App.OnResume error in Xamarin forms on Android and IOS devices - xamarin

Question

We are using xamarin forms. After an Android or IOS device resumes from background, we are making a REST call in .net that is being triggered by a timer. The first attempt on IOS returns a "The Descriptor is not a socket" error and the Android returns a "Connection refused" error. The same code works fine in Windows. Future attempts (every few seconds) in all 3 platforms work fine. Has anyone seen this and have a fix?
Code
//app on resume event
protected async override void OnResume()
{
// Handle when your app resumes
if (MainPage is RootPage)
{
RootPage mainPage = MainPage as RootPage;
if (mainPage.Detail is NavigationPage)
{
NavigationPage nvPage = mainPage.Detail as NavigationPage;
if(nvPage.CurrentPage is ThingsPage)
{
ThingsPage thPage = nvPage.CurrentPage as ThingsPage;
thPage.TurnOnTimer();
}
}
}
}
//code on the page
public void TurnOnTimer()
{
if (viewModel != null)
{
viewModel.ContinueTimer = true;
viewModel.StartAnotherTimer();
}
}
//code in view model
public async void StartAnotherTimer()
{
while (ContinueTimer)
{
try
{
DevicesUpdate devicesUpdate = await DataSource.GetDevices(LocationID, ControllerID, lastDevicesUpdateReceivedAt);
}
catch (Exception ex)
{
}
// Update the UI (because of async/await magic, this is still in the UI thread!)
if (ContinueTimer)
{
await Task.Delay(TimeSpan.FromSeconds(3));
}
}
}
public static async Task<DevicesUpdate> GetDevices(Guid locationID, Guid controllerID, DateTime lastUpdateReceivedAt)
{
DevicesUpdate devicesUpdate = await GetLastUpdatedDevices(controllerID, lastUpdateReceivedAt);
}
//code in view model
public static async Task<DevicesUpdate> GetLastUpdatedDevices(Guid controllerID,
DateTime lastUpdate)
{
System.Net.Http.HttpClient client = new System.Net.Http.HttpClient();
string url = string.Format("http://appname.azurewebsites.net/api/devices?controllerid={1}&lastUpdate={2}"
, Constants.WebServerURL, controllerID, lastUpdate);
System.Net.Http.HttpResponseMessage response = await client.GetAsync(new Uri(url));
string result = await response.Content.ReadAsStringAsync();
if (response.IsSuccessStatusCode)
{
DevicesUpdate devices = JSONHelper.Deserialize<DevicesUpdate>(result);
return devices;
}
else
{
if (response.ReasonPhrase == "UserException")
{
throw new UserException(result);
}
else
{
//throw error because the response from rest api is not a success
throw new System.Net.Http.HttpRequestException(result);
}
}
}

Answer 1

You might have a few things happening here that's causing problems.
GetDevices doesn't return anything. (I hope you just left out the return for brevity sake)
You are never setting ContinueTimer to false.
What iOS version are you on? In later versions, you HAVE to use HTTPS or explicitly allow non-secure connections. This shouldn't be a problem because Azure has ssl.
If you plan on running this in the background, you need to register your app as a background process.
If you don't plan on running this in the background, you might have issues with previous attempts being ran (or still trying to execute, or just have failed) and then calling more.
What is the reason for calling the 3 second timer for the network calls? What if the call takes more than 3 seconds (then you are making duplicate calls even though the first might succeed).
If you want to make your network calls more robust, check out this Blog Post by Rob Gibbons about resilient network calls.
First thing I would do is remove it from the timer because it seems like the underlying sockets are having issues cross-thread.