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Skill: R8 Code Shrinking

Enable R8 for Android to shrink, optimize, and obfuscate native code.

Quick Config

// android/app/build.gradle
def enableProguardInReleaseBuilds = true

android {
    buildTypes {
        release {
            minifyEnabled true
            shrinkResources true
        }
    }
}

When to Use

  • Android app size too large
  • Want to obfuscate code for security
  • Building release APK/AAB

What is R8?

R8 replaces ProGuard in Android: - Shrinks: Removes unused code - Optimizes: Improves bytecode - Obfuscates: Renames classes/methods

Compatibility: Uses ProGuard configuration format.

Step-by-Step Instructions

1. Enable R8

Edit android/app/build.gradle:

def enableProguardInReleaseBuilds = true

This sets minifyEnabled = true for release builds.

2. Enable Resource Shrinking (Optional)

Further reduces size by removing unused resources:

android {
    buildTypes {
        release {
            minifyEnabled true
            shrinkResources true  // Requires minifyEnabled

            proguardFiles getDefaultProguardFile('proguard-android-optimize.txt'), 'proguard-rules.pro'
        }
    }
}

3. Configure ProGuard Rules (If Needed)

Edit android/app/proguard-rules.pro. React Native defaults are usually sufficient—only add rules when specific libraries break after enabling R8.

Only add if using Firebase (@react-native-firebase/*):

-keep class io.invertase.firebase.** { *; }
-dontwarn io.invertase.firebase.**

Only add if using Retrofit:

-keepattributes Signature
-keepattributes *Annotation*
-keep class retrofit2.** { *; }
-dontwarn retrofit2.**

See Common Library Rules and Troubleshooting for more examples.

4. Build and Test

cd android
./gradlew assembleRelease
# or
./gradlew bundleRelease

Critical: Test thoroughly! R8 can remove code it thinks is unused.

ProGuard Rules Reference

Rule Effect
-keep class X Don't remove class X
-keepclassmembers Keep members but allow rename
-keepnames Keep names but allow removal if unused
-dontwarn X Suppress warnings for X
-dontobfuscate Disable obfuscation

Keep Entire Package

-keep class com.mypackage.** { *; }

Keep Classes with Annotation

-keep @interface com.facebook.proguard.annotations.DoNotStrip
-keep @com.facebook.proguard.annotations.DoNotStrip class *
-keepclassmembers class * {
    @com.facebook.proguard.annotations.DoNotStrip *;
}

Disable Obfuscation (If Needed)

# proguard-rules.pro
-dontobfuscate

Use when: - Debugging crashes (stack traces more readable) - Library requires class names

Size Impact

Example from guide: - Without R8: 9.5 MB - With R8: 6.3 MB - Savings: 33%

Larger apps may see 20-30% reduction.

Troubleshooting

App Crashes After R8

Usually means needed class was removed.

Debug steps:

  1. Check crash log for class name
  2. Add keep rule: 
    -keep class com.example.CrashedClass { *; }
  3. Rebuild and test

Library Specific Rules

Many libraries provide ProGuard rules. Check: - Library README - Library's consumer-proguard-rules.pro - Stack Overflow for library + proguard

Common Library Rules

# Hermes (usually auto-included)
-keep class com.facebook.hermes.unicode.** { *; }

# React Native
-keep class com.facebook.react.** { *; }

# Gson
-keepattributes Signature
-keep class com.google.gson.** { *; }

# OkHttp
-dontwarn okhttp3.**
-dontwarn okio.**

Verification

Check APK Size

# Build
./gradlew assembleRelease

# Check size
ls -la android/app/build/outputs/apk/release/

Use Ruler for Detailed Analysis

See bundle-analyze-app.md.

Verify Obfuscation

Decompile APK to check class names are obfuscated:

# Using jadx or similar
jadx android/app/build/outputs/apk/release/app-release.apk

Common Pitfalls

  • Not testing release build: Always QA with R8 enabled
  • Missing library rules: Check library docs
  • Over-keeping: Too many keep rules negates benefits
  • Reflection: Code using reflection may break