當ObservableCollection對象發生變化時，ListView不會更新

Question

要自動更新Xamarin.Forms ListView的ItemsSource屬性，我嘗試在代碼隱藏文件中使用ObservableCollection對象。然後，我將該ObservableCollection對象分配給ListView.ItemsSource屬性。但是我只是在代碼隱藏文件的構造函數中完成一項任務。

例1用一個計時器，更新ObservableCollection對象會自動更改Xaml文件中的視圖。 正如我期望的那樣，示例1的作品使用了ObservableCollection。

示例2 不起作用當按下名爲「Set 2」的按鈕時。 xaml文件中的視圖不會隨着該按鈕的新數據事件處理程序在後面的代碼中更新而更改。 請解釋爲什麼即使我使用的ObservableCollection相同的概念與例1

在下面是實施例1的代碼的工作，因爲我希望例2不工作的原因。

using System; 
using System.Collections.ObjectModel; 
using Xamarin.Forms; 

namespace ObservableLogger 
{ 
    public partial class ObservableLoggerPage : ContentPage 
    { 
     public ObservableLoggerPage() 
     { 
      InitializeComponent(); 

      ObservableCollection<DateTime> list = new ObservableCollection<DateTime>(); // ObservableCollection<DateTime> object is used 
      listView.ItemsSource = list; // ********** bind 

      Device.StartTimer(TimeSpan.FromSeconds(1),() => 
      { 
       list.Add(DateTime.Now); // *** changing the ObservableCollection<DateTime> object automatically makes ListView's ItemsSource is rebound and ListView's UI changed. 
       return true; 
      }); 
     } 
    } 
} 

<?xml version="1.0" encoding="utf-8" ?> 
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms" 
      xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml" 
      x:Class="ObservableLogger.ObservableLoggerPage"> 
    <ContentPage.Padding> 
     <OnPlatform x:TypeArguments="Thickness" 
        iOS="10, 20, 10, 0" 
        Android="10, 0" 
        WinPhone="10, 0" /> 
    </ContentPage.Padding> 

    <ListView x:Name="listView" /> 
</ContentPage> 

以下是例2的代碼的如我所料不工作。當命名爲「Set 2」的按鈕被擊中時，其事件處理程序會更改ObservableCollection人員（私人字段），但ListView不會在UI中更新。

using System; 
using System.Collections.Generic; 
using System.Collections.ObjectModel; 
using System.Linq; 
using System.Text; 
using System.Threading.Tasks; 

using Xamarin.Forms; 

namespace Mvvm1 
{ 
    public partial class Mvvm1Page : ContentPage 
    { 
     private ObservableCollection<Person> _people; // private field 

     public Mvvm1Page() 
     { 
      InitializeComponent(); 

      _people = getPeople(SetOption.Set1); 

      listViewPeople.ItemsSource = _people; // Only one time of assigning 
     } 

     public void ButtonSet1OnClicked(object sender, EventArgs e) 
     { 
      _people = getPeople(SetOption.Set1); // *** do change the ObservableCollection<Person> but ListView is not updated in UI. 
     } 

     public void ButtonSet2OnClicked(object sender, EventArgs e) 
     { 
      _people = getPeople(SetOption.Set2); // *** do change the ObservableCollection<Person> but ListView is not updated in UI. 
     } 

     private ObservableCollection<Person> getPeople(SetOption op) 
     { 
      var list = new ObservableCollection<Person>(); 

      var p1 = new Person { Id = 1, Age = 19, FirstName = "Anna", LastName = "Larson" }; 
      var p2 = new Person { Id = 2, Age = 23, FirstName = "Beri", LastName = "Slovik" }; 
      var p3 = new Person { Id = 3, Age = 65, FirstName = "Ron", LastName = "Prelosi" }; 
      var p4 = new Person { Id = 4, Age = 32, FirstName = "William", LastName = "Maxel" }; 
      var p5 = new Person { Id = 5, Age = 71, FirstName = "Fred", LastName = "Lipez" }; 
      var p6 = new Person { Id = 6, Age = 44, FirstName = "Dave", LastName = "Vanoviz" }; 

      switch (op) 
      { 
       case SetOption.Set1: 
        list.Add(p1); 
        list.Add(p2); 
        list.Add(p3); 
        list.Add(p4); 
        break; 
       case SetOption.Set2: 
        list.Add(p5); 
        list.Add(p6); 
        break; 
      } 

      return list; 
     } 
    } 
} 

<?xml version="1.0" encoding="utf-8" ?> 
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms" 
      xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml" 
      x:Class="Mvvm1.Mvvm1Page"> 
    <ContentPage.Padding> 
    <OnPlatform x:TypeArguments="Thickness" iOS="0,20,0,0" /> 
    </ContentPage.Padding> 


    <Grid> 
    <Grid.ColumnDefinitions> 
     <ColumnDefinition Width="*"></ColumnDefinition> 
     <ColumnDefinition Width="0"></ColumnDefinition> 
    </Grid.ColumnDefinitions> 
    <Grid.RowDefinitions> 
     <RowDefinition Height="Auto"></RowDefinition> 
     <RowDefinition Height="*"></RowDefinition> 
    </Grid.RowDefinitions> 

    <StackLayout VerticalOptions="StartAndExpand" Orientation="Horizontal" Grid.Row="0" Grid.Column="0"> 
     <Button Text="Set 1" Clicked="ButtonSet1OnClicked"></Button> 
     <Button Text="Set 2" Clicked="ButtonSet2OnClicked"></Button> 
    </StackLayout> 

    <ListView x:Name="listViewPeople" HasUnevenRows="True" Grid.Column="0" Grid.Row="1" Header="People"> 

     <ListView.ItemTemplate> 
     <DataTemplate> 
      <ViewCell> 
      <ContentView Padding="5"> 
       <Frame OutlineColor="Accent" Padding="10"> 
       <StackLayout Orientation="Horizontal"> 
        <StackLayout> 
        <Label Text="{Binding Id}"></Label> 
        <Label Text="{Binding FirstName}"></Label> 
        <Label Text="{Binding LastName}"></Label> 
        <Label Text="{Binding Age}"></Label> 
        </StackLayout> 
       </StackLayout> 
       </Frame> 
      </ContentView> 
      </ViewCell> 
     </DataTemplate> 
     </ListView.ItemTemplate> 
    </ListView> 
    </Grid> 
</ContentPage> 

using System.Text; 
using System.Threading.Tasks; 

namespace Mvvm1 
{ 
    public class Person 
    { 
     public int Id { get; set; } 
     public string FirstName { get; set; } 
     public string LastName { get; set; } 

     public int Age { get; set; } 
    } 

    public enum SetOption 
    { 
     Set1 = 1, 
     Set2 = 2 
    } 
} 

Answer 1

在按鈕單擊處理程序調用

_people = getPeople(SetOption.Set1); 

不不修改現有的集合實例，而是創建一個新的，並在listViewPeople.ItemsSource不會因此不變化的項目。

每次創建新集合時都必須設置ItemsSource屬性。然後，你就不需要在本地_people變量都：

public Mvvm1Page() 
{ 
    InitializeComponent(); 
    listViewPeople.ItemsSource = getPeople(SetOption.Set1); 
} 

public void ButtonSet1OnClicked(object sender, EventArgs e) 
{ 
    listViewPeople.ItemsSource = getPeople(SetOption.Set1); 
} 

public void ButtonSet2OnClicked(object sender, EventArgs e) 
{ 
    listViewPeople.ItemsSource = getPeople(SetOption.Set2); 
} 

或者，你可以一次分配的ItemsSource，隨後添加和刪除項目/從現有集合，例如像_people.Add(new Person { ... });

---

除此之外，listView.ItemsSource = list;是沒有約束力的，但只是一個普通的任務。