Porting LWUIT Part I: Basics Of The Porting Layer

This series is intends for hackers who wish to port LWUIT itself, not for developers wishing to port applications written in LWUIT. LWUIT is portable on two fronts, the first allows us to run applications developed in LWUIT in many platforms. The second allows us to adapt LWUIT to additional platforms such as CDC, Android etc...

LWUIT's portability is achieved by abstracting the underlying platform in a porting layer which consists of 3 main classes and a few optional classes: Graphics, Implementation and SystemFont. All of these classes are very system specific and require the porting to additional platforms to take them into account, additionally a port might want to either remove or adapt the classes: MediaComponent, Transition3D, M3G and Log.

Since the other classes in LWUIT make use of these classes all we need to do in order to port LWUIT is create a build.xml file which copies the existing LWUIT code base and deletes these files. We then need to compile with our version of these files that makes use of the new API, e.g. when porting to CDC we no longer have GameCanvas of which the Implementation class derives, so we derive from java.awt.Component which is the closest counterpart.

The implementation class is completely hidden within LWUIT since it contains many hidden implementation details that are subject to change. However, several of its methods are used by LWUIT itself and this allows us to ease the porting work considerably. Since the class is hidden its API isn't "documented" and so it is likely that the task of porting will change in some ways in future versions of LWUIT (e.g. as we add functionality to LWUIT the implementation class would be modified/enhanced).

To understand this better the Implementation class in the Android port is based on the android.view.View class, while the LWUIT Graphics class delegates everything to the android.graphics.Canvas class. In the Java SE port, the implementation class is a JComponent while the Graphics class delegates to the java.awt.Graphics2D class.