Xamarin.iOS 应用请求权限时来自 http 请求的 OperationCanceledException答案

【问题标题】：OperationCanceledException from http request when Xamarin.iOS app asking permissionsXamarin.iOS 应用请求权限时来自 http 请求的 OperationCanceledException
【发布时间】：2021-01-05 10:17:26
【问题描述】：

我在使用 Xamarin.iOS 应用时遇到问题。在启动时，我向服务器发送请求以进行授权。同时，应用程序会询问用户发送通知的权限。看起来应用程序进入前台并使用 OperationCanceledException 中断连接。由于这个问题，我无法验证用户。对于解决方法，我在捕获 OperationCanceledException 时提出了第二个请求。因此，有时用户会被授权两次。在这种情况下如何发送请求而不取消？

    private async Task<string> PostAndHandleHttpRequestAsync(Dictionary<string, string> content)
    {
        var cancellationTokenSource = new CancellationTokenSource();
        cancellationTokenSource.CancelAfter(15000); 
        CancellationToken token = cancellationTokenSource.Token;

        var newClient = new HttpClient();
        var contentSerialize = JsonConvert.SerializeObject(content);
        try
        {
            var postAsyncResult = await newClient.PostAsync(GlobalSettings.urlToPHP, new StringContent(contentSerialize), token);
            var responseBody = await postAsyncResult.Content.ReadAsStringAsync();
            return responseBody;
        }
        catch (OperationCanceledException c)
        {
            // onetime retry
            if (cancellationCounter == 0)
            {
                //Console.WriteLine("second");
                cancellationCounter = 1;
                var postAsyncResult = await newClient.PostAsync(GlobalSettings.urlToPHP, new StringContent(contentSerialize), token);
                var responseBody = await postAsyncResult.Content.ReadAsStringAsync();
                return responseBody;
            }
            else
            {
                cancellationCounter = 0;
                return "Cancel";
            }
            return "Cancel";
        }
        catch (Exception e)
        {
            Console.WriteLine("Error PostAndHandleHttpRequestAsync " + e.Message);
            return "Error";
        }
    }

【问题讨论】：

标签： ios xamarin push-notification httprequest httpclient

【解决方案1】：

当App从后台进入前台时，您可以再次发布请求。

在 AppDelegate.cs 中

public override void WillEnterForeground(UIApplication application)
        {
            base.WillEnterForeground(application);

            if(!UIDevice.CurrentDevice.CheckSystemVersion(13,0))
            {
                NSNotificationCenter.DefaultCenter.PostNotificationName("willActive", null);
            }          
        }

在场景委托中

[Export ("sceneWillEnterForeground:")]
public void WillEnterForeground (UIScene scene)
{
    NSNotificationCenter.DefaultCenter.PostNotificationName("willActive",null);

    // Called as the scene transitions from the background to the foreground.
    // Use this method to undo the changes made on entering the background.
}

在您的视图控制器中

    public override void ViewWillAppear(bool animated)
    {
        base.ViewWillAppear(animated);

        NSNotificationCenter.DefaultCenter.AddObserver(new NSString("willActive"),(notification)=> {
        
           if (cancellationCounter == 1)
            {
                //request again
            }
        
        });

    }


    public override void ViewWillDisappear(bool animated)
    {
        base.ViewWillDisappear(animated);

        NSNotificationCenter.DefaultCenter.RemoveObserver(this, "willActive");

    }

catch (OperationCanceledException c)
{
    cancellationCounter =1;
}

【讨论】：