Sharing 100% of the code would be great, but it is not entirely possible; we still require some implementation from platform-specific features (for example different platform services, hardware, cameras). A way to tackle this problem is via an IoC container. Using the IoC principle, we use an abstraction for the functionality in our shared code and pass an implementation of the abstraction into our shared code. Our IoC containers handle the instantiation of an object's dependency tree. We can register objects to their inherited interfaces and allow containers to pass registered objects as their abstracted interfaces all the way down the dependency tree (all the way to PCL).

What if I needed view models to call methods to a native Bluetooth service in a PCL project?

To put it simply, we can't. Our PCL projects know nothing about Bluetooth services on the native side. We create an interface that sits in the PCL project, create a class that inherits this interface, and define the methods and access all the native features required. We then register this class to the inherited interface through our IoC container, and finally resolve this abstracted interface in our PCL project. When we call functions from this interface down in the PCL, it will be calling the registered class function definitions described on the native side:

Now back to our SpeechTalk application. Because the PCL project cannot share code from the native side text-to-speech services, we will have to use IoC to access the native-side features from our PCL. Let's start by declaring an interface for our text to speech service, creating a new folder called Services, and adding a new ITextToSpeech.cs file for the interface:

public interface ITextToSpeech 
    { 
        void Speak (string msg) 
    }