REST api Call Retrofit using Callback in Kotlin

7 min readJul 11, 2022

In android many type of data storing and operations. That means Create Read Update Delete ( CRUD ). Data storing in android internal storage using like for SQLite and Room. If that data is small amout of data and its a key value pair used shared preference and Datastore. This are in connected with internal storage. But here store a limit amout of data and that not in a network based.

for example,

We have lot of data in online shopping like images of products and its category,name,price,quantity etc. That huge data stored in a Room DB. But how can access this datas in another device use as shopping app. Its not possible so we are using backend technologies. That means the backend is actually a computer in that we are store the huge amount of data and to store data using SQL or NoSQL database technologies, also contain backend programs (php, Laravel , Node Js, Node Js Express , python Django, Java hibernate etc…) for CRUD operations. This type webtechnology developers called backend developers. This backend developers provide a URL when we call that URL and returns data. This kind of URL is called API.

So,

In android this type Api called and perform action in server like CRUD operations. And manage huge amount of data.Here its very faster and user friendly application is possible. In kotlin avalable to call Api lot of dependacies. The most popular two dependancies are Volley and Retrofit. We will discuss volley in next time.

Retrofit is actually a dependacy to help call api and perform actions (CRUD ). Retrofit contains two type of api calling first one callback method. Its an old method and another one is response method.Here we are discussing about callback method.

Here we using open api

https://gorest.co.in/public/v1/users

Steps for Api call below,

1. Add Dependancies

in build.gradle,

plugins {
id ‘com.android.application’
id ‘org.jetbrains.kotlin.android’
id ‘kotlin-android-extensions’
}

android {
compileSdk 32

defaultConfig {
applicationId “com.riyaspullur.learningrestapimethods”
minSdk 21
targetSdk 32
versionCode 1
versionName “1.0”

testInstrumentationRunner “androidx.test.runner.AndroidJUnitRunner”
}

buildTypes {
release {
minifyEnabled false
proguardFiles getDefaultProguardFile(‘proguard-android-optimize.txt’), ‘proguard-rules.pro’
}
}
compileOptions {
sourceCompatibility JavaVersion.VERSION_1_8
targetCompatibility JavaVersion.VERSION_1_8
}
kotlinOptions {
jvmTarget = ‘1.8’
}
}

dependencies {

implementation ‘androidx.core:core-ktx:1.7.0’
implementation ‘androidx.appcompat:appcompat:1.4.1’
implementation ‘com.google.android.material:material:1.6.0’
implementation ‘androidx.constraintlayout:constraintlayout:2.1.3’

//Retrofit Dependancy for Process actions using api
implementation ‘com.squareup.retrofit2:retrofit:2.9.0’

//Gson convertor to fetch Json data from api and convert in to Kotlin objects
implementation ‘com.squareup.retrofit2:converter-gson:2.9.0’

//depndacy for lifeccycle viwmodel
implementation ‘androidx.lifecycle:lifecycle-extensions:2.2.0’

//logging Interceptor Depndancy used For monitoring data.
implementation ‘com.squareup.okhttp3:logging-interceptor:5.0.0-alpha.7’

testImplementation ‘junit:junit:4.13.2’
androidTestImplementation ‘androidx.test.ext:junit:1.1.3’
androidTestImplementation ‘androidx.test.espresso:espresso-core:3.4.0’
}

2.Test Api

Just call api through browser of any api testing tools like Postman and test api is it working or not if it succesfull we can see json data and its arrays and so on.

3. Copy Json Data And Generate Kotlin Data class.( generate kotlin calss is plugin download.)

4. Activate permission of Internet.

AndroidManifest.xml

<?xml version=”1.0" encoding=”utf-8"?>

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

</application>

</manifest>

5. Create layout to perform actions in api

activity_create_new_user.xml

<?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=”.CreateNewUserActivity”>

<EditText
android:id=”@+id/editTextName”
android:layout_width=”match_parent”
android:layout_height=”wrap_content”
android:layout_marginStart=”20dp”
android:layout_marginEnd=”20dp”
android:hint=”Enter Name”
android:textColor=”@color/black”
android:textSize=”20dp”

app:layout_constraintBottom_toBottomOf=”parent”
app:layout_constraintEnd_toEndOf=”parent”
app:layout_constraintHorizontal_bias=”0.0"
app:layout_constraintStart_toStartOf=”parent”
app:layout_constraintTop_toTopOf=”parent”
app:layout_constraintVertical_bias=”0.07" />

<EditText
android:id=”@+id/editTextEmail”
android:layout_width=”match_parent”
android:layout_height=”wrap_content”
android:layout_marginStart=”20dp”
android:layout_marginEnd=”20dp”
android:hint=”Enter Email”
android:textColor=”@color/black”
android:textSize=”20dp”

<Button
android:id=”@+id/createButton”
android:layout_width=”wrap_content”
android:layout_height=”wrap_content”
android:backgroundTint=”@color/black”
android:text=”create”
android:textSize=”20sp”
android:textStyle=”bold”

app:layout_constraintBottom_toBottomOf=”parent”
app:layout_constraintEnd_toEndOf=”parent”
app:layout_constraintHorizontal_bias=”0.115"
app:layout_constraintStart_toStartOf=”parent”
app:layout_constraintTop_toTopOf=”parent”
app:layout_constraintVertical_bias=”0.436" />

<Button
android:id=”@+id/deleteButton”
android:layout_width=”wrap_content”
android:layout_height=”wrap_content”
android:backgroundTint=”@color/black”
android:text=”delete”
android:textSize=”20sp”
android:textStyle=”bold”
android:visibility=”gone”

app:layout_constraintBottom_toBottomOf=”parent”
app:layout_constraintEnd_toEndOf=”parent”
app:layout_constraintHorizontal_bias=”0.851"
app:layout_constraintStart_toStartOf=”parent”
app:layout_constraintTop_toTopOf=”parent”
app:layout_constraintVertical_bias=”0.434" />

</androidx.constraintlayout.widget.ConstraintLayout>

6. Create Row card for Recycler

recycler_row_list.xml

<?xml version=”1.0" encoding=”utf-8"?>
<LinearLayout xmlns:android=”http://schemas.android.com/apk/res/android"
android:layout_width=”match_parent”
android:layout_height=”wrap_content”
android:orientation=”vertical”
android:padding=”5dp”
android:layout_margin=”4dp”
android:background=”#FFD7D7">
<TextView
android:layout_width=”match_parent”
android:background=”@color/black”
android:gravity=”center”
android:layout_height=”wrap_content”
android:textStyle=”bold”
android:textColor=”@color/white”
android:text=”name”
android:textSize=”20sp”
android:id=”@+id/textViewName”/>
<TextView
android:layout_width=”match_parent”
android:background=”@color/black”
android:gravity=”center”
android:layout_height=”wrap_content”
android:textStyle=”bold”
android:textColor=”@color/white”
android:text=”email”
android:textSize=”18sp”
android:id=”@+id/textViewEmail”/>
<TextView
android:layout_width=”match_parent”
android:background=”@color/black”
android:gravity=”center”
android:layout_height=”wrap_content”
android:textStyle=”bold”
android:textColor=”@color/white”
android:text=”status”
android:textSize=”16sp”
android:id=”@+id/textViewStatus”/>
<TextView
android:layout_width=”match_parent”
android:background=”@color/black”
android:gravity=”center”
android:layout_height=”wrap_content”
android:textStyle=”bold”
android:textColor=”@color/white”
android:text=”status”
android:textSize=”16sp”
android:id=”@+id/textViewGender”/>
<TextView
android:layout_width=”match_parent”
android:background=”@color/black”
android:gravity=”center”
android:layout_height=”wrap_content”
android:textStyle=”bold”
android:textColor=”@color/white”
android:text=”ID number”
android:textSize=”16sp”
android:id=”@+id/textViewID”/>

</LinearLayout>

7.Design activity_main

activity_main.xml

<com.google.android.material.textfield.TextInputEditText
android:id=”@+id/searchEditText”
android:shadowRadius=”@integer/material_motion_duration_long_1"
android:layout_width=”match_parent”
android:layout_height=”wrap_content”
android:layout_marginStart=”10dp”
android:layout_marginEnd=”10dp”
android:padding=”5dp”
android:textSize=”18dp”
android:background=”#F1C5EA”
android:hint=”Search User”
app:layout_constraintBottom_toBottomOf=”parent”
app:layout_constraintEnd_toEndOf=”parent”
app:layout_constraintHorizontal_bias=”0.037"
app:layout_constraintStart_toStartOf=”parent”
app:layout_constraintTop_toTopOf=”parent”
app:layout_constraintVertical_bias=”0.048" />

<androidx.recyclerview.widget.RecyclerView
android:id=”@+id/recyclerView”
android:layout_width=”match_parent”
android:layout_height=”455dp”
android:layout_margin=”10dp”
android:padding=”5dp”
android:background=”@color/white”
app:layout_constraintBottom_toBottomOf=”parent”
app:layout_constraintEnd_toEndOf=”parent”
app:layout_constraintHorizontal_bias=”0.515"
app:layout_constraintStart_toStartOf=”parent”
app:layout_constraintTop_toTopOf=”parent”
app:layout_constraintVertical_bias=”0.8" />

</androidx.constraintlayout.widget.ConstraintLayout>

8. When Generated Data calss Name ‘UserList’ so other class Auto Generate.

//data class to hold data

data class UserList( val data:List<User>)

data class User( var id:String?, var name:String?, var email:String?, var gender:String?, var status:String?)

data class UserResponse(val code:Int?, val meta:String?, val data:User?)

9. Create Instance class named ‘RetrofitInstance’

class RetrofitInstance {

var BASE_URL=”https://gorest.co.in/public/v1/"

fun getRetroInstance():Retrofit{

val logg=HttpLoggingInterceptor() logg.level=(HttpLoggingInterceptor.Level.BODY)

val client=OkHttpClient.Builder()

client.addInterceptor(logg)

return Retrofit.Builder() .baseUrl(BASE_URL) .client(client.build())

.addConverterFactory(GsonConverterFactory.create()) .build()

}

10. create Interface for call HTTP methods

interface RetroService.kt

//users is sub url

@GET(“users”)

@Headers(“Accept:application/json”,”Content-Type:application/json”)

fun getUserList():Call<UserList>

interface RetroService {

//https://gorest.co.in/public/v2/users?name=a

@GET(“users”)

@Headers(“Accept:application/json”,”Content-Type:application/json”)

fun searchUsers(@Query(“name”) searchText:String):Call<UserList>

//https://gorest.co.in/public/v2/users/3838

@GET(“users/{user_id}”)

@Headers(“Accept:application/json”,”Content-Type:application/json”)

fun getUsers(@Path(“user_id”) user_id:String):Call<UserResponse>

@POST(“users”)

@Headers(“Accept:application/json”,”Content-Type:application/json”, “Authorization: Bearer 93e0d54a7328dabcbed3f20c726427178bba5eef1ce095251dcf1d44c573a16c”)

fun createUser(@Body params:User):Call<UserResponse>

@PATCH(“users/{user_id}”)

@Headers(“Accept:application/json”,”Content-Type:application/json”, “Authorization: Bearer 93e0d54a7328dabcbed3f20c726427178bba5eef1ce095251dcf1d44c573a16c”)

fun updateUser(@Path(“user_id”) user_id:String,@Body params:User):Call<UserResponse>

@DELETE(“users/{user_id}”)

@Headers(“Accept:application/json”,”Content-Type:application/json”, “Authorization: Bearer 93e0d54a7328dabcbed3f20c726427178bba5eef1ce095251dcf1d44c573a16c”)

fun deleteUser(@Path(“user_id”) user_id:String):Call<UserResponse>

}

11. Create view model named

CreateNewUserViewModel.kt

class CreateNewUserViewModel :ViewModel(){

lateinit var createNewUserLiveData:MutableLiveData<UserResponse?>

lateinit var loadUserData:MutableLiveData<UserResponse?>

lateinit var deleteUserLiveData:MutableLiveData<UserResponse?>

init {

createNewUserLiveData= MutableLiveData()

loadUserData= MutableLiveData()

deleteUserLiveData= MutableLiveData()

}

fun getCreateNewUserObservable():MutableLiveData<UserResponse?>{

return createNewUserLiveData

}

fun getLoadUserObservable():MutableLiveData<UserResponse?>{

return loadUserData

}

fun getdeleteUserObservable():MutableLiveData<UserResponse?>{

return deleteUserLiveData

}

fun createUser(user:User){

val retroInstance=RetrofitInstance.getRetroInstance()

.create(RetroService::class.java)

val call=retroInstance.createUser(user)

call.enqueue(object:Callback<UserResponse?> {

override fun onResponse(call: Call<UserResponse?>, response:Response<UserResponse?>) {

if (response.isSuccessful){ createNewUserLiveData.postValue(response.body())

}else{

createNewUserLiveData.postValue(null)

}

override fun onFailure(call: Call<UserResponse?>, t: Throwable) {

createNewUserLiveData.postValue(null)

}

})

}

fun updateUser(user_id:String,user:User){

val retroInstance=RetrofitInstance.getRetroInstance()

.create(RetroService::class.java)

val call=retroInstance.updateUser(user_id,user)

call.enqueue(object:Callback<UserResponse?> {

override fun onResponse(call: Call<UserResponse?>, response: Response<UserResponse?>) {

if (response.isSuccessful){ createNewUserLiveData.postValue(response.body())

}else{

createNewUserLiveData.postValue(null)

}

override fun onFailure(call: Call<UserResponse?>, t: Throwable) { createNewUserLiveData.postValue(null)

}

})

}

fun deleteUser(user_id:String?){

val retroInstance=RetrofitInstance.getRetroInstance()

.create(RetroService::class.java)

val call=retroInstance.deleteUser(user_id!!)

call.enqueue(object:Callback<UserResponse?> {

override fun onResponse(call: Call<UserResponse?>, response: Response<UserResponse?>) {

if (response.isSuccessful){

deleteUserLiveData.postValue(response.body())

}else{

deleteUserLiveData.postValue(null)

}

override fun onFailure(call: Call<UserResponse?>, t: Throwable) { deleteUserLiveData.postValue(null)

}

})

}

fun getUserData(user_id:String?){

val retroInstance=RetrofitInstance.getRetroInstance()

.create(RetroService::class.java)

val call=retroInstance.getUsers(user_id!!)

call.enqueue(object:Callback<UserResponse?> {

override fun onResponse(call: Call<UserResponse?>, response: Response<UserResponse?>) {

if (response.isSuccessful){

loadUserData.postValue(response.body())

}else{

loadUserData.postValue(null)

}

override fun onFailure(call: Call<UserResponse?>, t: Throwable) { loadUserData.postValue(null)

}

})

}

12. Create kotlin class for activity_create_new_user

CreateNewUserActivity.kt

class CreateNewUserActivity : AppCompatActivity() {

lateinit var viewModel: CreateNewUserViewModel

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

var user_id=intent.getStringExtra(“user_id”)

initViewModel()

createUserObservable()

if (user_id!=null){

loadUserData(user_id)

}

var aa=1111

createButton.setOnClickListener {

aa++

user_id=”$aa”

createUser(user_id!!)

}

deleteButton.setOnClickListener {

deleteUser(user_id)

}

private fun deleteUser(user_id:String?){ viewModel.getdeleteUserObservable().observe(

this, Observer <UserResponse?>{

if (it==null){

Toast.makeText(this,”Failed to Delete”,Toast.LENGTH_LONG).show()

}else{

Toast.makeText(this,”Successfully Deleted User”,Toast.LENGTH_LONG).show()

finish()

} //Toast.makeText(this,”Failed to create”,Toast.LENGTH_LONG).show()

})

viewModel.deleteUser(user_id!!)

}

private fun loadUserData(user_id:String?){ viewModel.getLoadUserObservable().observe(

this, Observer <UserResponse?>{

if (it!=null){

editTextName.setText(it.data?.name)

editTextEmail.setText(it.data?.email) editTextGender.setText(it.data?.gender)

createButton.setText(“Update”)

deleteButton.visibility=View.VISIBLE

} //Toast.makeText(this,”Failed to create”,Toast.LENGTH_LONG).show()

})

viewModel.getUserData(user_id)

}

private fun createUser(user_id:String) {

val user=User(“”,editTextName.text.toString(),

editTextEmail.text.toString(),

editTextGender.text.toString(),”Active”

)

if (user_id==null) viewModel

.createUser(user) else

viewModel.updateUser(user_id,user)

}

private fun initViewModel() { viewModel=ViewModelProvider(this).get(

CreateNewUserViewModel::class.java )

}

private fun createUserObservable(){ viewModel.getCreateNewUserObservable().observe(

this, Observer <UserResponse?>{

if (it==null){

Toast.makeText(this,”Failed to create / Update”,Toast.LENGTH_LONG).show()

}else{

Toast.makeText(this,”Succesfully created / Updated User”,Toast.LENGTH_LONG

).show()

finish()

}

})

}

13. Create view model for main activity

MainActivityViewModel.kt

class MainActivityViewModel:ViewModel() {

lateinit var recyclerListData:MutableLiveData<UserList>

init {

recyclerListData= MutableLiveData()

}

fun getUserListObservable():MutableLiveData<UserList>{

return recyclerListData

}

fun getUserList(){

val retroInstance=RetrofitInstance.getRetroInstance().create(

RetroService::class.java

)

val call=retroInstance.getUserList()

call.enqueue(object:Callback<UserList>{

override fun onResponse(call: Call<UserList>, response: Response<UserList>) {

if (response.isSuccessful){

recyclerListData.postValue(response.body())

}else{

recyclerListData.postValue(null)

}

override fun onFailure(call: Call<UserList>, t: Throwable) { recyclerListData.postValue(null)

}

})

}

fun searchUserList(searchText:String){

val retroInstance=RetrofitInstance.getRetroInstance().create(

RetroService::class.java

)

val call=retroInstance.searchUsers(searchText)

call.enqueue(object:Callback<UserList>{

override fun onResponse(call: Call<UserList>, response: Response<UserList>) {

if (response.isSuccessful){

recyclerListData.postValue(response.body())

}else{

}

override fun onFailure(call: Call<UserList>, t: Throwable) { recyclerListData.postValue(null)

}

})

}

14. Create adapter for Recyclerview

RecyclerViewAdapter.kt

class RecyclerViewAdapter(

val clickListener:OnItemClickListener) :

RecyclerView.Adapter<RecyclerViewAdapter.MyViewHolder>() {

var userList= mutableListOf<User>()

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

val textViewName=view.textViewName val textViewEmail=view.textViewEmail

val textViewStatus=view.textViewStatus

val textViewGender=view.textViewGender

val textViewId=view.textViewID

fun bind(data:User){

textViewName.text=data.name

textViewEmail.text=data.email

textViewStatus.text=data.status

textViewGender.text=data.gender

textViewId.text=”id = ${data.id}”

}

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

val inflater=LayoutInflater.from(parent.context)

.inflate(R.layout.recycler_row_list,parent,false)

return MyViewHolder(inflater)

}

override fun onBindViewHolder(holder: MyViewHolder, position: Int) { holder.bind(userList[position])

holder.itemView.setOnClickListener { clickListener.onItemEditClick(userList[position])

}

override fun getItemCount(): Int

return userList.size

}

interface OnItemClickListener{

fun onItemEditClick(user:User)

}

15 . activity_main.xml for create Main activity class

MainActivity.kt

class MainActivity : AppCompatActivity(),RecyclerViewAdapter.OnItemClickListener {

lateinit var reclerViewAdapter:RecyclerViewAdapter

lateinit var viewModel:MainActivityViewModel

override fun onCreate(savedInstanceState: Bundle?) { super.onCreate(savedInstanceState)

setContentView(R.layout.activity_main)

initRecycleView()

initViewModel()

searchUser()

createUserButton.setOnClickListener { startActivity(Intent(this,CreateNewUserActivity::class.java))

}

private fun searchUser() {

search_button.setOnClickListener {

if (!TextUtils.isEmpty(searchEditText.toString())){ viewModel.searchUserList(searchEditText.text.toString())

}else{

viewModel.getUserList()

}

private fun initRecycleView() {

recyclerView.apply { layoutManager=LinearLayoutManager(this@MainActivity)

val decoration=DividerItemDecoration(

this@MainActivity,DividerItemDecoration.VERTICAL)

addItemDecoration(decoration) reclerViewAdapter= RecyclerViewAdapter(this@MainActivity)

adapter=reclerViewAdapter

}

fun initViewModel(){

viewModel = ViewModelProvider(this)

.get(MainActivityViewModel::class.java)

viewModel.getUserListObservable()

.observe(this, Observer<UserList>{

if (it==null){ Toast.makeText(this,”no result found…”,Toast.LENGTH_LONG).show()

}else{

reclerViewAdapter.userList=it.data.toMutableList() reclerViewAdapter.notifyDataSetChanged()

}

})

viewModel.getUserList()

}

override fun onItemEditClick(user: User) {

val intent=Intent(this,CreateNewUserActivity::class.java) intent.putExtra(“user_id”,user.id) startActivityForResult(intent,1000)

}

override fun onActivityResult(requestCode: Int, resultCode: Int, data: Intent?) {

super.onActivityResult(requestCode, resultCode, data) if (requestCode==1000){ viewModel.getUserList()

}

Callback method completed…

Just look

HTTP methods

1 GET

The GET method is used to retrieve information from the given server using a given URI. Requests using GET should only retrieve data and should have no other effect on the data.

2 HEAD

Same as GET, but transfers the status line and header section only.

3 POST

A POST request is used to send data to the server, for example, customer information, file upload, etc. using HTML forms.

4 PUT

Replaces all current representations of the target resource with the uploaded content.

5 DELETE

Removes all current representations of the target resource given by a URI.

6 CONNECT

Establishes a tunnel to the server identified by a given URI.

7 OPTIONS

Describes the communication options for the target resource.

8 TRACE

Performs a message loop-back test along the path to the target resource.

Http response codes.

Informational responses (100–199)
Successful responses (200–299)
Redirection messages (300–399)
Client error responses (400–499)
Server error responses (500–599)

Thank you for reading.

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REST api Call Retrofit using Callback in Kotlin

1. Add Dependancies

2.Test Api

3. Copy Json Data And Generate Kotlin Data class.( generate kotlin calss is plugin download.)

4. Activate permission of Internet.

5. Create layout to perform actions in api

activity_create_new_user.xml

6. Create Row card for Recycler

recycler_row_list.xml

7.Design activity_main

activity_main.xml

8. When Generated Data calss Name ‘UserList’ so other class Auto Generate.

9. Create Instance class named ‘RetrofitInstance’

10. create Interface for call HTTP methods

interface RetroService.kt

11. Create view model named

CreateNewUserViewModel.kt

12. Create kotlin class for activity_create_new_user

CreateNewUserActivity.kt

13. Create view model for main activity

MainActivityViewModel.kt

14. Create adapter for Recyclerview

RecyclerViewAdapter.kt

15 . activity_main.xml for create Main activity class

MainActivity.kt

HTTP methods

Http response codes.

Written by Riyas Pullur