This document summarizes Android resource compilation and the overlay mechanism. It discusses: 1. How the aapt tool compiles resources into a Resources.arsc file which stores resource IDs, and generates R.java with resource ID constants. 2. The structure of compiled binary XML files and Resources.arsc. 3. The static and dynamic overlay mechanisms. Static overlay places additional resources in overlay directories, while dynamic overlay uses idmap files to map resource IDs at runtime. 4. Recommends using static overlay for stability, avoiding new compilation issues when rebasing AOSP. It suggests always adding new resources to the overlay rather than AOSP.

1. Android resource and
overlay study

2. Agenda
• Compile resource.
• Init resource.
• Load resource.
• Overlay mechanism.
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3. What is resource in Android
Android resources makes it easy to update various
characteristics of your application without modifying
code.
Resource type
Assets
Res
 Animator
 Anim
 Color
 Drawable
 Layout
 Menu
 Raw
 Values
 xml
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4. Compile resource
Aapt tool is used to compile the resource
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Res/values will be compiled into Resources.arsc

5. Compile resource
• No modification
The resource files under assets and res/raw.
The Bitmap files under drawable directory.(*.png,*.bmp,*.jpg)
• Every resource(excepts assets) will be assigned a
identical ID.
• Resources.arsc is created to store the resource
index.
• R.java is created to store the resource ID constant.
• Other xml files(includes of AndroidManifests.xml)
will be compiled into binary xml file.
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Every resource(excepts assets) will be assigned a
identical ID

6. Compile resource
• The structure of every binary xml
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Every xml will be compiled into a binary xml

7. Compile resource
• The structure of Resources.arsc
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8. Scan package
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A Idmap file will be created if it is a overlay package

9. Init resource
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Only the string pool will be loaded into java layer

10. Resource structure
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11. Load resource
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Load the best matching resource in APK or overlay APK

12. Overlay mechanism
Static overlay
Dynamic overlay
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13. Overlay mechanism - Static overlay
 Overlay resource is placed under device/intel/***
PRODUCT_PACKAGE_OVERLAYS :=
DEVICE_PACKAGE_OVERLAYS :=
 The overlay directory list will be used by the aapt tool.
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14. Overlay mechanism - Dynamic overlay
 Create the pure overlay package using AAPT with the –o argument.
<overlay targetPackage="com.target.package" priority="1234"/>
 A idmap file of every overlay apk is created when it is scanned.
The idmap file is used to map the resId original package in to the resId in overlay apk.
It is stored under "/data/resource-cache/“ and named
@vendor@intel@overlay@settings@settings.apk@idmap.
( vendor/intel/overlay/settings/settings.apk)
 Load the idmap into Header in ResTable when the resource is loaded.
two variable is added in Header for idmap.
resourceIDMap
resourceIDMapSize
 Query the resource id from all packages according to the priority below.
High priority overlay apk -> low priority overlay apk -> original apk
It only can be used to replace the existing resource but
no can be used to add a new resource.
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15. Conclusion
Dynamic overlay is used to change the existing
resource without compilation.
It is not suitable for our requirement,
Don’t port the patch of Sony from Google.
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16. Conclusion
static overlay.
 How to resolve the compile issue when we do
pdk rebase.(Some new resource is added in aosp)
 Keep AOSP clean and add all new resources into overlay directory.
 Add a argument named when we use AAPT tool.
LOCAL_AAPT_FLAGS += --auto-add-overlay
 How to enable overlay directory in different
platform(ARM and emulator)
Add DEVICE_PACKAGE_OVERLAYS in the mk for platform
Add the resource into overlay dir.
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17. Reference
https://android-
review.googlesource.com/#/c/46739/
http://developer.android.com/reference/android/c
ontent/res/Resources.html
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