Xamarin 的新手，在我下面的代碼示例中，我試圖弄清楚如何從我的視圖模型中傳遞 Players 以在視圖中進行系結。我已經確認 GetPlayers() 獲得了正確的資料。只是不確定我需要對 PlayersViewModel 進行哪些更新以傳遞 List 播放器進行系結。提前致謝！
PlayersViewModel.cs：
namespace App.ViewModels
{
public class PlayersViewModel : BaseViewModel
{
public ICommand GetPlayersCommand { get; set; }
public PlayersViewModel()
{
Title = "Players";
GetPlayersCommand = new Command(async () => await GetPlayers());
GetPlayersCommand.Execute(null);
}
private readonly string _yearDateFormat = "yyyy";
public List<Player> Players { get; private set; } = new List<Player>();
async Task GetPlayers()
{
IsRefreshing = true;
List<Player> players = await ApiManager.GetPlayersAsync(DateTime.Now.Year.ToString(_yearDateFormat));
IsRefreshing = false;
}
}
}
玩家頁面.xaml.cs
App.Views
{
[DesignTimeVisible(false)]
public partial class PlayersPage : ContentPage
{
readonly PlayersViewModel viewModel;
public PlayersPage()
{
InitializeComponent();
BindingContext = viewModel = new PlayersViewModel();
}
}
}
玩家頁面.xaml：
<?xml version="1.0" encoding="utf-8" ?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
xmlns:d="http://xamarin.com/schemas/2014/forms/design"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" xmlns:ffimageloading="clr-namespace:FFImageLoading.Forms;assembly=FFImageLoading.Forms"
mc:Ignorable="d"
x:Class="App.Views.PlayersPage"
Title="{Binding Title}"
x:Name="BrowsePlayersPage">
<ListView x:Name="PlayersView" ItemsSource="{Binding Players}">
<ListView.ItemTemplate>
<DataTemplate>
<TextCell Text="{Binding FirstName}" />
</DataTemplate>
</ListView.ItemTemplate>
</ListView>
</ContentPage>
uj5u.com熱心網友回復：
您系結到Players從未設定為任何內容的屬性。
嘗試InitData()在建構式或任何你喜歡的地方呼叫：
public PlayersViewModel()
{
Title = "Players";
GetPlayersCommand = new Command(async () => await GetPlayers());
GetPlayersCommand.Execute(null);
InitData();
}
private async void InitData()
{
Players = await GetPlayers();
}
uj5u.com熱心網友回復：
sabsab 的回答確實顯示了錯誤的本質（OP 沒有設定Players，所以它是空的。）
但是，如果您在構建 PlayersViewModel 時嘗試執行長時間運行的操作，則可能會遇到一些微妙的問題。
使用以下兩種策略之一是最安全的：
顯示沒有播放器的頁面，然后在它們可用時添加它們。（稍加改動，可以等待一次全部添加。）
在創建 PlayersViewModel 之前執行緩慢的異步操作。將其作為引數傳遞給建構式。
解決方案1：
public partial class PlayersPage : ContentPage
{
public PlayersPage()
{
InitializeComponent();
BindingContext = PlayersViewModel.CreateDeferred();
}
}
public class PlayersViewModel : BindableObject
{
/// <summary>
/// Players starts empty. It is filled on a background thread.
/// This allows page to display quickly, though at first it lacks Players.
/// </summary>
/// <returns></returns>
public static PlayersViewModel CreateDeferred()
{
PlayersViewModel vm = new PlayersViewModel();
// Empty list for initial display.
vm.Players = new ObservableCollection<Player>();
// This version runs on Main Thread.
Device.BeginInvokeOnMainThread(async () => await vm.LoadPlayers());
// Returns before LoadPlayers is finished.
return vm;
}
public ObservableCollection<Player> Players { get; set; }
/// <summary>
/// "private". Only call this via static method.
/// </summary>
private PlayersViewModel()
{
}
/// <summary>
/// This touches Players, which is UI-visible, so only call from Main Thread.
/// aka "GetPlayers"; renamed to "Load"Players, because it does not "return" players to caller.
/// </summary>
/// <returns></returns>
async Task LoadPlayers()
{
Players.Clear();
for (int iPlayer = 0; iPlayer < 2; iPlayer )
{
// Simulate query time. Remove from production code.
await Task.Delay(2000);
Player player = new Player();
Players.Add(player);
}
}
}
解決方案 1 的替代方案，直到所有玩家都被提取后才會顯示。這使用了解決方案 2 中的“PrefetchPlayers”：
// Alternative version, that waits until all players are fetched.
async Task LoadPlayers()
{
List<Player> players = await PrefetchPlayers();
Players = new ObservableCollection<Player>(players);
OnPropertyChanged(nameof(Players));
}
解決方案2：
public partial class App : Application
{
public App()
{
InitializeComponent();
// This shows when app starts, while your data is loading.
//TODO MainPage = new MyLoadingPage();
}
protected override async void OnStart()
{
List<Player> players = await PlayersViewModel.PrefetchPlayers();
var page = new PlayersPage();
page.SetPlayers(players);
MainPage = page;
}
}
public partial class PlayersPage : ContentPage
{
public PlayersPage()
{
InitializeComponent();
// No Players added yet.
BindingContext = new PlayersViewModel();
}
public void SetPlayers(List<Player> players)
{
((PlayersViewModel)BindingContext).Players = new ObservableCollection<Player>(players);
}
}
public class PlayersViewModel : BindableObject
{
public static async Task<List<Player>> PrefetchPlayers()
{
var players = new List<Player>();
for (int iPlayer = 0; iPlayer < 2; iPlayer )
{
// Simulate query time. Remove from production code.
await Task.Delay(2000);
Player player = new Player();
players.Add(player);
}
return players;
}
public ObservableCollection<Player> Players { get; set; }
public PlayersViewModel()
{
// Optional. Start with an empty list.
Players = new ObservableCollection<Player>();
}
}
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標籤：C＃ 沙马林 xamarin.forms 虚拟机