Core

The core library is intended to provide a set of common use utilities.

Background iOS HttpClient requests

Have you ever noticed that when the user backgrounds the app on iOS, the app is suspended, and the network requests fails due to The network connection was lost exception?

var client = new HttpClient();
var responseTask = client.GetAsync("https://myservice.foo/my-endpoint");
// --> i.e. App goes to background because user receives a phone call
// ...
// --> After a while the user comes back to the app
using var response = await responseTask; // <-- Exception: The network connection was lost
var content = await response.Content.ReadAsStringAsync();

This is really annoying: it forces us to implement complex retry logic, especially considering that the request may have already hit the server and potentially made changes there.

To solve this issue, we provide a NSUrlBackgroundSessionHttpMessageHandler to be used in your HttpClient to allow http request to continue even when the app is in the background.

#if IOS
    var client = new HttpClient(new NSUrlBackgroundSessionHttpMessageHandler());
#else
    var client = new HttpClient();
#endif

To make this work, you need to change your AppDelegate as follows:

[Export("application:handleEventsForBackgroundURLSession:completionHandler:")]
public virtual void HandleEventsForBackgroundUrl(UIApplication application, string sessionIdentifier, Action completionHandler)
    => NSUrlBackgroundSessionHttpMessageHandler.HandleEventsForBackgroundUrl(application, sessionIdentifier, completionHandler);

If your app is in the background and the response is ready, the system will wake up the app complete the request.

using var response = await responseTask; // <-- No exception :)

So we can finally process our response:

var content = await response.Content.ReadAsStringAsync();
// do something with the content 

Make sure to close the using block to acknowledge the system that the response has been processed. This needs to be completed within a few seconds while the app is in the background, or iOS will terminate the app.

What if my app crashes (or is terminated by the system) while a request is in progress?

No worries! The system will automatically continue your request and restart your app in the background to handle the response. Problem is: given the app is not running, there’s no await to wait for the response, and the system will discard it.

To handle this use case we need to do a couple of things:

1. We need a way to identify the request that was in progress
2. We need a way to react to the lost response

To identify the request, we have to add a special header to the request:

httpRequestMessage.Headers.Add(NSUrlBackgroundSessionHttpMessageHandler.RequestIdentifierHeaderName, requestIdentifier);

We can then register a singleton service implementing the INSUrlBackgroundSessionLostMessageHandler interface to handle the lost response.

public class NSUrlBackgroundSessionLostMessageHandler : INSUrlBackgroundSessionLostMessageHandler
{
    public async Task HandleLostMessageAsync(NSUrlBackgroundResponseHandle responseHandle)
    {
        try {
            var requestIdentifier = responseHandle.RequestIdentifier;
            using var response = await responseHandle.GetResponseAsync();
            var content = await response.Content.ReadAsStringAsync();
            // store content in a safe place related to the requestIdentifier
        } catch (Exception ex) {
            // handle exception
        }
    }
}