# Copyright (C) 2008 The Android Open Source Project


- Description -
---------------

Layoutlib_create generates a JAR library used by the Eclipse graphical layout editor
to perform layout.


- Usage -
---------

 ./layoutlib_create path/to/android.jar destination.jar


- Design Overview -
-------------------

Layoutlib_create uses the "android.jar" containing all the Java code used by Android
as generated by the Android build, right before the classes are converted to a DEX format.

The Android JAR can't be used directly in Eclipse:
- it contains references to native code (which we want to avoid in Eclipse),
- some classes need to be overridden, for example all the drawing code that is
  replaced by Java 2D calls in Eclipse.
- some of the classes that need to be changed are final and/or we need access
  to their private internal state.

Consequently this tool:
- parses the input JAR,
- modifies some of the classes directly using some bytecode manipulation,
- filters some packages and removes some that we don't want to end in the output JAR,
- injects some new classes,
- and generates a modified JAR file that is suitable for the Android plugin
  for Eclipse to perform rendering.

The ASM library is used to do the bytecode modification using its visitor pattern API.

The layoutlib_create is *NOT* generic. There is no configuration file. Instead all the
configuration is done in the main() method and the CreateInfo structure is expected to
change with the Android platform as new classes are added, changed or removed.

The resulting JAR is used by layoutlib_bridge (a.k.a. "the bridge"), also part of the
platform, that provides all the necessary missing implementation for rendering graphics
in Eclipse.



- Implementation Notes -
------------------------

The tool works in two phases:
- first analyze the input jar (AsmAnalyzer class)
- then generate the output jar (AsmGenerator class),


- Analyzer
----------

The goal of the analyzer is to create a graph of all the classes from the input JAR
with their dependencies and then only keep the ones we want.

To do that, the analyzer is created with a list of base classes to keep -- everything
that derives from these is kept. Currently the one such class is android.view.View:
since we want to render layouts, anything that is sort of the view needs to be kept.

The analyzer is also given a list of class names to keep in the output.
This is done using shell-like glob patterns that filter on the fully-qualified
class names, for example "android.*.R**" ("*" does not matches dots whilst "**" does,
and "." and "$" are interpreted as-is).
In practice we almost but not quite request the inclusion of full packages.

With this information, the analyzer parses the input zip to find all the classes.
All classes deriving from the requested bases classes are kept.
All classes which name matched the glob pattern are kept.
The analysis then finds all the dependencies of the classes that are to be kept
using an ASM visitor on the class, the field types, the method types and annotations types.
Classes that belong to the current JRE are excluded.

The output of the analyzer is a set of ASM ClassReader instances which are then
fed to the generator.


- Generator
-----------

The generator is constructed from a CreateInfo struct that acts as a config file
and lists:
- the classes to inject in the output JAR -- these classes are directly implemented
  in layoutlib_create and will be used to interface with the renderer in Eclipse.
- specific methods to override (see method stubs details below).
- specific methods to remove based on their return type.
- specific classes to rename.

Each of these are specific strategies we use to be able to modify the Android code
to fit within the Eclipse renderer. These strategies are explained beow.

The core method of the generator is transform(): it takes an input ASM ClassReader
and modifies it to produce a byte array suitable for the final JAR file.

The first step of the transformation is changing the name of the class in case
we requested the class to be renamed. This uses the RenameClassAdapter to also rename
all inner classes and references in methods and types. Note that other classes are
not transformed and keep referencing the original name.

The TransformClassAdapter is then used to process the potentially renamed class.
All protected or private classes are market as public.
All classes are made non-final.
Interfaces are left as-is.

If a method has a return type that must be erased, the whole method is skipped.
Methods are also changed from protected/private to public.
The code of the methods is then kept as-is, except for native methods which are
replaced by a stub. Methods that are to be overridden are also replaced by a stub.

Finally fields are also visited and changed from protected/private to public.


- Method stubs
--------------

As indicated above, all native and overridden methods are replaced by a stub.
We don't have the code to replace with in layoutlib_create.
Instead the StubMethodAdapter replaces the code of the method by a call to
OverrideMethod.invokeX(). When using the final JAR, the bridge can register
listeners from these overridden method calls based on the method signatures.

The listeners are currently pretty basic: we only pass the signature of the
method being called, its caller object and a flag indicating whether the
method was native. We do not currently provide the parameters. The listener
can however specify the return value of the overridden method.

An extension being worked on is to actually replace these listeners by
direct calls to a delegate class, complete with parameters.


- Strategies
------------

We currently have 4 strategies to deal with overriding the rendering code
and make it run in Eclipse. Most of these strategies are implemented hand-in-hand
by the bridge (which runs in Eclipse) and the generator.


1- Class Injection

This is the easiest: we currently inject 4 classes, namely:
- OverrideMethod and its associated MethodListener and MethodAdapter are used
  to intercept calls to some specific methods that are stubbed out and change
  their return value.
- CreateInfo class, which configured the generator. Not used yet, but could
  in theory help us track what the generator changed.


2- Overriding methods

As explained earlier, the creator doesn't have any replacement code for
methods to override. Instead it removes the original code and replaces it
by a call to a specific OveriddeMethod.invokeX(). The bridge then registers
a listener on the method signature and can provide an implementation.


3- Renaming classes

This simply changes the name of a class in its definition, as well as all its
references in internal inner classes and methods.
Calls from other classes are not modified -- they keep referencing the original
class name. This allows the bridge to literally replace an implementation.

An example will make this easier: android.graphics.Paint is the main drawing
class that we need to replace. To do so, the generator renames Paint to _original_Paint.
Later the bridge provides its own replacement version of Paint which will be used
by the rest of the Android stack. The replacement version of Paint can still use
(either by inheritance or delegation) all the original non-native code of _original_Paint
if it so desires.

Some of the Android classes are basically wrappers over native objects and since
we don't have the native code in Eclipse, we need to provide a full alternate
implementation. Sub-classing doesn't work as some native methods are static and
we don't control object creation.

This won't rename/replace the inner static methods of a given class.


4- Method erasure based on return type

This is mostly an implementation detail of the bridge: in the Paint class
mentioned above, some inner static classes are used to pass around
attributes (e.g. FontMetrics, or the Style enum) and all the original implementation
is native.

In this case we have a strategy that tells the generator that anything returning, for
example, the inner class Paint$Style in the Paint class should be discarded and the
bridge will provide its own implementation.


- References -
--------------


The JVM Specification 2nd edition:
  http://java.sun.com/docs/books/jvms/second_edition/html/VMSpecTOC.doc.html

Understanding bytecode:
  http://www.ibm.com/developerworks/ibm/library/it-haggar_bytecode/

Bytecode opcode list:
  http://en.wikipedia.org/wiki/Java_bytecode_instruction_listings

ASM user guide:
  http://download.forge.objectweb.org/asm/asm-guide.pdf


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