react-native-config/android/dotenv.gradle

import java.util.regex.Matcher
import java.util.regex.Pattern

def getCurrentFlavor() {
    Gradle gradle = getGradle()
    String tskReqStr = gradle.getStartParameter().getTaskRequests()[0].args

    Matcher matcher = Pattern.compile("([A-Z]\\w+)").matcher( tskReqStr )
    if( matcher.find() ) {
        return matcher.group(1).toLowerCase()
    } else {
        return "";
    }
}

def readDotEnv = {
    def envFile = ".env"

    if (System.env['ENVFILE']) {
      envFile = System.env['ENVFILE'];
    } else if (project.hasProperty("envConfigFiles")) {
      def possibleFile = project.envConfigFiles.get(getCurrentFlavor())
      if (possibleFile) {
        envFile = possibleFile;
      }
    }

    def env = [:]
    println("Reading env from: $envFile")
    try {
      new File("$project.rootDir/../$envFile").eachLine { line ->
          def (key, val) = line.tokenize('=')
          if (key && val && key.substring(0, 1) != "#") {
              env.put(key, val)
          }
      }
    } catch (Exception ex) {
      println("**************************")
      println("*** Missing .env file ****")
      println("**************************")
    }
    project.ext.set("env", env)
}

readDotEnv()

android {
    defaultConfig {
        project.env.each { k, v ->
            def escaped = v.replaceAll("%","\\\\u0025")
            buildConfigField "String", k, "\"$v\""
            resValue "string", k, "\"$escaped\""
        }
    }
}