How to add protobuf into flutter/kotlin android project

Question

I'm trying to add protobuf to my flutter project. Here's the tree ls flutter_project:

android  docker  ios   protobuf      Readme.md         test
assets   docs    lib   pubspec.lock  
build    gen     oboe  pubspec.yaml  

.proto files are in the protobuf folder

I followed the instructions on https://github.com/google/protobuf-gradle-plugin. Here are my build.gradle files:

flutter_project/android/build.gradle :

buildscript {
    ext.kotlin_version = '1.3.50'
    repositories {
        google()
        jcenter()
        mavenCentral()
    }

    dependencies {
        classpath 'com.android.tools.build:gradle:4.1.1'
        classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
        classpath 'com.google.protobuf:protobuf-gradle-plugin:0.8.14'
    }
}

//...

flutter_project/android/app/build.gradle :

//...

apply plugin: 'com.android.application'
apply plugin: 'com.google.protobuf'
apply plugin: 'kotlin-android'

apply from: "$flutterRoot/packages/flutter_tools/gradle/flutter.gradle"

android {
    compileSdkVersion 29

    sourceSets {
        main.java.srcDirs += 'src/main/kotlin'
        main {
            proto {
                srcDir '../../protobuf/proto_java/'
            }
        }
    }

//..

The project syncs ok, but I cannot import any of the proto files in my MainActivity.kt. For example, the protobuf file:

syntax = "proto3";
package proto_pack
enum MessageResponseCode {

}

I cannot do import proto_pack on MainActivity.kt neither typing MessageResponseCode suggests anything.

Solution

First of I don't see your configuration for the protobuf compiler, if it is not set it will search for it in the system path (Is it available ?). Usually to avoid issues I'm using the distributed version of the compiler by configuring the protobuf configuration in the app gradle build file. Also i'm using the lite runtime as it is the recommended way for Android.

dependencies {
    implementation  "com.google.protobuf:protobuf-javalite:3.10.0"
}

protobuf {
    protoc {
        artifact = "com.google.protobuf:protoc:3.10.0"
    }

    generateProtoTasks {
        all().each { task ->
            task.builtins {
                java {
                    option 'lite'
                }
            }
        }
    }
}

Try to compile your application, you should see the java generated file in your flutter_project/build/app/generated/source/proto/debug/java/proto_pack

If so your class should be available in the MainActivity.

Last point, your proto file is not syntactically correct but i assume that it is was just for the example.

Comments

[Gatonito]

where do I put the protobuf sources? Remember that they are in the root dir, not android dir

[bcordier]

You have configured the proto source directory to be 'protobuf/proto_java' in the root directory of your project (flutter_project/), so it should be where you put them.

[Gatonito]

how much of my gradle files and yours should I preserve? I'm lost

[Gatonito]

I pasted your code to mine and I got protoc: stdout: . stderr: datasets/google_message3/benchmark_message3_1.proto: File not found. after trying to compile my app. Please look, I opened a bounty

[bcordier]

For the configuration you just needed to add the block in your android/app/build.gradle file without changing your configuration. As for your error, it seems that you are trying to include datasets/google_message3/benchmark_message3_1.proto in a proto file and it is not found in the configured proto source directory or your are trying to use some file that includes datasets/google_message3/benchmark_message3_1.proto which again do not exist. Without the complete proto file it would be difficult to debug.