Retrofit test with interceptor, clear communication and show data to TextView

Retrofit is not a simplest way to create communication from Android and API, from .NET-developer point of View this is HttpClient with embedded deserializer like newtonsoft. In other case Retrofit can not allow analyze DISCO endpoint or /help endpoint of API and can not build interface and classes automatically, you must define and describe endpoint manually by own hand. This combination of function (compose JSON converter with HTTPClient to one package but automatically building API interface classes is absent at all) look strange for .NET Developer, but in Android platform this is standard point of view.

However, there are more familiar solution for .NET Developer - Volley, see detail instruction how to work with Volley for example in this video https://www.youtube.com/watch?v=xPi-z3nOcn8.

In this page I describe how to use Retrofit step-by-step. Full code has been published to github https://github.com/Alex-1347/RetrofitTest

• Create empty project.
  
• Set up project property.
  
• Add project to GIT tracking.
  
• Add Retrofit library to project https://github.com/square/retrofit.

     1:  plugins {

     2:      id 'com.android.application'

     3:      id 'kotlin-android'

     4:  }

     5:   

     6:  android {

     7:      compileSdkVersion 30

     8:      buildToolsVersion "30.0.3"

     9:   

    10:      defaultConfig {

    11:          applicationId "com.example.retrofittest"

    12:          minSdkVersion 30

    13:          targetSdkVersion 30

    14:          versionCode 1

    15:          versionName "1.0"

    16:   

    17:          testInstrumentationRunner "androidx.test.runner.AndroidJUnitRunner"

    18:      }

    19:   

    20:      buildTypes {

    21:          release {

    22:              minifyEnabled false

    23:              proguardFiles getDefaultProguardFile('proguard-android-optimize.txt'), 'proguard-rules.pro'

    24:          }

    25:      }

    26:      compileOptions {

    27:          sourceCompatibility JavaVersion.VERSION_1_8

    28:          targetCompatibility JavaVersion.VERSION_1_8

    29:      }

    30:      kotlinOptions {

    31:          jvmTarget = '1.8'

    32:      }

    33:  }

    34:   

    35:  dependencies {

    36:   

    37:      implementation "org.jetbrains.kotlin:kotlin-stdlib:$kotlin_version"

    38:      implementation 'androidx.core:core-ktx:1.3.2'

    39:      implementation 'androidx.appcompat:appcompat:1.2.0'

    40:      implementation 'com.google.android.material:material:1.3.0'

    41:      implementation 'androidx.constraintlayout:constraintlayout:2.0.4'

    42:      implementation 'com.squareup.retrofit2:retrofit:2.9.0'

    43:      implementation 'com.squareup.retrofit2:converter-gson:2.9.0'

    44:      implementation 'com.squareup.okhttp3:logging-interceptor:4.9.1'

    45:      testImplementation 'junit:junit:4.+'

    46:      androidTestImplementation 'androidx.test.ext:junit:1.1.2'

    47:      androidTestImplementation 'androidx.test.espresso:espresso-core:3.3.0'

    48:  }

  
• Set permission to application - clearCommunication and uses

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

     2:  <manifest xmlns:android="http://schemas.android.com/apk/res/android"

     3:      package="com.example.retrofittest">

     4:   

     5:      <application

     6:          android:allowBackup="true"

     7:          android:icon="@mipmap/ic_launcher"

     8:          android:label="@string/app_name"

     9:          android:roundIcon="@mipmap/ic_launcher_round"

    10:          android:supportsRtl="true"

    11:          android:usesCleartextTraffic="true"

    12:          android:theme="@style/Theme.RetrofitTest">

    13:          <activity android:name=".MainActivity">

    14:              <intent-filter>

    15:                  <action android:name="android.intent.action.MAIN" />

    16:   

    17:                  <category android:name="android.intent.category.LAUNCHER" />

    18:              </intent-filter>

    19:          </activity>

    20:      </application>

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

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

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

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

    25:  </manifest>

  
• Create activity (form)

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

     2:  <androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"

     3:      xmlns:app="http://schemas.android.com/apk/res-auto"

     4:      xmlns:tools="http://schemas.android.com/tools"

     5:      android:layout_width="match_parent"

     6:      android:layout_height="match_parent"

     7:      android:padding="8dp"

     8:      tools:context=".MainActivity">

     9:   

    10:      <androidx.core.widget.NestedScrollView

    11:          android:layout_width="match_parent"

    12:          android:layout_height="match_parent">

    13:   

    14:      <TextView

    15:          android:id="@+id/text_view_result"

    16:          android:textColor="@color/black"

    17:          android:layout_width="wrap_content"

    18:          android:layout_height="wrap_content"

    19:          app:layout_constraintBottom_toBottomOf="parent"

    20:          app:layout_constraintLeft_toLeftOf="parent"

    21:          app:layout_constraintRight_toRightOf="parent"

    22:          app:layout_constraintTop_toTopOf="parent" />

    23:   

    24:      </androidx.core.widget.NestedScrollView>

    25:   

    26:  </androidx.constraintlayout.widget.ConstraintLayout>

  
• Check data and create interface.

     1:  package com.example.retrofittest

     2:   

     3:  public class OneItem {

     4:      public var City: String=""

     5:      public var CityID: String=""

     6:      public var Country: String=""

     7:  }

  

  This is main point, this interface allow us Android Studio analyzing and intelliSense and support type reference to data.

  

     1:  package com.example.retrofittest

     2:   

     3:  import retrofit2.Call

     4:  import retrofit2.http.*

     5:   

     6:  interface JsonPlaceholder {

     7:      @GET("GetCities")

     8:      fun GetCities(): Call<MutableList<OneItem>>

     9:  }

  
• And finally create program.

   1:  package com.example.retrofittest

   2:   

   3:  import androidx.appcompat.app.AppCompatActivity

   4:  import android.os.Bundle

   5:  import android.widget.TextView

   6:  import okhttp3.OkHttpClient

   7:  import okhttp3.logging.HttpLoggingInterceptor

   8:  import retrofit2.Call

   9:  import retrofit2.Callback

  10:  import retrofit2.Response

  11:  import retrofit2.Retrofit

  12:  import retrofit2.converter.gson.GsonConverterFactory

  13:   

  14:  class MainActivity : AppCompatActivity() {

  15:   

  16:      override fun onCreate(savedInstanceState: Bundle?) {

  17:          super.onCreate(savedInstanceState)

  18:          setContentView(R.layout.activity_main)

  19:   

  20:          val Res = findViewById(R.id.text_view_result) as TextView

  21:   

  22:         //logging  https://stackoverflow.com/questions/32514410/logging-with-retrofit-2

  23:          val Logger = HttpLoggingInterceptor()

  24:          Logger.setLevel(HttpLoggingInterceptor.Level.BODY)

  25:          val Client = OkHttpClient.Builder().addInterceptor(Logger).build()

  26:   

  27:          val RF: Retrofit = Retrofit.Builder()

  28:              .baseUrl("http://192.168.0.102/Backend/LocationService.svc/") //

  29:              .client(Client)

  30:              .addConverterFactory(GsonConverterFactory.create())

  31:              .build()

  32:   

  33:          Res.setText(RF.baseUrl().toString() + System.lineSeparator())

  34:   

  35:          val SRV = RF.create(JsonPlaceholder::class.java)

  36:   

  37:          val CityList = SRV.GetCities()

  38:   

  39:          //another way -  https://stackoverflow.com/questions/43233025/use-retrofit-methods-more-expressive-way

  40:          CityList.enqueue(object : Callback<MutableList<OneItem>> {

  41:              override fun onFailure(call: Call<MutableList<OneItem>>?, t: Throwable?) {

  42:                  if (t != null) {

  43:                      Res.setText(t.message.toString())

  44:                  }

  45:              }

  46:   

  47:              override fun onResponse(

  48:                  call: Call<MutableList<OneItem>>?,

  49:                  response: Response<MutableList<OneItem>>?

  50:              ) {

  51:                  if (response != null) {

  52:                      if (!response.isSuccessful) {

  53:                          Res.setText(response.code().toString())

  54:                          return

  55:                      } else {

  56:                          val DT = response.body()

  57:                          if (DT != null) {

  58:                              val Str1=StringBuilder()

  59:                              //or more correctly StringBuilder

  60:                              DT.forEach { one -> Str1.append( "${one.City}(${one.Country})${System.lineSeparator()}" )}

  61:                              Res.setText(Str1.toString())

  62:                          }

  63:                      }

  64:                  }

  65:              }

  66:          })

  67:      }

  68:  }

• Start program and debug it.

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