Xamarin的形式，與OnPropertyChanged正與一個int值

Question

我有我的MasterDetailPage一個按鈕，改變對一個INT（名爲App.value1），這取決於你點擊尋找這樣的事情的價值：

void click1 (object s, EventArgs a) 
{ 
    if (App.value1 == 0) { 

     App.value1 = App.value1 + 1; 

    } else { 
     App.value1 = 0; 
    } 
} 

而且我想要這個點擊功能立即改變我的StartPage上的值（另一個ContentPage）。所以，我創建了一個視圖模型看起來像這樣，在那裏我嘗試用電流值的工作：

public event PropertyChangedEventHandler PropertyChanged; 

protected virtual void OnPropertyChanged (string propertyName) 
{ 
    var changed = PropertyChanged; 
    if (changed != null) { 

     PropertyChanged (this, new PropertyChangedEventArgs (propertyName));  
    } 
} 

public int currentValue { 

     get { 
      return App.value1; 
     } 

     set { 

      if (App.value1 == 0) { 
       App.value1 = 0; 

      } else { 

       App.value1 = 1; 
      } 

     } 


    } 

而這正是我想要的INT的值立即更新取決於你在點擊了什麼StartPageMasterDetailView。

public StartPage() 
{ 
     var ourView = new StartPageViewModel(); 

     ourCurrentValue = ourView.currentValue; 
} 

protected async override void OnAppearing() 
{ 
    LoadData(); 
} 


private async Task<List<Pin>> LoadData() //I work with pins here (not showing that code though as it is irrelavant. 
{ 
    if (ourCurrentValue == 0) { 
     System.Diagnostics.Debug.WriteLine ("Value is 0"); 
    } 

    else { 
     System.Diagnostics.Debug.WriteLine ("Value is 1"); 
    } 
} 

現在我只在我的日誌中看到「值爲0」。當我點擊MasterDetailPage上的按鈕時，沒有任何更新。

更新的代碼：

public class StartPageViewModel : INotifyPropertyChanged 
{ 
    private ICommand clickCommand; 
    private int currentValue; 

    public event PropertyChangedEventHandler PropertyChanged; 

    protected virtual void OnPropertyChanged (string propertyName) 
    { 
     var changed = PropertyChanged; 
     if (changed != null) 
     { 
      PropertyChanged (this, new PropertyChangedEventArgs (propertyName));  
     } 
    } 


    public StartPageViewModel() 
    { 
     ClickCommand = new Command(() => CurrentValue = CurrentValue + 1); 
    } 

    public ICommand ClickCommand 
    { 
     get { return clickCommand; } 
     set 
     { 
      clickCommand = value; 
      OnPropertyChanged("ClickCommand"); 
     } 
    } 
    public int CurrentValue 
    { 
     get { return currentValue; } 
     set 
     { 
      currentValue = value; 
      OnPropertyChanged("CurrentValue"); 
     } 
    } 

} 

而且起始頁：

public StartPage() 
    { 
     App.PropertyChanged += (sender, args) => OnPropertyChanged("currentValue"); // ERROR: `An object reference is requiered to access non-static member 'Xamarin.Forms.BindableObject.PropertyChanged` 
    } 

Answer 1

您可以用類似的東西着手：

製作以下更改App類和類中value1屬性：

public static event PropertyChangedEventHandler PropertyChanged; 

private static void OnPropertyChanged (string propertyName) 
{ 
    var changed = PropertyChanged; 
    if (changed != null) 
    { 
     PropertyChanged (null, new PropertyChangedEventArgs (propertyName));  
    } 
} 

private static int _value1; 
public static int value1 
{ 
    get { return _value1; } 
    set 
    { 
     _value1 = value; 
     OnPropertyChanged("value1"); 
    } 
} 

，這一行添加到您的StartPageViewModel構造：

App.PropertyChanged += (sender, args) => OnPropertyChanged("currentValue"); 

在該代碼中你只是利用PropertyChanged自己的目的（你甚至可以創建你自己的事件）。 我的意思是StartPageViewModel訂閱PropertyChanged事件App類，所以它會在value1更改時通知。並且當它真的發生時，那麼它正在調用他自己的PropertyChanged來通知查看關於currentValue的更改。

但是，我要說的更好的辦法是MasterDetailPage和StartPage之間共享視圖模型，因爲使用全局狀態，使您的解決方案很難理解：

public class SharedViewModel : INotifyPropertyChanged 
{ 
    private ICommand clickCommand; 
    private int currentValue; 

    /* INotifyPropertyChanged implementation */ 

    public SharedViewModel() 
    { 
     ClickCommand = new Command(() => CurrentValue = CurrentValue + 1); 
    } 

    public ICommand ClickCommand 
    { 
     get { return clickCommand; } 
     set 
     { 
      clickCommand = value; 
      OnPropertyChanged("ClickCommand"); 
     } 
    } 
    public int CurrentValue 
    { 
     get { return currentValue; } 
     set 
     { 
      currentValue = value; 
      OnPropertyChanged("CurrentValue"); 
     } 
    } 
} 

而且你需要使用的SharedViewModel相同的實例中MasterDetailPage以及StartPage