#Kotlin #Android Retrofit2 singleton instance sample

TmdbApi.kt

package rubenpla.develop.privtmdbendlesslist.data.api

import retrofit2.Retrofit
import retrofit2.converter.gson.GsonConverterFactory
import retrofit2.http.GET
import retrofit2.http.Query
import rubenpla.develop.privtmdbendlesslist.data.model.MoviesResultsItem


interface TmdbApi {

    @GET("movie/popular")
    fun getPopularMovies(@Query("api_key") api_key: String,
                         @Query("page") page: Int): List<MoviesResultsItem>


    companion object Factory {
        @Volatile
        private var retrofit : Retrofit? = null
        
        private const val BASE_URL: String = "https://api.themoviedb.org/3/"

        @Synchronized
        fun getInstance(): TmdbApi? {
            retrofit ?: synchronized(this) {
                retrofit ?: buildRetrofit()
            }

            return retrofit?.create(TmdbApi::class.java)
        }
        
        private fun buildRetrofit() = retrofit2.Retrofit.Builder()
                .baseUrl(BASE_URL)
                .addConverterFactory(GsonConverterFactory.create())
                .build() 
    }
}