Debugging Xamarin app network calls on OSX with iOS Simulator

This blog is a quick tip for people trying to debug network calls in their Xamarin/Xamarin.Forms app while on OSX.

While we take it for granted to debug network calls in our web browsers these days, when it comes to a native app it’s not so apparent, especially if you’re on OSX. Note*, this post is for OSX if your using Windows this forum post might help.

In my research I couldn’t find a free debugging proxy but the old faithful Charles Debugging proxy works well.

While you think it might be as simple as opening up Charles there’s actually a few quirks.

Using Charles with iOS Simulator

Thankfully the latest version of Charles proxy can automatically register the certificate on your simulator devices. To do this open Charles and from the Help menu option select ‘Install Charles CA SSL Certificate in iOS Simulators’.

… run the app in the iOS simulator and that should work right? wrong!

If you’re using Xamarin/Xamarin.Forms and the standard System.Net.HttpClient then this code is fully managed code and will not communicate with the local OS to discover the proxy settings. Luckily we have a little saving grace in that the HttpClient allows for different underlying network implementations, in iOS the native network library is called CFNetwork library. Miguel has previously blogged about this.

Now that’s great but how do I get it working in my Xamarin.Forms app? Well considering the CFNetwork is a Native iOS binding we can’t actually reference it from our Forms PCL, we need to create a dependancy that’s sets it up for us.

It’s the standard way to create a dependancy.

1. Define a interface in our PCL

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public interface IHttpClientHandlerFactory
{
    HttpClientHandler GetHttpClientHandler();
}

public interface IHttpClientHandlerFactory
{
    HttpClientHandler GetHttpClientHandler();
}

2. Create the Native implementation of the interface

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[assembly: Xamarin.Forms.Dependency(typeof(HttpClientHandlerFactory))]
 
namespace App.iOS
{
    public class HttpClientHandlerFactory : IHttpClientHandlerFactory
    {
        public HttpClientHandler GetHttpClientHandler()
        {
            return new System.Net.Http.HttpClientHandler {
                Proxy = CoreFoundation.CFNetwork.GetDefaultProxy(),
                UseProxy = true
            };
        }
    }
}

[assembly: Xamarin.Forms.Dependency(typeof(HttpClientHandlerFactory))]

namespace App.iOS
{
    public class HttpClientHandlerFactory : IHttpClientHandlerFactory
    {
        public HttpClientHandler GetHttpClientHandler()
        {
            return new System.Net.Http.HttpClientHandler {
                Proxy = CoreFoundation.CFNetwork.GetDefaultProxy(),
                UseProxy = true
            };
        }
    }
}

3. Use the Native implementation

Note* as I’ve only implemented this in my iOS app I’ve got logic which only uses the custom HttpClientHandler when the factory exists.

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var handlerFactory = Xamarin.Forms.DependencyService.Get<IHttpClientHandlerFactory> ();
if (handlerFactory == null) 
{
    client = new HttpClient() {
        BaseAddress = new Uri (ServiceUrl),
    };
} 
else 
{
    var handler = handlerFactory.GetHttpClientHandler ();
    client = new HttpClient(handler) {
        BaseAddress = new Uri (ServiceUrl),
    };
}  

var handlerFactory = Xamarin.Forms.DependencyService.Get<IHttpClientHandlerFactory> ();
if (handlerFactory == null) 
{
    client = new HttpClient() {
        BaseAddress = new Uri (ServiceUrl),
    };
} 
else 
{
    var handler = handlerFactory.GetHttpClientHandler ();
    client = new HttpClient(handler) {
        BaseAddress = new Uri (ServiceUrl),
    };
}  

Once you’v done that, first run Charles and then debug your app in the simulator and you should see the network calls come in.

Happy Debugging…