INotifyPropertyChanged Xamarin字符串值只是消失

Question

我有一個Xamarin.Forms項目與一個ListView填充ObservableCollection。作爲ObservableCollection中每個項目的類（對象）實現INotifyPropertyChanged。 Color屬性在用戶界面中正常切換，但字符串屬性消失並且永不返回。

我從web服務獲得初始值，但只是做一個完全靜態的值的變化，用於調試，我仍然無法弄清楚。

在我ContentPage類的頂部我有這樣的：

public ObservableCollection<GroceryItem> oc; 

後我的web服務已與我把數據的的ObservableCollection返回的數據並作出，該的ItemsSource爲ListView。像這樣：

lvGroceries.ItemsSource = oc; 

這一切都很好。

的XAML

<ListView x:Name="lvGroceries" ItemTapped="GroceryPdsItemTapped" >    
    <ListView.ItemTemplate > 
    <DataTemplate> 
     <ViewCell>    
     <AbsoluteLayout VerticalOptions="Fill"> 
      <Label Text="{Binding GroceryName}" AbsoluteLayout.LayoutBounds="0,0,200,40" ></Label> 
      <Label Text="{Binding strHomeLoc}" AbsoluteLayout.LayoutBounds="200,0,100,40" ></Label> 
      <Label Text="{Binding isNeeded}" AbsoluteLayout.LayoutBounds="300,0,50,40" ></Label>     
      <Label Text="someText" BackgroundColor="{Binding myBackgroundColor}" AbsoluteLayout.LayoutBounds="350,0,50,40" ></Label> 
     </AbsoluteLayout>   
     </ViewCell> 
    </DataTemplate> 
    </ListView.ItemTemplate> 
</ListView> 

類 - GroceryItem

public class GroceryItem : INotifyPropertyChanged 
{ 
    public GroceryItem() { } 
    public event PropertyChangedEventHandler PropertyChanged; 

    private string privateIsNeeded; 
    public string isNeeded 
    { 
     get { return privateIsNeeded; } 
     set 
     { 
      privateIsNeeded = value; 
      OnPropertyChanged(); 
     } 
    } 

    private Color theColor; 
    public Color myBackgroundColor 
    { 
     get { return theColor; } 
     set 
     { 
      theColor = value; 
      OnPropertyChanged(); 
     } 
    } 

    protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null) 
    { 
     PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName)); 
    } 

} 

單擊處理程序。我從ObservableCollection中獲取一個項目並更改這兩個屬性。

public void GroceryPdsItemTapped(object obj, ItemTappedEventArgs e) 
{ 
    if (e.Item == null) 
    { 
     return; 
    } 
    var g = ((GroceryItem)e.Item); 
    foreach (var gr in oc) 
    { 
     if (gr.GroceryId == "27769") 
     { // the UI changes because the myBackgroundColor property in the GroceryItem class is watching for a value change 
      gr.myBackgroundColor = (gr.myBackgroundColor == Color.Yellow) ? Color.Blue : Color.Yellow; 
      gr.isNeeded = (gr.isNeeded == "true" || gr.isNeeded == "blah" || gr.isNeeded == "false") ? "notblah" : "blah"; 
     } 
    } 
} 

顏色切換優良的UI，但isNeeded字符串值消失的第一次敲擊和永不再出現

想法？

Answer 1

這裏有一些問題。首先，你需要讓你的ObservableCollection運行OnPropertyChanged()時，它已經改變了，像這樣：

private ObservableCollection<GroceryItem> _oc; 

public ObservableCollection<GroceryItem> oc { 
    get { return _oc ?? (_oc = new ObservableCollection<GroceryItem>()); } 
    set { 
     if(_oc != value) { 
      _oc = value; 
      OnPropertyChanged(); 
     } 
    } 
} 

現在你要真有這一切在一個視圖模型，但既然你不這樣做，你需要設置你的ContentPage爲從內部代碼隱藏BindingContext這樣的：

public partial class MyGroceryPage : ContentPage { 

    public MyGroceryPage() { BindingContext = this; } 
} 

您還需要分配給它的有約束力的ObservableCollection您ListView.ItemSource代替。這看起來是這樣的：

<ListView ItemSource="{Binding oc}"/> 

如果你做到上面，然後你進入後面的代碼並執行lvGroceries.ItemsSource = oc;那麼這將覆蓋綁定您在XAML一樣，所以不做。相反，當您從網絡服務中獲取數據，你只需將其分配給現有的ObservableCollection：

public async Task GetGroceryData() { 
    List<GroceryItem> myData = await GroceryService.GetGroceriesAsync(); 

    oc = new ObservableCollection<GroceryItem>(myData); 
} 

嘗試所有的第一，如果你的項目，仍沒有更新，你可能會想嘗試從消除他們的ObservableCollection，更改屬性，然後將它們添加回來：

public void GroceryPdsItemTapped(object obj, ItemTappedEventArgs e) { 

    if (e.Item == null) { return; } 

    var g = ((GroceryItem)e.Item); 

    foreach (var gr in oc.ToList()) { 

     if (gr.GroceryId == "27769") { // the UI changes because the myBackgroundColor property in the GroceryItem class is watching for a value change 
      oc.Remove(gr); 
      gr.myBackgroundColor = (gr.myBackgroundColor == Color.Yellow) ? Color.Blue : Color.Yellow; 
      gr.isNeeded = (gr.isNeeded == "true" || gr.isNeeded == "blah" || gr.isNeeded == "false") ? "notblah" : "blah"; 
      oc.Add(gr); 
     } 
    } 
}