Overview

Retrofit is a RESTful network request framework for Android. Under the hood, it relies on OkHttp for actual HTTP communication while providing a clean abstraction layer for API interface definitions. Retrofit handles request parameters, headers, URLs, and response parsing through annotated interface methods.

Benefits

Comparing with raw OkHttp usage, several pain points emerge:

1. Tedious interface configuration, especially when handling request bodies, headers, and multiple parameters
2. Manual response body parsing that lacks reusability across different endpoints
3. No automatic thread switching for UI updates

Retrofit addresses these issues by providing a declarative interface-based approach to network calls.

Implementation

Important: Ensure the BaseUrl ends with a forward slash, otherwise an exception will be thrown. The following demonstrates a GET request example. Endpoint: ${BASE_URL}test?name=" "&age=""

3.1 API Interface

Define the network endpoint contract with annotated method signatures:

interface UserApiService {
   @GET("test")
   suspend fun fetchUser(
       @Query("name") userName: String,
       @Query("age") userAge: String
   ): ApiResponse<List<User>>
}

User data model:

data class User(
   val name: String = "",
   val age: String = ""
)

Standardized response wrapper:

data class ApiResponse<T>(
   val code: String,
   val data: T
)

3.2 RetrofitManager

Configure OkHttpClient instance and build Retrofit with required converters and base URL:

private val httpClient: OkHttpClient
   get() = OkHttpClient.Builder()
       .addInterceptor(loggingInterceptor)
       .connectTimeout(CONNECTION_TIMEOUT_SECONDS, TimeUnit.SECONDS)
       .build()

fun <T> createService(serviceClass: Class<T>, baseUrl: String? = null): T {
   return Retrofit.Builder()
       .client(httpClient)
       .addConverterFactory(GsonConverterFactory.create())
       .baseUrl(BASE_URL)
       .build()
       .create(serviceClass)
}

3.3 DataRepository

Implement the API interface and provide a centralized access point for network operations:

object NetworkRepository : UserApiService {
   private val apiInstance by lazy { RetrofitManager.createService(UserApiService::class.java) }

   override suspend fun fetchUser(
       userName: String,
       userAge: String
   ): ApiResponse<List<User>> {
       return apiInstance.fetchUser(userName, userAge)
   }
}

3.4 ViewModel Integration

Execute network requests within coroutine scope for proper lifecycle management:

fun loadUserData(name: String, age: String) {
   viewModelScope.launch {
       val response = NetworkRepository.fetchUser(name, age)
       Log.d("network", "result: $response")
   }
}

3.5 UI Layer Call

loadUserData("", "")