如何使用mvvm模式更改標籤內容

Question

我嘗試做一些簡單的事情，但似乎我錯過了一些東西。當我點擊一個按鈕時，我嘗試更改標籤的內容。我使用MVVM模式來做到這一點。這裏是我的代碼：

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" 
     /> 

視圖模型：

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; 

     } 

    } 

最初的標籤是「etape1」在構造函數中，但後來當我點擊下一步按鈕值不會改變。 ps：改變作品的功能，因爲我把一個斷點看。感謝您的幫助

Answer 1

你必須使用的財產，不支持字段。

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; 

    } 

這將提高通知的變化

編輯

它也更好地使用ViewModelBase您的視圖模型

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

在這裏你可以定義

替代

按this評論，我們可以引入CallerMemberName

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

另一個有用的變化是基於一個選擇，當你需要從不同的提高它的依賴屬性 setter

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線程上引發屬性已更改的事件。

更換

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

與

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

還爲path值不_path

更新

正如評論，DISPA指出tcher.Invoke在這種情況下不是必需的，因爲它已經在UI線程中。

只需撥打

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

Answer 3

您a再在改變值：

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; 

    } 

但是當你正在設置_path，在path設置器還沒有被調用：

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

更改爲：

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

}