Question

您好我无法弄清楚为什么这种异步操作不会等待

private async void scroller_ViewChanged(object sender, ScrollViewerViewChangedEventArgs e)
    {
        if (isLoadingMore)
            return;
        if (scroller.VerticalOffset > scrollContent.ActualHeight - (Frame.ActualHeight + 300))
            await LoadMoreItems();
    }
    bool isLoadingMore = false;
    private async Task LoadMoreItems()
    {
        isLoadingMore = true;
        try
        {
            List<Items> itemsList = await App.ServiceClient.GetItems("bananas");//<-------Doesn't wait
            foreach (Items in itemsList)
                listView.Items.Add(c);
        }
        catch { }
        isLoadingMore = false;
    }

这是另一个异步任务的后端调用

public async Task<List<Items>> GetItems(string selector)
    {
        HttpClient client = new HttpClient();
        var request = new HttpRequestMessage()
        {
            RequestUri = new Uri(myUrl),
            Method = HttpMethod.Get,
        };
        request.Headers.Add("Accept", "application/json");
        List<Items> items = null;
        var task = await client.SendAsync(request).ContinueWith(
            async (s) =>
            {
                try
                {
                    string Result = await s.Result.Content.ReadAsStringAsync();
                    if (s.Result.IsSuccessStatusCode)
                        items= JsonConvert.DeserializeObject<List<Items>>(Result);
                }
                catch
                { }
            });
        return items;
    }

问题是嵌套等待或在scoll视图中。我发现this但我不认为情况就是这样。

Answer 1

事件处理程序为async void，因此用户界面永远不会等待这一点，如果您无法处理多个ViewChanged事件，则有两个选项：

1 - 在执行异步调用时禁用滚动条：

private async void scroller_ViewChanged(object sender, ScrollViewerViewChangedEventArgs e)
{
    scroller.Enabled = false;
    try
    {
        if (isLoadingMore)
            return;
        if (scroller.VerticalOffset > scrollContent.ActualHeight - (Frame.ActualHeight + 300))
            await LoadMoreItems();
    }
    finally{ scroller.Enabled = true; }
}

如果还有另一个事件处理程序正在运行，那么不要执行代码：

bool runnning = false

private async void scroller_ViewChanged(object sender, ScrollViewerViewChangedEventArgs e)
{
    if(running)
        return;

    runnning = true;
    try
    {
        if (isLoadingMore)
            return;
        if (scroller.VerticalOffset > scrollContent.ActualHeight - (Frame.ActualHeight + 300))
            await LoadMoreItems();
    }
    finally{ running = false; }
}

C＃异步操作不等wp8.1

1 个答案: