- Qt 6.11
- Qt Jenny: Java Code Generator
- How callbacks work
- Qt 6.11.0 Reference Documentation

# 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.`