Hooking into Android Studio's Gradle build process

If you want to automate steps to re-build data-files or assets for an Android app every time the app gets built in Android Studio, you need to hook into the Gradle based build process. While Gradle itself has impressive documentation, it was not very clear initially how to hook into an Android app’s build process. This article shows how I managed to make it work.

Issue

I want to make an Android app which uses a WebView to display a webapp built on React Framework. Create React App is a simple way to bootstrap a react app using WebPack and BabelJS to translate JSX to plain JS and bundle the code. The Create React App scripts provides a simple live-rebuild and webserver feature for local development and makes an npm run build command available to generate an optimized build.

Rather than host this webapp on a server, I would like to make it available locally under the assets folder in the Android app to minimize latency. I would also like the webapp automatically rebuilt everytime I compile the android app and refreshed in the assets folder.

Solution

The Android Gradle Plugin unfortunately uses fancy features of the Gradle DSL to build a task based on build target configurations at runtime making it difficult to hook onto one of the :assemble or :build tasks while compiling.

However, one of the early tasks that runs during Android App compilation with Gradle is called :preBuild and allows us to reliably hook into it for running or own asset generation routines.

To do this, I appended the following at the bottom of build.gradle of the android application. I first create a Gradle Exec task which runs the npm run build command in the directory containing the webapp sources. I then create a Gradle Sync that copies the generated webapp files to the assets folder. The Sync task depends on the Exec task. Finally I make the Android build’s :preBuild task depend on our Sync task so that the assets are build and copied before the Android app is compiled.

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task buildWebApp(type:Exec) {
    workingDir '../../webapp'
    commandLine 'npm', 'run', 'build'
}

task syncWebApp(type: Sync, dependsOn:'buildWebApp') {
    from '../../webapp/build'
    into 'src/main/assets/build'
}

preBuild.dependsOn(syncWebApp)