Refresh the layoutlib_create doc.

Change-Id: I43e92c33d824ace9edd77d90a1b36a5f69d85e7f

Author: ralf-at-android

tools/layoutlib/create/README.txt

@@ -4,68 +4,196 @@
 - Description -
 ---------------
 
-makeLayoutLib generates a library used by the Eclipse graphical layout editor
+Layoutlib_create generates a JAR library used by the Eclipse graphical layout editor
 to perform layout.
 
 
-
 - Usage -
 ---------
 
- ./makeLayoutLib path/to/android.jar destination.jar
+ ./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 goal of makeLayoutLib is to list all the classes from the input jar and create a new
-jar that only keeps certain classes and create stubs for all their dependencies.
+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
+-----------
 
-First the input jar is parsed to find all the classes defined.
+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.
 
-In the Main(), the following list of classes are hardcoded (TODO config file later):
-- keep all classes that derive from android.view.View.
-- keep all classes in the android.view and android.widget packages (sub-packages excluded).
-- keep specific classes such as android.policy.PhoneLayoutInflater.
+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.
 
-For each class to keep, their dependencies are examined using BCEL.
-A dependency is defined as a class needed to instantiate the given class that should be kept,
-directly or indirectly. So a dependency is a class that is used by the input class, that is
-defined in the input jar and that is not part of the current JRE.
+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.
 
-Dependencies are computed recursively.
+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.
 
-Once all dependencies are found, the final jar can be created.
-There are three kind of classes to write:
-- classes that are to be kept as-is. They are just dumped in the new jar unchanged.
-- classes that are to be kept yet contain native methods or fields.
-- classes that are just dependencies. We don't want to expose their implementation in the final
-  jar.
+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.
 
-The implementation of native methods and all methods of mock classes is replaced by a stub
-that throws UnsupportedOperationException.
+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.
 
-Incidentally, the access level of native and mock classes needs to be changed in order for
-native methods to be later overridden. Methods that are "final private native" must become
-non-final, non-native and at most protected. Package-default access is changed to public.
-Classes that are final are made non-final. Abstract methods are left untouched.
+Finally fields are also visited and changed from protected/private to public.
 
 
+- Method stubs
+--------------
 
-----
-20080617 Replace Class
+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.
 
-Some classes are basically wrappers over native objects.
-Subclassing doesn't work as most methods are either static or we don't
-control object creation. In this scenario the idea is to be able to
-replace classes in the final jar.
+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.
 
-Example: android.graphics.Paint would get renamed to OriginalPaint
-in the generated jar. Then in the bridge we'll introduce a replacement
-Paint class that derives from OriginalPaint.
+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.
+
 
+--
+end