How callbacks work

Introduction

There are various situations where you need to implement a callback when using a Java API. Practical examples of these are callbacks that listen to content changes via a ContentObserver, and callbacks that implement Java interfaces such as Handler.Callback.

QtJenny allows implementing such callbacks both in the case of implementing a Java interface, and extending a Java class, depending on what the Java API uses. Many of the cross-platform Java APIs that are also provided by Android use callbacks that are interfaces, but Android also uses abstract base classes in some situations, a prominent example being ContentObserver.

Implementing Java interfaces

The following diagram illustrates the structure and logical division between different parts of implementing Java interfaces in C++.

The way it works is that we use a java.reflect.Proxy, which wraps a Java interface, and dispatches calls of the interface methods to the generic invoke() method of an InvocationHandler.

The InvocationHandler interface is implemented by a project-specific NativeInvocationHandler (provided by your application), and that implementation calls a native generic invocation method that is also project-supplied. The native method calls a generic qt_invoke() virtual function in a generated C++ class that mimics the Java interface.

Finally, the user code in the Qt C++ project inherits that generated class, and overrides the virtual functions in it.

Extending Java classes

The following diagram illustrates the structure and logical division between different parts of extending Java classes in C++.

The way it works is that we generate a Java subclass, which then invokes a generic invoke() method of a NativeInvocationHandler], which is a project-supplied Java class. The \c {NativeInvocationHandler} calls a native generic invocation method that is also project-supplied. The native method calls a generic \c {qt_invoke()} virtual function in a generated C++ class that mimics the Java class. Finally, the user code in the Qt C++ project inherits that generated class, and overrides the virtual functions in it.