Serialization of Kotlin using Kotlinx.Serialization

After working on a multiplatform library that collected .framework and .aar artifacts, I came to the conclusion that there are a lot of useful things already in Kotlin, but many of us never knew about them.

One of the things that you must take care when creating a multiplatform project is the libraries that you use when developing. It is best to adhere to the solutions provided by Kotlin out of the box.
So, when I got into a situation when it became necessary to serialize a JSON document that needed to be used on two platforms (iOS and Android), there were problems in compiling an iOS project. After a bit of searching, I found the Kotlinx Serializtion library .
To be frank, I never knew about this library, so this publication is more for people who, like me, did not know about this tool.

The process of setting up a project for using a plug-in is quite well described inrepositories on Github . But I will customize the project both for Android and for multi-platform use.

The only thing that needs to be done in multiplatform compilation is to add dependencies to the end of dependencies in your native Grundle file.

implementation org.jetbrains.kotlinx:kotlinx-serialization-runtime-native:0.9.1

Grundle example for multiplatform use

plugins {
    id 'kotlin-multiplatform' version '1.3.11'
    id 'kotlinx-serialization' version '1.3.10'
}
repositories {
    google()
    jcenter()
    mavenCentral()
    maven { url "https://kotlin.bintray.com/kotlinx" }
}
apply plugin: 'com.android.library'
apply plugin: 'kotlin-android-extensions'
android {
    compileSdkVersion 28
    defaultConfig {
        minSdkVersion 15
        targetSdkVersion 28
        versionCode 1
        versionName "1.0"
        testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"
    }
    buildTypes {
        release {
            minifyEnabled false
        }
    }
}
dependencies {
    implementation fileTree(dir: 'libs', include: ['*.jar'])
    implementation 'com.android.support:appcompat-v7:28.0.0'
    implementation 'com.android.support.constraint:constraint-layout:1.1.3'
    androidTestImplementation 'com.android.support.test:runner:1.0.2'
}
kotlin {
    targets {
        fromPreset(presets.android, 'android')
        // Пресет для эмулятора iPhone// Поменяйте гп presets.iosArm64 (или iosArm32) чтобы собрать библиотеку для iPhone
        fromPreset(presets.iosX64, 'ios') {
            compilations.main.outputKinds('FRAMEWORK')
        }
    }
    sourceSets {
        commonMain {
            dependencies {
                implementation 'org.jetbrains.kotlin:kotlin-stdlib-common'
                implementation 'org.jetbrains.kotlinx:kotlinx-serialization-runtime:0.9.1'
            }
        }
        commonTest {
            dependencies {
        		implementation 'org.jetbrains.kotlin:kotlin-test-common'
        		implementation 'org.jetbrains.kotlin:kotlin-test-annotations-common'
            }
        }
        androidMain {
            dependencies {
                implementation 'org.jetbrains.kotlin:kotlin-stdlib'
            }
        }
        androidTest {
            dependencies {
            }
        }
        iosMain {
            dependencies{
                implementation "org.jetbrains.kotlinx:kotlinx-serialization-runtime-native:0.9.1"
            }
        }
        iosTest {
        }
    }
}

For Android

apply plugin: 'com.android.application'
apply plugin: 'kotlin-android'
apply plugin: 'kotlin-android-extensions'
apply plugin: 'kotlinx-serialization'
android {
    compileSdkVersion 28
    defaultConfig {
        applicationId "com.example.smile.kotlinxretrosample"
        minSdkVersion 16
        targetSdkVersion 28
        versionCode 1
        versionName "1.0"
        testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"
    }
    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
        }
    }
}
dependencies {
    implementation fileTree(dir: 'libs', include: ['*.jar'])
    implementation "org.jetbrains.kotlin:kotlin-stdlib-jdk7:$kotlin_version"
    implementation "org.jetbrains.kotlinx:kotlinx-serialization-runtime:0.9.1"
    implementation 'com.android.support:appcompat-v7:28.0.0'
    implementation 'com.android.support.constraint:constraint-layout:1.1.3'
    implementation 'com.android.support:design:28.0.0'
    implementation 'com.squareup.retrofit2:retrofit:2.5.0'
    implementation 'com.squareup.okhttp3:okhttp:3.12.0'
    testImplementation 'junit:junit:4.12'
    androidTestImplementation 'com.android.support.test:runner:1.0.2'
    androidTestImplementation 'com.android.support.test.espresso:espresso-core:3.0.2'
}

Serialization

To serialize a class, simply add an annotation before it. @Serializable

import kotlinx.serialization.Serializable
@SerializableclassField{
    var length: Int = 0var hint: String = ""var required: Boolean = false
}

Serialization works with data classes

Now, let's write a small example to convert JSON to an object and back.

/*
* {
    length = 20
    hint = "example"
    required= false
}
*/funtoObject(stringValue: String): Field {
        return JSON.parse(Field.serializer(), stringValue)
    }
    funtoJson(field: Field): String {
        // Обратите внимание, что мы вызываем Serializer, который автоматически сгенерирован из нашего класса// Сразу после того, как мы добавили аннотацию @Serializerreturn JSON.stringify(Field.serializer(), field)
    }

@Transient and @Optional

Two more annotations about which are worth telling this:

• @Transient - Shows Serializer that the field needs to be ignored.
• @Optional - Serializer will not stop and will not throw an error if the field is absent, but at the same time the default value should still be set.

@Optionalvar isOptional: Boolean = false@Transientvar isTransient: Boolean = false

An example for Android using Retrofit

For those who want to use this plugin in the development for Android, Retrofit 2 has an adapter. Link to the adapter .

A bit of code:

un createRetrofit(): Retrofit {
        val contentType = MediaType.get("application/json")
        return Retrofit.Builder()
            .addConverterFactory(serializationConverterFactory(contentType, JSON))
            .baseUrl(BASE_URL)
            .client(provideOkhttpClient())
            .build()
    }

If your class already has annotations, then after sending the request your class should turn into a JSON object.

In general, serialization in Kotlin is an excellent addition to any project and makes the process of storing data in a string or a JSON object much simpler and less labor-intensive.

List of repositories

1. KotlinxRetrofit - a small working example of using serialization on Android
2. kotlinx.serialization - The main library repository

JakeWharton / retrofit2-kotlinx serialization-converter - adapter for Retrofit