Android: Add some developer documentation to AndroidBackendRegister

A quick blurb to explain the usecase and limitations of
AndroidBackendRegister, meant for future developers.

Also documents the two public-facing functions, getInterface()
and callInterface().

Change-Id: I79f07d4a19fdb1f4a53529ab42a8663999759f85
Reviewed-by: Assam Boudjelthia <assam.boudjelthia@qt.io>

src/plugins/platforms/android/androidbackendregister.h

@@ -19,6 +19,57 @@ QT_BEGIN_NAMESPACE
 
 Q_DECLARE_LOGGING_CATEGORY(lcAndroidBackendRegister)
 
+/*
+    \internal
+
+    This class is used to [un]register QJniObjects which implement specific interfaces. These
+    objects can then be fetched or directly called by using that interface.
+
+    This is intended to decouple the Qt C++ code from the underlying Java implementation, as Qt now
+    has multiple separate usecases, each of which may have different implementations and support
+    different features.
+
+    To use this register, the interface must be declared as a JNI class via Q_DECLARE_JNI_CLASS:
+
+    Q_DECLARE_JNI_CLASS(ImaginaryInterface, "org/qtproject/qt/android/ImaginaryInterface")
+
+    Where ImaginaryInterface is a Java interface like this:
+
+    @UsedFromNativeCode
+    interface ImaginaryInterface {
+        void doSomething(int imaginary, int imaginary2);
+    }
+
+    After that, the features provided by that interface can be used in the C++ code in two ways:
+
+    Use the convenience method callInterface() to call a method directly:
+
+    AndroidBackendRegister *reg = QtAndroid::backendRegister();
+    int imaginary, imaginary2;
+    reg->callInterface<QtJniTypes::ImaginaryInterface, void>("doSomething", imaginary, imaginary2);
+
+    Or get the QJniObject directly and use it as you would any other QJniObject:
+    AndroidBackendRegister *reg = QtAndroid::backendRegister();
+    auto imaginary = reg->getInterface<QtJniTypes::ImaginaryInterface>();
+    // ... do whatever with QJniObject
+
+    In order to register a new interface on the Java side, the BackendRegister class must be used,
+    with its native functions registerBackend() and unregisterBackend():
+
+    BackendRegister.registerBackend(ImaginaryInterface.class, imaginaryInterfaceObject);
+
+    and
+
+    BackendRegister.unregisterBackend(ImaginaryInterface.class);
+
+    Note that only one object can be registered for each interface. If multiple objects are
+    registered, only the latest one is kept. Thus, you only need to declare the interface you want
+    to unregister, not the object that implements the interface as well.
+
+    If the interface needs to be available as soon as possible, it should be registered immediately
+    after Qt has started, by using the QtNative app state listener functionality.
+*/
+
 template <typename T>
 using ValidInterfaceType = std::enable_if_t<std::is_base_of_v<QtJniTypes::JObjectBase, T>, bool>;
 
@@ -27,6 +78,13 @@ class AndroidBackendRegister
 public:
     static bool registerNatives();
 
+    /*
+        \internal
+
+        Returns a QJniObject which is registered for the given interface.
+        Requires the type of the requested interface to be registered via
+        Q_DECLARE_JNI_CLASS. (see ValidInterfaceType).
+    */
     template <typename T, ValidInterfaceType<T> = true>
     [[nodiscard]] T getInterface()
     {
@@ -39,6 +97,14 @@ public:
             typename std::disjunction<std::is_base_of<QJniObject, Object>,
                                       std::is_base_of<QtJniTypes::JObjectBase, Object>>;
 
+    /*
+        \internal
+
+        Convenience function that calls getInterface<Interface>() and then QJniObject::callMethod()
+        on the resulting object, forwarding the rest of the parameters to that function.
+
+        If the interface is not registered, a warning is printed and an empty object is returned.
+    */
     template <typename Interface, typename Ret, typename... Args,
               ValidInterfaceType<Interface> = true>
     auto callInterface(const char *func, Args... args)