xamarin.forms:Observablecollection，聊天中的列表视图:当新消息添加到顶部时，列表视图会

Question

我正在用xamarin表单聊天。我使用一个可观察的集合，通过重载适配器将项添加到该集合中。逻辑起作用，所有消息都被加载，例如，当消息被添加(到列表的顶部，而不是底部！(时，listview总是将该项显示为第一项。但这意味着，由于重新加载工作"；向后"；加载下一个项目，列表视图再次向上移动，然后加载下一项目，依此类推。当然，这意味着一旦启用第一次重新加载，它将下载所有项目并移动聊天的顶部。那个ofc太可怕了。

通常，您会将项目添加到可观察集合的末尾，这样做后，即使添加了项目，列表视图也会保持原位。但当添加到顶部时，这种行为就不会发生。或者更确切地说，这就是问题所在：添加到顶部的每个新项目都会使列表视图呈现在顶部，但由于聊天是向后的，因此总是显示列表的顶部而不是底部。

以下是我做的

private async Task ReloadChatMessages(bool scrollToBeginning)
{
if (IsBusy)
return;
isBusy = true;
//reset List, cause we only want our new messages on here
unsortedMessages = new List<ChatType>();
// keep track of download iterations, skipping the first
if (result.Count != 0)
amountOfDownloads++;
if(amountOfDownloads * Constants.NUMBEROFIADSTOLOADATONCECHAT >= totalNumberOfMessagesAvailable)
{
// end reached
listview_AllChats.ItemAppearing -= Handle_ItemAppearing;
endReached = true; 
//load last remaining messages
int amountOfRemainingMSGs = (amountOfDownloads * Constants.NUMBEROFIADSTOLOADATONCECHAT) - totalNumberOfMessagesAvailable;
//final load
unsortedMessages = await Chats.GetAllChats(uniqueChatID, 0, amountOfRemainingMSGs);

}
else // regular reload 
{
// download messages in reverse order 
unsortedMessages = await Chats.GetAllChats(uniqueChatID,
(totalNumberOfMessagesAvailable - (Constants.NUMBEROFIADSTOLOADATONCECHAT * amountOfDownloads)),
Constants.NUMBEROFIADSTOLOADATONCECHAT);
}


// since we insert on loads after second we need to reverse list first 
if (result.Count >= Constants.NUMBEROFIADSTOLOADATONCECHAT)
unsortedMessages.Reverse();
//add to all chats, so we are always up to speed with all messages
allChats.AddRange(unsortedMessages);
// format message to our visible chat type
for (int i = 0; i < unsortedMessages.Count; i++)
{
if (result.Count < Constants.NUMBEROFIADSTOLOADATONCECHAT)
result.Add(FormattedChatMessage(unsortedMessages[i]));
else
{
result.Insert(0, FormattedChatMessage(unsortedMessages[i]));
}
}

//traverse list backwards and add number to messsage
int counter = result.Count; 
for(int i = totalNumberOfMessagesAvailable; i > (totalNumberOfMessagesAvailable-result.Count); i--)
{
result[counter-1].numberOfMessage = i;
counter--;
}
isBusy = false;

// add to chatview if not already added 
if (listview_AllChats.ItemsSource == null)
listview_AllChats.ItemsSource = result;

if(amountOfDownloads < 2 || scrollToBeginning)
{
if (result.Count == 0)
return;
VisibleChatType target = new VisibleChatType();
try
{
target = result[result.Count];
}
catch
{
target = result[result.Count-1];
}
try
{
listview_AllChats.ScrollTo(target, ScrollToPosition.End, false);
}
catch
{

}

}
}

和重新加载适配器：

private async void Handle_ItemAppearing(object sender, ItemVisibilityEventArgs e)
{
try
{
var item = e.Item as VisibleChatType;
if (isBusy)
return;

//offset important 
if (item.numberOfMessage  == totalNumberOfMessagesAvailable + 1 - (result.Count))
{
//LoadMore
await ReloadChatMessages(false);
}
}
catch { }

}

你们有没有可能知道我是怎么解决的？

Answer 1

在重新加载后将其添加到：

listview_AllChats.ScrollTo(item，ScrollToPosition.Start，false(；

修好了！