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About a year we made a tutorial on using RxJava and Retrofit in Android.

Android working with RxJava 2 and Retrofit

Today we are going to make a similar tutorial in Kotlin. We will be parsing JSON data from URL and display it in a RecyclerView using Retrofit and RxJava. We parse the JSON array from the URL.

https://learn2crack-json.herokuapp.com/api/android

Creating Project

Here I have created an Android Studio project with package com.learn2crack.retrofitkotlin also Activity as MainActivity and layout as activity_main. Don’t forget to check Include Kotlin support option while creating a new project.

Adding Dependencies

The dependencies we require are,

implementation 'com.android.support:recyclerview-v7:26.1.0' 
implementation 'com.android.support:cardview-v7:26.1.0'
implementation 'com.android.support.constraint:constraint-layout:1.0.2'

implementation 'com.squareup.retrofit2:retrofit:2.3.0'
implementation 'com.squareup.retrofit2:converter-gson:2.3.0' 
implementation 'com.squareup.retrofit2:adapter-rxjava2:2.3.0' 
implementation 'io.reactivex.rxjava2:rxjava:2.1.6'
implementation 'io.reactivex.rxjava2:rxandroid:2.0.1'

We have added the dependencies for RxJava, Retrofit, Card View, Recycler View and ConstraintLayout.

build.gradle

apply plugin: 'com.android.application'

apply plugin: 'kotlin-android'

apply plugin: 'kotlin-android-extensions'

android {
    compileSdkVersion 26
    defaultConfig {
        applicationId "com.learn2crack.retrofitkotlin"
        minSdkVersion 21
        targetSdkVersion 26
        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-jre7:$kotlin_version"
    implementation 'com.android.support:appcompat-v7:26.1.0'
    implementation 'com.android.support:recyclerview-v7:26.1.0'
    implementation 'com.android.support:cardview-v7:26.1.0'
    implementation 'com.android.support.constraint:constraint-layout:1.0.2'

    implementation 'com.squareup.retrofit2:retrofit:2.3.0'
    implementation 'com.squareup.retrofit2:converter-gson:2.3.0'
    implementation 'com.squareup.retrofit2:adapter-rxjava2:2.3.0'
    implementation 'io.reactivex.rxjava2:rxjava:2.1.6'
    implementation 'io.reactivex.rxjava2:rxandroid:2.0.1'

    testImplementation 'junit:junit:4.12'
    androidTestImplementation 'com.android.support.test:runner:1.0.1'
    androidTestImplementation 'com.android.support.test.espresso:espresso-core:3.0.1'
}

Adding Permissions to Manifest

We need permission to access the Internet.

<uses-permission android:name="android.permission.INTERNET"/>

Creating Layout

Our main layout has only one RecyclerView widget.

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context="com.learn2crack.retrofitkotlin.MainActivity">

    <android.support.v7.widget.RecyclerView
        android:id="@+id/rv_android_list"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

</android.support.constraint.ConstraintLayout>

The next layout is for RecylerView list item. It has three TextView widgets to display Android version name, version number, and API level.

recycler_view_row.xml

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:layout_marginBottom="1dp"
    android:foreground="?android:attr/selectableItemBackground">

    <TextView
        android:id="@+id/tv_name"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:layout_marginEnd="8dp"
        android:layout_marginStart="8dp"
        android:layout_marginTop="8dp"
        android:textColor="@android:color/white"
        android:textSize="24sp"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <TextView
        android:id="@+id/tv_version"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:layout_marginEnd="8dp"
        android:layout_marginStart="8dp"
        android:layout_marginTop="8dp"
        android:textColor="@android:color/white"
        android:textSize="18sp"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/tv_name" />

    <TextView
        android:id="@+id/tv_api_level"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:layout_marginBottom="8dp"
        android:layout_marginEnd="8dp"
        android:layout_marginStart="8dp"
        android:layout_marginTop="8dp"
        android:textColor="@android:color/white"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/tv_version" />
</android.support.constraint.ConstraintLayout>

Creating Model class

We create Kotlin data class to store Android version name, version number, and API level.

Android.kt

package com.learn2crack.retrofitkotlin.model

data class Android(val name : String, val version : String, val apiLevel : String)

A single line of definition is enough. We need to add data before the class definition. That’s all, It takes care of everything required. This Android class is created in the model package.

Creating Retrofit Interface

The getData() method is defined to fetch data and an Observable is returned. The endpoint is defined as “api/android” with GET HTTP method. This Retrofit Interface is created in the network package.

RequestInterface.kt

package com.learn2crack.retrofitkotlin.network

import com.learn2crack.retrofitkotlin.model.Android
import io.reactivex.Observable
import retrofit2.http.GET

interface RequestInterface {

    @GET("api/android")
    fun getData() : Observable<List<Android>>
}

Creating RecyclerView Adapter

We pass the data as an ArrayList in constructor along with a Listener which we use to track RecyclerView item clicks. We also defined an array of hexadecimal colors which we set as a background in RecyclerView Item. We have created a ViewHolder class which takes care of binding of data in RecyclerView Item Views. Setting text to TextView is as simple as,

itemView.tv_name.text = android.name

where tv_name is the Id of the TextView and it is set using text property. This Adapter kotlin class is created in the adapter package.

DataAdapter.kt

package com.learn2crack.retrofitkotlin.adapter

import android.support.v7.widget.RecyclerView
import android.view.LayoutInflater
import android.view.View
import android.view.ViewGroup
import android.graphics.Color

import com.learn2crack.retrofitkotlin.R
import com.learn2crack.retrofitkotlin.model.Android

import kotlinx.android.synthetic.main.recycler_view_row.view.*

class DataAdapter (private val dataList : ArrayList<Android>, private val listener : Listener) : RecyclerView.Adapter<DataAdapter.ViewHolder>() {

    interface Listener {

        fun onItemClick(android : Android)
    }

    private val colors : Array<String> = arrayOf("#EF5350", "#EC407A", "#AB47BC", "#7E57C2", "#5C6BC0", "#42A5F5")

    override fun onBindViewHolder(holder: ViewHolder, position: Int) {

        holder.bind(dataList[position], listener, colors, position)
    }

    override fun getItemCount(): Int = dataList.count()

    override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): ViewHolder {

        val view = LayoutInflater.from(parent.context).inflate(R.layout.recycler_view_row, parent, false)

        return ViewHolder(view)
    }

    class ViewHolder(view : View) : RecyclerView.ViewHolder(view) {

        fun bind(android: Android, listener: Listener, colors : Array<String>, position: Int) {

            itemView.tv_name.text = android.name
            itemView.tv_version.text = android.version
            itemView.tv_api_level.text = android.apiLevel
            itemView.setBackgroundColor(Color.parseColor(colors[position % 6]))

            itemView.setOnClickListener{ listener.onItemClick(android) }
        }
    }
}

Creating Activity

We initialize a CompositeDisposable object and add all the RxJava disposable to it. During onDestroy() we can clear the disposables so that it prevents the memory leak.

The RecyclerView is initialised using initRecyclerView() method. The Retrofit request is defined using

val requestInterface = Retrofit.Builder()
        .baseUrl(BASE_URL)
        .addCallAdapterFactory(RxJava2CallAdapterFactory.create())
        .addConverterFactory(GsonConverterFactory.create())
        .build().create(RequestInterface::class.java)

To use RxJava Observable along with Retrofit, we need to use a new Adapter Factory implementation. It is done by using addCallAdapterFactory() method.

The network operation is carried out by,

requestInterface.getData()
        .observeOn(AndroidSchedulers.mainThread())
        .subscribeOn(Schedulers.io())
        .subscribe(this::handleResponse, this::handleError)

The above code returns a Disposable. The observeOn() and subscribeOn() defines, in which thread we need to handle the results and perform the network operation. Here we have defined the results should be delivered and handled in the main thread and the network operation should be handled in IO thread. The handleResponse() and handleError() methods are called for success and failure. A Toast is displayed when an item is clicked in the RecyclerView.

MainActivity.kt

package com.learn2crack.retrofitkotlin

import android.support.v7.app.AppCompatActivity
import android.os.Bundle
import android.support.v7.widget.LinearLayoutManager
import android.support.v7.widget.RecyclerView
import android.util.Log
import android.widget.Toast

import io.reactivex.disposables.CompositeDisposable
import io.reactivex.schedulers.Schedulers
import io.reactivex.android.schedulers.AndroidSchedulers

import retrofit2.converter.gson.GsonConverterFactory
import retrofit2.adapter.rxjava2.RxJava2CallAdapterFactory
import retrofit2.Retrofit

import com.learn2crack.retrofitkotlin.network.RequestInterface
import com.learn2crack.retrofitkotlin.adapter.DataAdapter
import com.learn2crack.retrofitkotlin.model.Android

import kotlinx.android.synthetic.main.activity_main.*

class MainActivity : AppCompatActivity(), DataAdapter.Listener {

    private val TAG = MainActivity::class.java.simpleName

    private val BASE_URL = "https://learn2crack-json.herokuapp.com"

    private var mCompositeDisposable: CompositeDisposable? = null

    private var mAndroidArrayList: ArrayList<Android>? = null

    private var mAdapter: DataAdapter? = null

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        mCompositeDisposable = CompositeDisposable()

        initRecyclerView()

        loadJSON()
    }

    private fun initRecyclerView() {

        rv_android_list.setHasFixedSize(true)
        val layoutManager : RecyclerView.LayoutManager = LinearLayoutManager(this)
        rv_android_list.layoutManager = layoutManager
    }

    private fun loadJSON() {

        val requestInterface = Retrofit.Builder()
                .baseUrl(BASE_URL)
                .addCallAdapterFactory(RxJava2CallAdapterFactory.create())
                .addConverterFactory(GsonConverterFactory.create())
                .build().create(RequestInterface::class.java)

        mCompositeDisposable?.add(requestInterface.getData()
                .observeOn(AndroidSchedulers.mainThread())
                .subscribeOn(Schedulers.io())
                .subscribe(this::handleResponse, this::handleError))

    }

    private fun handleResponse(androidList: List<Android>) {

        mAndroidArrayList = ArrayList(androidList)
        mAdapter = DataAdapter(mAndroidArrayList!!, this)

        rv_android_list.adapter = mAdapter
    }

    private fun handleError(error: Throwable) {

        Log.d(TAG, error.localizedMessage)

        Toast.makeText(this, "Error ${error.localizedMessage}", Toast.LENGTH_SHORT).show()
    }

    override fun onItemClick(android: Android) {

        Toast.makeText(this, "${android.name} Clicked !", Toast.LENGTH_SHORT).show()
    }

    override fun onDestroy() {
        super.onDestroy()
        mCompositeDisposable?.clear()
    }
}

Complete Project Files

You can download the complete project as zip or fork from our Github repository.
Download (Github)
Fork

Screenshots

retrofit kotlin
retrofit kotlin
Raj Amal

Raj Amal

Developer at Learn2Crack
Raj Amal is an Android Developer. He Loves to code and explores new technologies. He also authored a book Learning Android Google Maps
Raj Amal
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