Android平台Overlay机制
Android overlay 机制允许在不修改packages中apk的情况下，来自定义 framework和package中的资源文件，实现资源的定制。来达到显示不同的UI得目的（如MIUI）。
以下几类能够通过该机制定义：
(1)，Configurations (string, bool, bool-array)
(2)，Localization (string, string-array)
(3)，UI Appearance (color, drawable, layout, style, theme, animation)
(4)，Raw resources (audio, video, xml)
For detailed introduction on Android application resources, please refer to:
http://developer.android.com/guide/topics/resources/available-resources.html1 为产品添加Overlay目录
1.1 Product Overlays与Device Overlays
有两种不同的overaly目录定义，来影响最终的效果:
PRODUCT_PACKAGE_OVERLAYS: used by a particular product
DEVICE_PACKAGE_OVERLAYS: used several products that share a common device model
如果包含同一资源，那么 PRODUCT_PACKAGE_OVERLAYS 将覆盖 DEVICE_PACKAGE_OVERLAYS 中的, 这两个定义如下:
build/core/package.mk (Line: 93)
1 LOCAL_RESOURCE_DIR := /2 $(wildcard $(foreach dir, $(PRODUCT_PACKAGE_OVERLAYS), /3 $(addprefix $(dir)/, $(LOCAL_RESOURCE_DIR)))) /4 $(wildcard $(foreach dir, $(DEVICE_PACKAGE_OVERLAYS), /5 $(addprefix $(dir)/, $(LOCAL_RESOURCE_DIR)))) /6 $(LOCAL_RESOURCE_DIR)
PRODUCT_PACKAGE_OVERLAYS & DEVICE_PACKAGE_OVERLAYS 功能是一样的，只是优先级不一样：PRODUCT_PACKAGE_OVERLAYS 优先于 DEVICE_PACKAGE_OVERLAYS。
1.2 改变makefile来添加overlays的编译项
为了添加一个overlay目录, 需要修改产品的makefile：
(for example: device/vendor-name/device-name/product-name.mk)
添加以下几行:
PRODUCT_PACKAGE_OVERLAYS := device/vendor-name/device-name/product-name/overlay
$(PRODUCT_PACKAGE_OVERLAYS)
Or：
DEVICE_PACKAGE_OVERLAYS := device/vendor-name/device-name/common/overlay
$(DEVICE_PACKAGE_OVERLAYS)
(如: device/vendor-name/device-name/device_base.mk)中添加：
LOCAL_PATH := device/vendor-name/device-name
DEVICE_PACKAGE_OVERLAYS := $(LOCAL_PATH)/overlay
如果要定义多个overlays目录，需要用空格隔开。如果有多个目录，并且都包含同一资源的定义，那么将使用第一个定义的目录中的资源。
1.3 在overlay目录下创建资源文件
想覆盖Android系统自带package中资源文件, 那么在overlay目录下必须包含和要替换package相同的路径, 该路径是Android源码目录的相对路径.
For example, 如果我们想要替换以下目录的资源文件:
packages/apps/Settings/res/
那么在overlay目录下面必须创建一样的目录:
....../overlay目录/packages/apps/Settings/res/
然后放入想要替换的资源（必须和系统package相同路径和文件名）。
注意:
(1)，For color, bool, string, array, style/theme types, the resource values are identifed by their keys, so for these types, there is no need to put the resources in a file with the same name as in the original base package.
(2)，For layout, animation, picture drawables and raw types, the resources are indentifed by their file name, and overlay for these resources should keep the file name same as in the base packages.
2 在APK中检测资源
通过overlay改变apk资源文件并生成apk后，一般要检测生成的apk的资源是否已经改变了.
2.1 使用AAPT检测
Usage: aapt l[ist] [-v] [-a] file.{zip,jar,apk} List contents of Zip-compatible archive.
aapt d[ump] [--values] WHAT file.{apk} [asset [asset ...]]
badging Print the label and icon for the app declared in APK.
permissions Print the permissions from the APK.
resources Print the resource table from the APK.
configurations Print the configurations in the APK.
xmltree Print the compiled xmls in the given assets.
xmlstrings Print the strings of the given compiled xml assets.
For example:
1. To dump string, bool values: aapt dump resources Settings.apk
2. To dump a raw xml file: aapt dump xmltree Settings.apk res/xml/appwidget_info.xml
3. To dump the current configurations/localizations: aapt dump configurations Settings.apk
2.2 使用apktools检测 Reference:http://code.google.com/p/android-apktool/
Apktool v1.5.0.5a056e3 - a tool for reengineering Android apk files Copyright 2010 Ryszard Wi??niewski with smali v1.3.4-ibot8, and baksmali v1.3.4-ibot8.
Updated by iBotPeaches Apache License 2.0 (http://www.apache.org/licenses/LICENSE-2.0)
Usage: apktool [-q|--quiet OR -v|--verbose] COMMAND [...]
COMMANDs are:
d[ecode] [OPTS] [ ]
Decode to
OPTS:
-s, --no-src Do not decode sources.
-r, --no-res Do not decode resources.
-d, --debug Decode in debug mode. Check project page for more info.
-f, --force Force delete destination directory.
-t , --frame-tag Try to use framework files tagged by .
--keep-broken-res Use if there was an error and some resources were dropped, e.g.:"Invalid config flags detected. Dropping resources", but you want to decode them anyway, even with errors. You will have to fix them manually before building.
b[uild] [OPTS] [] [] Build an apk from already decoded application located in . It will automatically detect, whether files was changed and perform needed steps only.If you omit then current directory will be used. If you omit then /dist/ will be used.
OPTS:
-f, --force-all Skip changes detection and build all files.
-d, --debug Build in debug mode. Check project page for more info.
if|install-framework [] Install framework file to your system.
For additional info, see:https://github.com/iBotPeaches/brut.apktoolFor smali/baksmali info, see:http://code.google.com/p/smali/.
3 More on AAPT and Overlay
3.1 How overlay works
While building the package APKs, the overlay directories are passed to aapt command lines using -S options in the same order as they are defined in PRODUCT_PACKAGE_OVERLAYS and DEVICE_PACKAGE_OVERLAYS.
For example, while building the Settings APK, the following command are executed:
out/host/linux-x86/bin/aapt package -u -z /
-M packages/apps/Settings/AndroidManifest.xml /
-S device/vendor-name/device-name/product-name/overlay/packages/apps/Settings/res /
-S vendor/vendor-name/media/common/overlay/packages/apps/Settings/res -S packages/apps/Settings/res /
-I out/target/common/obj/APPS/framework-res_intermediates/package-export.apk /
--min-sdk-version 16 --target-sdk-version 16 --product default /
--version-code 16 --version-name 4.1.2-eng.xxxx.20121121.152327 /
-F out/target/product/product-name/obj/APPS/Settings_intermediates/package.apk
Note: some overlay directories that don't contain the Settings resources will be filtered first, and do not appear in the above command line.
3.2 Add extra resources in Overlay
Though not recommanded, we can add new resources in overlay directory, for example, if base package Settings doesn't define a bool value with key no_such_key, we can add it in the overlay file bool.xml like this:
... ...
true
... ...
If the add-resource line is missing, aapt tool will complain while creating the apk file: device/vendor-name/device-name/product-name/overlay/packages/apps/Settings/res/values/bools.xml:30: error: Resource at no_such_key appears in overlay but / not in the base package; use to add.
Another way to avoid the complaint is to run aapt with the option: --auto-add-overlay Automatically add resources that are only in overlays.
