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# Incremental Install
Incremental Install is a way of building & deploying an APK that tries to
minimize the time it takes to make a change and see that change running on
device. They work best with `is_component_build=true`, and do *not* require a
rooted device.
## Building
Add the gn arg:
incremental_install = true
This causes all apks to be built as incremental except for denylisted ones.
## Running
It is not enough to `adb install` them. You must use the generated wrapper
script:
out/Debug/bin/your_apk run
out/Debug/bin/run_chrome_public_test_apk # Automatically sets --fast-local-dev
# How it Works
## Overview
The basic idea is to sideload .dex and .so files to `/data/local/tmp` rather
than bundling them in the .apk. Then, when making a change, only the changed
.dex / .so needs to be pushed to the device.
Faster Builds:
* No `final_dex` step (where all .dex files are merged into one)
* No need to rebuild .apk for code-only changes (but required for resources)
* Apks sign faster because they are smaller.
Faster Installs:
* The .apk is smaller, and so faster to verify.
* No need to run `adb install` for code-only changes.
* Only changed .so / .dex files are pushed. MD5s of existing on-device files
are cached on host computer.
Slower Initial Runs:
* The first time you run an incremental .apk, the `DexOpt` needs to run on all
.dex files. This step is normally done during `adb install`, but is done on
start-up for incremental apks.
* DexOpt results are cached, so subsequent runs are faster.
* The slowdown varies significantly based on the Android version. Android O+
has almost no visible slow-down.
Caveats:
* Isolated processes (on L+) are incompatible with incremental install. As a
work-around, isolated processes are disabled when building incremental apks.
* Android resources, assets, and `loadable_modules` are not sideloaded (they
remain in the apk), so builds & installs that modify any of these are not as
fast as those that modify only .java / .cc.
* Since files are sideloaded to `/data/local/tmp`, you need to use the wrapper
scripts to uninstall them fully. E.g.:
```shell
out/Default/bin/chrome_public_apk uninstall
```
## The Code
All incremental apks have the same classes.dex, which is built from:
//build/android/incremental_install:bootstrap_java
They also have a transformed `AndroidManifest.xml`, which overrides the the
main application class and any instrumentation classes so that they instead
point to `BootstrapApplication`. This is built by:
//build/android/incremental_install/generate_android_manifest.py
Wrapper scripts and install logic is contained in:
//build/android/incremental_install/create_install_script.py
//build/android/incremental_install/installer.py
Finally, GN logic for incremental apks is sprinkled throughout.