如何使用 mvvm 模式更改标签内容

Question

我尝试做一些简单的事情，但似乎我错过了什么。单击按钮时，我尝试更改标签的内容。我使用 MVVM 模式来做到这一点。这是我的代码：

查看：

    <Button x:Name="buttonNext" 
        HorizontalAlignment="Center" 
        VerticalAlignment="Center"
        Grid.Column="1"
        Grid.Row="2"
        Width="85" 
        Height="35"
        Style="{StaticResource AccentedSquareButtonStyle}"
        Command="{Binding Path=Next}">
        <TextBlock Text="Next"
               TextWrapping="Wrap"
               TextAlignment="Center"
               HorizontalAlignment="Center"
               VerticalAlignment="Center"/>
    </Button>
<Label Name="Path"
       Grid.ColumnSpan="2"
       HorizontalAlignment="Center"
       VerticalAlignment="Center"
       Content="{Binding path}"
       FontWeight="Bold"
       Foreground="DeepSkyBlue"
       />

视图模型：

    public ICommand Next { get; set; } 
    private string _path;
    public string path
    {
        get
        {
            return _path;
        }
        set
        {
            _path = value;

            RaisePropertyChanged("path");
        }
    }

    public event PropertyChangedEventHandler PropertyChanged = null;

    protected virtual void RaisePropertyChanged(string propName)
    {
        if (PropertyChanged != null)
        {
            Task.Run(() => PropertyChanged(this, new PropertyChangedEventArgs(propName)));
        }
    }

    public page4ViewModel(NavigationViewModel navigationViewModel)
    {
      _path = "etape1";
       Next = new BaseCommand(GoNext);
    }

    private void GoNext(object obj)
    {
        switch (_path)
        {
            case "etape1":
                _path = "etape2";
                break;
            case : "etape2"
                _path = "etape3";
                break;
            case "etape3":
                _path = "etape4";
                break;
            default:
                _path = " ";
                break;

        }

    }

起初标签是构造函数中的“etape1”，但是当我单击下一个按钮时，值不会改变。 ps：更改的功能有效，因为我放了一个断点来查看。感谢您的帮助

Answer 1

您必须使用属性，而不是支持字段。

private void GoNext(object obj)
{
    switch (_path)
    {
        case "etape1":
            path = "etape2"; // without underscore
            break;
        case : "etape2"
            path = "etape3";
            break;
        case "etape3":
            path = "etape4";
            break;
        default:
            path = " ";
            break;

    }

这将引发更改通知

编辑

为您的 ViewModel 使用 ViewModelBase 也更好

public abstract class ViewModelBase : INotifyPropertyChanged
{
    public event PropertyChangedEventHandler PropertyChanged;

你可以在哪里定义

    protected virtual void RaisePropertyChanged(string propertyName)
    {
        this.VerifyPropertyName(propertyName);

        PropertyChangedEventHandler handler = this.PropertyChanged;
        if (handler != null)
        {
            var e = new PropertyChangedEventArgs(propertyName);
            handler(this, e);
        }
    }

替代品

根据this的评论，我们可以介绍CallerMemberName

protected virtual void RaisePropertyChanged<T>([CallerMemberName] string propertyName = null)

另一个有用的变体是基于选择器，当您需要从 不同的 setter 中为依赖属性提升它时

    protected virtual void RaisePropertyChanged<T>(Expression<Func<T>> selectorExpression)
    {
        if (selectorExpression == null)
            throw new ArgumentNullException("selectorExpression");
        MemberExpression body = selectorExpression.Body as MemberExpression;
        if (body == null)
            throw new ArgumentException("The body must be a member expression");
        RaisePropertyChanged(body.Member.Name);
    }

Answer 2

您需要在 ui 线程上引发属性更改事件。

替换

if (PropertyChanged != null)
    {
        Task.Run(() => PropertyChanged(this, new PropertyChangedEventArgs(propName)));
    }

与

if (PropertyChanged != null)
    {
        Application.Current.Dispatcher.Invoke(() => PropertyChanged(this, new PropertyChangedEventArgs(propName)));
    }

同时设置path 的值而不是_path

更新

正如 cmets 中所指出的，在这种情况下不需要 Dispatcher.Invoke，因为它已经在 UI 线程上。

只要打电话

PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));

Answer 3

您正在更改以下值：

private void GoNext(object obj)
{
    switch (_path)
    {
        case "etape1":
            _path = "etape2";
            break;
        case : "etape2"
            _path = "etape3";
            break;
        case "etape3":
            _path = "etape4";
            break;
        default:
            _path = " ";
            break;

    }

但是当你设置 _path 时，path 中的设置器永远不会被调用：

public string path
{
    set ///This is never called
    {
        _path = value;
        RaisePropertyChanged("path");
    }
}

改为：

switch (path)
{
    case "etape1":
        path = "etape2";
        break;
    case : "etape2"
        path = "etape3";
        break;
    case "etape3":
        path = "etape4";
        break;
    default:
        path = " ";
        break;

}