AssetsMocksHttpClient.kt

package com.example.android.testing.blueprint

import android.content.res.AssetManager
import okhttp3.Interceptor
import okhttp3.MediaType.Companion.toMediaType
import okhttp3.OkHttpClient
import okhttp3.Protocol
import okhttp3.Request
import okhttp3.Response
import okhttp3.ResponseBody.Companion.toResponseBody
import retrofit2.Retrofit

/**
 * Provides [OkHttpClient] that returns mocked data from `assets/[root]` folder
 * based on [Request] data.
 *
 * Use it as a client for [Retrofit]:
 * ```
 * Retrofit.Builder()
 *   .baseUrl("http://fake.url")
 *   .client(assetsMocksHttpClient(assets, "mocks"))
 *   .addConverterFactory(GsonConverterFactory.create())
 *   .build()
 * ```
 */
fun assetsMocksHttpClient(assets: AssetManager, root: String): OkHttpClient {
    val pathResolver: (Request) -> String = { request ->
        listOf(root)
                .plus(request.url.pathSegments)
                .joinToString("/", postfix = ".json")
    }
    val dataProvider: (String) -> MockData = { path ->
        MockData(assets.open(path).bufferedReader().readText())
    }
    return OkHttpClient.Builder()
            .addInterceptor(MockResponseInterceptor(pathResolver, dataProvider))
            .build()
}

private class MockResponseInterceptor(
        private val pathResolver: (Request) -> String,
        private val dataProvider: (String) -> MockData
) : Interceptor {
    override fun intercept(chain: Interceptor.Chain): Response = with(chain.request()) {
        try {
            val path = pathResolver(this)
            val (content, mediaType) = dataProvider(path)
            ok(content, mediaType)
        } catch (e: Exception) {
            error(e.message!!, "plain/text")
        }
    }

    private fun Request.ok(content: String, mediaType: String): Response =
            response(200, "OK", content, mediaType)

    private fun Request.error(content: String, mediaType: String): Response =
            response(500, "Internal Server Error", content, mediaType)

    private fun Request.response(
            code: Int,
            message: String,
            content: String,
            mediaType: String
    ) = Response.Builder()
            .request(this)
            .protocol(Protocol.HTTP_1_0)
            .code(code)
            .message(message)
            .body(content.toResponseBody(mediaType.toMediaType()))
            .build()

}

private data class MockData(val content: String, val mediaType: String = "application/json")