使用 Kotlin 协程 + Fuel 调用 REST API

JSON : Placeholder

JSON : Placeholder (https://jsonplaceholder.typicode.com/) 是一个用于测试的 REST API 网站。
以下使用 Kotlin 协程 + Fuel 调用该网站的 REST API，获取字符串以及 JSON 数据。

• GET /posts/1
• GET /posts
• POST /posts
• PUT /posts/1
• DELETE /posts/1

所有 GET API 都返回JSON数据，格式（JSON-Schema）如下：

{
  "type":"object",
  "properties": {
    "userId": {"type" : "integer"},
    "id": {"type" : "integer"},
    "title": {"type" : "string"},
    "body": {"type" : "string"}
  }
}

创建工程

打开 Intellij IDEA，File / New / Project...
在 New Project 向导的第1页，选 Kotlin, Project Template 选 JVM Application，Build System 选 Gradle Kotlin，Project JDK 选 1.8。
在向导的第2页 Template 选 Console Application
点击 Finish 按钮创建工程

build.gradle.kts 内容如下

import org.jetbrains.kotlin.gradle.tasks.KotlinCompile

plugins {
    kotlin("jvm") version "1.4.32"
    application
}

group = "me.xxxx"
version = "1.0-SNAPSHOT"

repositories {
    mavenCentral()
}

dependencies {
    testImplementation(kotlin("test-junit"))
}

tasks.test {
    useJUnit()
}

tasks.withType<KotlinCompile>() {
    kotlinOptions.jvmTarget = "1.8"
}

application {
    mainClassName = "MainKt"
}

将 dependencies 这部分的内容改为：

dependencies {
    testImplementation(kotlin("test-junit"))
    val fuel_version = "2.3.1"
    implementation("com.github.kittinunf.fuel:fuel:$fuel_version")
    implementation("com.github.kittinunf.fuel:fuel-coroutines:$fuel_version")
    implementation("com.github.kittinunf.fuel:fuel-gson:$fuel_version")
    implementation("org.jetbrains.kotlinx:kotlinx-coroutines-core:1.4.3-native-mt")
}

这一段引用了 Kotlin 协程和 Fuel 这两个库。
其中 Fuel 这个库中还包含了 Gson 的转换器。

Post 对象

jsonschema2pojo 可以将 JSON 数据或格式自动转换为 Java 的 POJO 类。

data class Post(val userId: Int, val id: Int, val title: String, val body: String) {
    override fun toString() =
        "Post {userId = $userId, id = $id, title = "$title", body = "${body.replace("
", "\n")}"}"
}

Post 对象负责 Kotlin 对象与 JSON 数据之间的相互转换。
由于两者字段名相同，这里不需要使用注解。

// 如果需要加上注解的话
data class Post(@SerializedName("userId") val userId: Int,
                @SerializedName("id") val id: Int,
                @SerializedName("title") val title: String,
                @SerializedName("body") val body: String) {
// ...
}

Fuel 序列化

class PostDeserializer : ResponseDeserializable<Post> {
    override fun deserialize(content: String): Post =
        Gson().fromJson(content, Post::class.java)
}

class PostListDeserializer : ResponseDeserializable<List<Post>> {
    override fun deserialize(content: String): List<Post> {
        val listType = object : TypeToken<List<Post>>() {}.type
        return Gson().fromJson(content, listType)
    }
}

调用 REST API

private val home = "https://jsonplaceholder.typicode.com/"

suspend fun getPostAsString(): String =
    Fuel.get("${home}posts/1").awaitString()

suspend fun getPostAsJson(): Post =
    Fuel.get("${home}posts/1").awaitObject(PostDeserializer())

suspend fun getPosts(n: Int): List<Post> =
    Fuel.get("${home}posts").awaitObject(PostListDeserializer()).take(n)

suspend fun createPost(): Post =
    Fuel.post("${home}posts", listOf("userId" to 101, "title" to "test title", "body" to "test body"))
        .awaitObject(PostDeserializer())

suspend fun updatePost(): Post =
    Fuel.put("${home}posts/1", listOf("userId" to 101, "id" to 1, "title" to "test title", "body" to "test body"))
        .awaitObject(PostDeserializer())

suspend fun deletePost(): String =
    Fuel.delete("${home}posts/1").awaitString()

• getPostAsString 函数取出第1个Post，返回字符串
• getPostAsJson 函数取出第1个Post，返回Post对象
• getPosts 函数取出前n个Post，返回n个Post对象
• createPost 函数创建1个Post，返回所创建的Post对象
• updatePost 函数更新第1个Post，返回所更新的Post对象
• deletePost 函数删除第1个Post，返回字符串

main 函数

fun main(args: Array<String>) = runBlocking {
    println(getPostAsString())
    println(getPostAsJson())
    println(getPosts(2))
    println(createPost())
    println(updatePost())
    println(deletePost())
}

输出结果

{
  "userId": 1,
  "id": 1,
  "title": "sunt aut facere repellat provident occaecati excepturi optio reprehenderit",
  "body": "quia et suscipit
suscipit recusandae consequuntur expedita et cum
reprehenderit molestiae ut ut quas totam
nostrum rerum est autem sunt rem eveniet architecto"
}
Post {userId = 1, id = 1, title = "sunt aut facere repellat provident occaecati excepturi optio reprehenderit", body = "quia et suscipit
suscipit recusandae consequuntur expedita et cum
reprehenderit molestiae ut ut quas totam
nostrum rerum est autem sunt rem eveniet architecto"}
[Post {userId = 1, id = 1, title = "sunt aut facere repellat provident occaecati excepturi optio reprehenderit", body = "quia et suscipit
suscipit recusandae consequuntur expedita et cum
reprehenderit molestiae ut ut quas totam
nostrum rerum est autem sunt rem eveniet architecto"}, Post {userId = 1, id = 2, title = "qui est esse", body = "est rerum tempore vitae
sequi sint nihil reprehenderit dolor beatae ea dolores neque
fugiat blanditiis voluptate porro vel nihil molestiae ut reiciendis
qui aperiam non debitis possimus qui neque nisi nulla"}]
Post {userId = 101, id = 101, title = "test title", body = "test body"}
Post {userId = 101, id = 1, title = "test title", body = "test body"}
{}