Retrofit Android Example using Kotlin language

 
Retrofit is http client used for android. This post shows you how to use retrofit networking library using kotlin in your android app. You can also checkout sample app at HERE.
 


Retrofit is type-safe REST client for Android which aims to make it easier to consume RESTful web services. Retrofit automatically serializes the JSON response using a POJO(Plain Old Java Object) which must be defined in advanced for the JSON Structure. In Retrofit2 you configure which converter is used for the data serialization. Typically for JSON you use GSon, but you can add custom converters to process XML or other protocols. Retrofit uses the OkHttp library for HTTP requests.
 
If you want to use retrofit2 library with mvvm architecture pattern, You can refer  this article.


 

Add Internet Permission in the AndroidManifest.xml

Internet permission is required to call apis in android applications. You need to add required permissions in manifest file.

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



Add the below dependencies in your app level build.gradle file

dependencies {
    implementation 'com.squareup.retrofit2:retrofit:2.5.0'
    implementation 'com.squareup.retrofit2:converter-gson:2.5.0'
    implementation 'com.squareup.okhttp3:logging-interceptor:3.9.1'
    implementation 'com.android.support:recyclerview-v7:28.1.1'
    implementation 'com.android.support:cardview-v7:28.1.1'
}



Create a ApiClient class

We are using retrofit builder class to create retrofit instance with base url of api in this api client class. HttpLoggingInterceptor is used to display logs related to api call. Retrofit2 does not include convertor. So we manually add popular Gson convertor for JSON mapping.

package com.example.retrofitsample.api

import com.google.gson.Gson
import com.google.gson.GsonBuilder
import okhttp3.OkHttpClient
import okhttp3.logging.HttpLoggingInterceptor
import retrofit2.Retrofit
import retrofit2.converter.gson.GsonConverterFactory


object ApiClient {

    private const val BASE_URL: String = "https://jsonplaceholder.typicode.com/"

    private val gson : Gson by lazy {
        GsonBuilder().setLenient().create()
    }

    private val interceptor : HttpLoggingInterceptor by lazy {
        HttpLoggingInterceptor().setLevel(HttpLoggingInterceptor.Level.BODY)
    }

    private val httpClient : OkHttpClient by lazy {
        OkHttpClient.Builder().addInterceptor(interceptor).build()
    }

    private val retrofit : Retrofit by lazy {
        Retrofit.Builder()
            .baseUrl(BASE_URL)
            .client(httpClient)
            .addConverterFactory(GsonConverterFactory.create(gson))
            .build()
    }

    val apiService :  ApiService by lazy{
        retrofit.create(ApiService::class.java)
    }

}



Create an interface for different API calls

ApiService interface defines api endpoints with retrofit annotations about the various parameters and request method. We also need to specify return type for specific type response.  Otherwise we can also define return type as Call<ResponseBody>.

package com.example.retrofitsample.api

import com.example.retrofitsample.response.User
import retrofit2.Call
import retrofit2.http.GET

interface ApiService {

    @GET("users")
    fun getUsers(): Call<MutableList<User>>

}



Create a response class to parse json data

package com.example.retrofitsample.response

import com.google.gson.annotations.SerializedName
import java.io.Serializable

data class User(
    @SerializedName("id")
    val id: Int? = null,

    @SerializedName("name")
    val name: String? = null,

    @SerializedName("username")
    val userName: String? = null,

    @SerializedName("email")
    val email: String? = null,

    @SerializedName("address")
    val addressObject : Address? = null,

    @SerializedName("phone")
    val phone: String? = null,

    @SerializedName("website")
    val website: String? = null
    ) : Serializable

package com.example.retrofitsample.response

import com.google.gson.annotations.SerializedName
import java.io.Serializable

data class Address(

    @SerializedName("street")
    val street: String? = null,

    @SerializedName("suite")
    val suite: String? = null,

    @SerializedName("city")
    val city: String? = null,

    @SerializedName("zipcode")
    val zipCode: String? = null

) : Serializable



Create a layout file to display RecyclerView in Activity

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.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=".activities.MainActivity">

    <androidx.recyclerview.widget.RecyclerView
        android:id="@+id/recycler_main"
        android:layout_width="0dp"
        android:layout_height="0dp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

</androidx.constraintlayout.widget.ConstraintLayout>



Create a layout file for adapter to display list row

<?xml version="1.0" encoding="utf-8"?>
<androidx.cardview.widget.CardView xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    app:cardCornerRadius="10dp"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:layout_marginTop="8dp"
    android:layout_marginBottom="8dp"
    android:layout_marginStart="15dp"
    android:layout_marginEnd="15dp">

    <androidx.constraintlayout.widget.ConstraintLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:padding="10dp">

        <TextView
            android:id="@+id/txt_user_name"
            app:layout_constraintStart_toStartOf="parent"
            app:layout_constraintTop_toTopOf="parent"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:textSize="22sp"
            android:textStyle="bold" />

        <TextView
            android:id="@+id/txt_user_info1"
            app:layout_constraintStart_toStartOf="parent"
            app:layout_constraintTop_toBottomOf="@+id/txt_user_name"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:textSize="16sp" />

        <TextView
            android:id="@+id/txt_user_info2"
            app:layout_constraintStart_toStartOf="parent"
            app:layout_constraintTop_toBottomOf="@+id/txt_user_info1"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:textSize="16sp" />

        <TextView
            android:id="@+id/txt_user_address"
            app:layout_constraintStart_toStartOf="parent"
            app:layout_constraintTop_toBottomOf="@+id/txt_user_info2"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:textSize="16sp" />

    </androidx.constraintlayout.widget.ConstraintLayout>

</androidx.cardview.widget.CardView>



Create a adapter class to bind view and data

package com.example.retrofitsample.adapters

import android.content.Context
import android.view.LayoutInflater
import android.view.View
import android.view.ViewGroup
import android.widget.TextView
import androidx.recyclerview.widget.RecyclerView
import com.example.retrofitsample.R
import com.example.retrofitsample.response.User

class UsersAdapter(private val context: Context, private var list: MutableList<User>) : RecyclerView.Adapter<UsersAdapter.MyViewHolder>() {

    override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): MyViewHolder {
        val inflater = LayoutInflater.from(context)
        val view: View = inflater.inflate(R.layout.user_row,parent,false)
        return MyViewHolder(view)
    }

    override fun getItemCount(): Int {
        return list.size
    }

    override fun onBindViewHolder(holder: MyViewHolder, position: Int) {
        val user = list.get(position)
        holder.name?.text = user.name
        holder.info1?.text = user?.userName + " | " + user?.email
        holder.info2?.text = user?.phone + " | " + user?.website
        val addressObj = user.addressObject
        holder.address?.text = addressObj?.suite + "," + addressObj?.street + "," + addressObj?.city + "," + addressObj?.zipCode
    }

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

        var name: TextView? = null
        var info1: TextView? = null
        var info2: TextView? = null
        var address: TextView? = null

        init {
            name = view.findViewById(R.id.txt_user_name)
            info1 = view.findViewById(R.id.txt_user_info1)
            info2 = view.findViewById(R.id.txt_user_info2)
            address = view.findViewById(R.id.txt_user_address)
        }

    }

}

 

Create a util class to define some functions to use for API

package com.example.retrofitsample.utils

import android.R
import android.app.Activity
import android.content.Context
import android.graphics.Color
import android.net.ConnectivityManager
import android.view.Gravity
import android.view.View
import android.view.ViewGroup
import android.widget.ProgressBar
import android.widget.RelativeLayout
import android.widget.TextView
import android.widget.Toast

object Utility {

    private var progressBar: ProgressBar? = null

    fun Context.isInternetAvailable(): Boolean {
        try {
            val cm = getSystemService(Context.CONNECTIVITY_SERVICE) as ConnectivityManager
            val netInfo = cm.activeNetworkInfo
            return if (netInfo != null &amp;&amp; netInfo.isConnected) {
                true
            } else {
                showErrorToast("Internet not available. Please try again!!")
                false
            }
        } catch (e: Exception) {
            e.printStackTrace()
        }
        return false
    }

    fun Context.showErrorToast(message: String?) {

        try {
            val toast = Toast.makeText(this, message, Toast.LENGTH_LONG)

            // set message color
            val textView = toast.view.findViewById(android.R.id.message) as? TextView
            textView?.setTextColor(Color.WHITE)
            textView?.gravity = Gravity.CENTER

            // set background color
            toast.view.setBackgroundColor(Color.RED)

            toast.setGravity(Gravity.TOP or Gravity.FILL_HORIZONTAL, 0, 0)

            toast.show()
        } catch (e: Exception) {
            e.printStackTrace()
        }

    }

    // show progressbar
    fun Context.showProgressBar() {
        try {
                val layout = (this as? Activity)?.findViewById<View>(android.R.id.content)?.rootView as? ViewGroup

                progressBar = ProgressBar(this, null, R.attr.progressBarStyleLarge)
                progressBar?.let { it1 ->
                    it1.isIndeterminate = true

                    val params = RelativeLayout.LayoutParams(
                        RelativeLayout.LayoutParams.MATCH_PARENT,
                        RelativeLayout.LayoutParams.MATCH_PARENT
                    )

                    val rl = RelativeLayout(this)

                    rl.gravity = Gravity.CENTER
                    rl.addView(it1)

                    layout?.addView(rl, params)

                    it1.visibility = View.VISIBLE
                }
        } catch (e: Exception) {
            e.printStackTrace()
        }
    }

    // hide progressbar
    fun hideProgressBar() {
        try {
            progressBar?.let {
                it.visibility = View.GONE
            }
        } catch (e: Exception) {
            e.printStackTrace()
        }
    }

}



Define a method to call API in Activity

package com.example.retrofitsample.activities

import androidx.appcompat.app.AppCompatActivity
import android.os.Bundle
import android.util.Log
import androidx.recyclerview.widget.LinearLayoutManager
import com.example.retrofitsample.R
import com.example.retrofitsample.adapters.UsersAdapter
import com.example.retrofitsample.api.ApiClient
import com.example.retrofitsample.response.User
import com.example.retrofitsample.utils.Utility.hideProgressBar
import com.example.retrofitsample.utils.Utility.isInternetAvailable
import com.example.retrofitsample.utils.Utility.showProgressBar
import kotlinx.android.synthetic.main.activity_main.*
import retrofit2.Call
import retrofit2.Callback
import retrofit2.Response

class MainActivity : AppCompatActivity() {

    private var listUsers: MutableList<User> = mutableListOf<User>()
    private var adapter: UsersAdapter? = null

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

        listUsers = mutableListOf()

        recycler_main.layoutManager = LinearLayoutManager(this@MainActivity)
        adapter = UsersAdapter(
            this,
            listUsers
        )
        recycler_main.adapter = adapter

        if (isInternetAvailable()) {
            getUsersData()
        }

    }

    private fun getUsersData() {

        showProgressBar()

        ApiClient.apiService.getUsers().enqueue(object : Callback<MutableList<User>> {
            override fun onFailure(call: Call<MutableList<User>>, t: Throwable) {
                hideProgressBar()
                Log.e("error", t.localizedMessage)
            }

            override fun onResponse(
                call: Call<MutableList<User>>,
                response: Response<MutableList<User>>
            ) {
                hideProgressBar()
                val usersResponse = response.body()
                listUsers.clear()
                usersResponse?.let { listUsers.addAll(it) }
                adapter?.notifyDataSetChanged()
            }

        })

    }

}




 
retrofit



3 thoughts on “Retrofit Android Example using Kotlin language

Leave a Reply