To enable code shrinking with ProGuard, add minifyEnabled true to the appropriate build type in your build.gradle file.

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

where:

• minifyEnabled true : enable code shrinking
• The getDefaultProguardFile(‘proguard-android.txt') method gets the default ProGuard settings from the Android SDK
• The proguard-rules.pro file is where you can add custom ProGuard rules

To enable resource shrinking, set the shrinkResources property to true in your build.gradle file.

android {
    ...

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

Pay attention because resource shrinking works only in conjunction with code shrinking.

You can customize which resources to keep or discard creating an XML file like this:

<?xml version=1.0" encoding="utf-8"?>
<resources xmlns:tools="http://schemas.android.com/tools"
    tools:keep="@layout/mylayout,@layout/custom_*"
    tools:discard="@layout/unused" />

Save this file in res/raw folder.

All libraries come with resources that are not necessary useful to your application. For example Google Play Services comes with translations for languages your own application don’t even support.

You can configure the build.gradle file to specify which resource you want to keep.
For example:

defaultConfig {
    // ...

    resConfigs "en", "de", "it"
    resConfigs "nodpi", "xhdpi", "xxhdpi", "xxxhdpi"
}