Android.OS.NetworkOnMainThreadException：在 async/await 块中

Question

我正在尝试从指向图像的 URL 获取可绘制对象。但问题是，在 android 中你不应该在主线程上执行网络任务。因此，我使用了 async/await 块，但错误仍然不断发生。这是代码：

page.Appearing += async(sender, ev3) =>
                    {
                        if ( GetToolbar == null ) return;

                        GetToolbar.Subtitle = viewModel?.SubTitle;

                        if ( viewModel == null ) return;

                        if ( viewModel.AvatarUrl.Contains("?") && !viewModel.AvatarUrl.Contains("gravatar") )
                            viewModel.AvatarUrl = viewModel.AvatarUrl
                                .Substring(0, viewModel.AvatarUrl.IndexOf("?", StringComparison.Ordinal));

                        //var stream = Context.ContentResolver.OpenInputStream(Android.Net.Uri.Parse(viewModel.AvatarUrl));

                        await SetLogo(viewModel);
 }

private async Task SetLogo(PublicRepositoryPageViewModel viewModel)
        {
            var url = new URL(viewModel.AvatarUrl);
            var connection = url.OpenConnection();
            var stream = connection.InputStream;
            var logo = await Drawable.CreateFromStreamAsync(stream, viewModel.Title + "_avatar");
            GetToolbar.Logo = logo;
        }

它恰好发生在这一行：

var stream = connection.InputStream;

如果我使用 OpenInputStream 而不是它，则会抛出 Java.IO.FileNotFoundException：没有内容提供程序：https ://avatars.githubusercontent.com/u/6516107

我已检查互联网许可。

这是 Xamarin.Forms Android 项目

回答：

await Task.Run(async() => {
                            var url = new URL(viewModel.AvatarUrl);
                            var connection = url.OpenConnection();
                            var stream = connection.InputStream;
                            var logo = await Drawable.CreateFromStreamAsync(stream, viewModel.Title + "_avatar");
                            Device.BeginInvokeOnMainThread(()=>GetToolbar.Logo = logo);
                        });

Answer 1

ATask.Run将使您离开主线程并进入默认线程池中的线程：

await Task.Run(async() => {
  var url = new URL(viewModel.AvatarUrl);
  var connection = url.OpenConnection();
  var stream = connection.InputStream;
  var logo = await Drawable.CreateFromStreamAsync(stream, viewModel.Title + "_avatar");
  GetToolbar.Logo = logo;
});