c# - Xamarin.iOS CollectionView - 添加新元素会导致整个列表被刷新

问题描述

我正在构建一个 xamarin.forms 聊天应用程序，并且只需要添加交付的新元素，在 Android 上我有预期的行为，但 iOS 会刷新所有导致糟糕的用户体验的 CollectionView。请查看我的代码（仅相关部分），让我知道我做错了什么，或者我错过了什么在 iOS 上也有这种行为。

我的 .xaml：

<CollectionView x:Name="ChatCollectionView" 
    SelectionMode="None" HorizontalScrollBarVisibility="Never" 
    VerticalScrollBarVisibility="Always" ItemsUpdatingScrollMode="KeepLastItemInView"
    ItemTemplate="{StaticResource MsgTemplateSelector}" Margin="5,0,5,10">
</CollectionView>

.cs 文件：


public static ReaderWriterLockSlim valuesLock = new ReaderWriterLockSlim();
void ObservableCollectionCallback(IEnumerable collection, object context, Action accessMethod, bool writeAccess)
{
    var collectionLock = (ReaderWriterLockSlim)context;
    Action enterLock = writeAccess ? new Action(collectionLock.EnterWriteLock) : new Action(collectionLock.EnterReadLock);
    Action exitLock = writeAccess ? new Action(collectionLock.ExitWriteLock) : new Action(collectionLock.ExitReadLock);

    enterLock();
    try { accessMethod(); } finally { exitLock(); }
}

ObservableCollection<Msg> ListMsg;

public Page(string userid, string chatkey, string titulo, string foto)
{
    // some code ...
    this.BindingContext = this;
    BindingBase.EnableCollectionSynchronization(ListMsg as IEnumerable, valuesLock,ObservableCollectionCallback);

    List<Msg> recipient = await Task.Run(()=> client.PostData<List<Msg>>("url/msg.php")); 
    //previous line returns a json and deserialize it to a list of "Msg"

    if (recipient != null)
    {
        ListMsg = new ObservableCollection<Msg>(recipient);
        
        ChatCollectionView.ItemsSource = ListMsg;
            
        if (ListMsg.Count > 0)
        {
            if(Device.RuntimePlatform==Device.iOS)
                await Task.Delay(2000);

            ChatCollectionView.ScrollTo(ListMsg.Count - 1, position: ScrollToPosition.End, animate: false);
        }
    }
    else
        ChatCollectionView.ItemsSource = new ObservableCollection<Msg>(); 

    MessagingCenter.Subscribe<MainPage, Msg>("Page", "scroll", async (sender, arg) =>
    {
        Task.WaitAll(tasks.ToArray());

        tasks.Add(Task.Run(async () =>
        {
            arg = await MsgPrepare(arg); //this only changes the element arg not the list
            _ = Device.InvokeOnMainThreadAsync(() =>
            {
                valuesLock.EnterWriteLock();
                try { ListMsg.Add(arg); }
                finally { valuesLock.ExitWriteLock(); }
            });
        }));
    }

}

标签： c#xamarinxamarin.formsxamarin.iosnscollectionview

c# - Xamarin.iOS CollectionView - 添加新元素会导致整个列表被刷新

问题描述

解决方案

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