As you can recall from Chapter 1, The Anatomy of an Android App, Android apps run within the Dalvik VM, and we now know that Mono apps run within the Mono CLR. So how does a Xamarin.Android app run? A simple answer is that it uses both the Mono CLR and the Dalvik VM. The following diagram depicts how the runtimes co-exist:
So, how do the Mono CLR and Dalvik VM work together in a Xamarin.Android app? The magic is accomplished through a concept called peer objects and a framework called the Java Native Interface (JNI).
Java Native Interface (JNI) is a framework that allows a non-Java code (such as C++ or C#) to call or be called by a Java code running inside a JVM. As you can see from the preceding diagram, JNI is a critical component in the overall Xamarin.Android architecture.
Peer objects are a pair of objects consisting of a managed object residing in the Mono CLR and a Java object residing in the Dalvik VM, which work together to perform the functions of a Xamarin.Android app.
Xamarin.Android is delivered with a set of assemblies called the Android binding libraries. Classes in the Android binding libraries correspond to the Java classes in the Android application framework, and the methods on the binding classes act as wrappers to call corresponding methods on Java classes. Binding classes are referred to as Managed Callable Wrappers (MCW). Anytime you create a C# class that inherits from one of these binding classes, a corresponding Java proxy class is generated at build time. The Java proxy contains a generated override for each overridden method in your C# class and acts as a wrapper to call the corresponding method on the C# class.
The creation of peer objects can be initiated from within the Dalvik VM by the Android application framework or from within the Mono CLR by the code you write in the overridden methods. A reference between the two peer objects is kept by each instance of a MCW and can be accessed through the Android.Runtime.IJavaObject.Handle property.
The following diagram depicts how peer objects collaborate:
In Chapter 1, The Anatomy of an Android App, we discussed Android packages (.apk files). Xamarin.Android creates the .apk files but also includes the following additional types of files:
- The C# code is stored as assemblies in the assembly folder of the archive
- The Mono runtime is packaged as native libraries