xamarin Code Sharing Between Projects The Bridge Pattern

Example

The Bridge pattern is one of the most basic Inversion of Control design patterns. For Xamarin, this pattern is used to reference platform-dependent code from a platform-independent context. For example: using Android's AlertDialog from a Portable Class Library or Xamarin Forms. Neither of those contexts knows what an AlertDialog object is, so you must wrap it in a box for them to use.

// Define a common interface for the behavior you want in your common project (Forms/Other PCL)
public interface IPlatformReporter
{
    string GetPlatform();
}


// In Android/iOS/Win implement the interface on a class
public class DroidReporter : IPlatformReporter
{
    public string GetPlatform()
    {
        return "Android";
    }
}


public class IosReporter : IPlatformReporter
{
    public string GetPlatform()
    {
        return "iOS";
    }
}


// In your common project (Forms/Other PCL), create a common class to wrap the native implementations
public class PlatformReporter : IPlatformReporter
{
    // A function to get your native implemenation
    public static func<IPlatformReporter> GetReporter;

    // Your native implementation
    private IPlatformReporter _reporter;

    // Constructor accepts native class and stores it
    public PlatformReporter(IPlatformReporter reporter)
    {
        _reporter = GetReporter();
    }

    // Implement interface behavior by deferring to native class
    public string GetPlatform()
    {
        return _reporter.GetPlatform();
    }
}


// In your native code (probably MainActivity/AppDelegate), you just supply a function that returns your native implementation
public class MainActivity : Activity
{
    protected override void OnCreate(Bundle bundle)
    {
        base.OnCreate(bundle);
        SetContentView(Resource.Layout.activity_main);

        PlatformReporter.GetReporter = () => { return new DroidReporter(); };
    }
}


public partial class AppDelegate : UIApplicationDelegate
{
    UIWindow window;

    public override bool FinishedLaunching(UIApplication app, NSDictionary options)
    {
        window = new UIWindow(UIScreen.MainScreen.Bounds);
        window.RootViewController = new UIViewController();
        window.MakeKeyAndVisible();

        PlatformReporter.GetReporter = () => { return new IosReporter(); };

        return true;
    }
}


// When you want to use your native implementation in your common code, just do as follows:
public void SomeFuncWhoCares()
{
    // Some code here...

    var reporter = new PlatformReporter();
    string platform = reporter.GetPlatform();

    // Some more code here...
}