this.DataContext = view_model;
}

当我输入时TextBox,Button永远不会启用.

Answer 1

ICommand接口的某些实现具有特殊方法来通知"CanExecute"是否已更改.RelayCommandclass(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:什么是CanExecuteChanged？