While building a project, I discovered the shrinkResources attribute, which removes unused resources from the APK. Here is how to configure it and what to watch out for.

Basic Configuration

shrinkResources requires minifyEnabled to be true, because resource shrinking runs after ProGuard/R8 removes unused code:

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android {
    ...
    buildTypes {
        release {
            minifyEnabled true
            shrinkResources true
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
        }
    }
}

Inspecting Results

Use --info logging to see overall reduction:

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./gradlew clean assembleRelease --info  > build-output.txt

...
:android:shrinkDebugResources
Removed unused resources: Binary resource data reduced from 2570KB to 1711KB: Removed 33%
...

View which specific files were skipped:

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$ ./gradlew clean assembleDebug --info | grep "Skipped unused resource"
Skipped unused resource res/anim/abc_fade_in.xml: 396 bytes
Skipped unused resource res/anim/abc_fade_out.xml: 396 bytes
Skipped unused resource res/anim/abc_slide_in_bottom.xml: 400 bytes

Keeping or Discarding Specific Resources

Resources referenced via reflection (common in third-party SDKs) can be mistakenly removed. Modern versions support keep.xml for fine-grained control.

Strict Mode

In res/raw/keep.xml:

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<?xml version="1.0" encoding="utf-8"?>
<resources xmlns:tools="http://schemas.android.com/tools"
    tools:shrinkMode="strict" />

Keeping Specific Resources

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<?xml version="1.0" encoding="utf-8"?>
<resources xmlns:tools="http://schemas.android.com/tools"
    tools:keep="@layout/umeng_*, @drawable/umeng_*, @anim/umeng_*" />

Forcing Removal

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<?xml version="1.0" encoding="utf-8"?>
<resources xmlns:tools="http://schemas.android.com/tools"
    tools:shrinkMode="safe"
    tools:discard="@layout/unused2" />

The build system also skips resources referenced via URLs like file:///android_res/drawable/ic_plus.png.

Language and Density Configuration

For apps that do not need multi-language support, use resConfigs to keep only specific languages or densities:

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android {
    defaultConfig {
        ...
        resConfigs "en", "zh"
    }
}

You can also restrict to specific densities like "nodpi", "hdpi".

Summary

Enable shrinkResources even if your app is relatively clean. The log output helps you understand which resources are in use and which might be incorrectly removed.

References