Kotlin Ktor client PoC

This project showcases the usage of the Ktor (JavaScript) client.

Installation

1. Clone this repository
2. Run the following command in the root directory of the project gradlew -t run this will start a webpack dev server in continuous mode (code changes will refresh the browser automatically).

About

The Ktor client allows you to make HTTP requests, the Ktor ktor-client-json library (in conjunction with kotlinx.serialization) allows for automatic (de)serialization. To make this possible you must first create the client and install the JsonFeature (features are like filter/interceptors):

private const val API_HOST = "jsonplaceholder.typicode.com"

val client = HttpClient {
   install(JsonFeature)
   defaultRequest {
       host = API_HOST
   }
}

Now you can use the client to make HTTP requests. As you can see a default host has also been provided, now you don't need to provide the host in individual requests but only the API path.

Below is an example of a POST (the client has support for all HTTP request methods) request.

suspend fun post(): Todo = client.post {
    url(API_PATH)
    body = Todo(1, "test", false)
    contentType(ContentType.Application.Json)
}

As you can see there is no need to manually (de)serialize, the request body will be serialized automatically because the content-type is application/json. And the response body will be deserialized automatically based on the function's return type.

NOTE: You must annotate your class (in this example Todo) with kotlinx.serialization's @Serializable annotation in order for this to work!

As you can see the function above is a suspend function which means it makes use of kotlin coroutines, so there is no need to use .then()

The library will not automatically deserialize a top-level JSON list as it doesnt acknowledge this as valid JSON, the problem is that a lot of APIs do return a top-level JSON list. I have created a workaround which allows this:

suspend fun getList(): List<Todo> = client.list {
    url(API_PATH)
}

The function above sends a GET request, you can use any HTTP method by setting the method value (along with any other Ktor request configurations):

suspend fun getList(): List<Todo> = client.list {
    url(API_PATH)
    method = HttpMethod.Post
}

The list method looks as follows:

suspend inline fun <reified T : Any> HttpClient.list(noinline block: HttpRequestBuilder.() -> Unit): List<T> =
    customSerializerRequest(T::class.serializer().list, block)

As you can see it calls the customSerializerRequest passing the list serializer of the class and theHtttpRequestBuilder configuration.

The customSerializerRequest looks as follows:

suspend fun <T : Any> HttpClient.customSerializerRequest(
   serializer: KSerializer<T>,
   block: HttpRequestBuilder.() -> Unit
): T {
   return request<HttpResponse> {
       apply(block)
   }.use {
       if (it.status.isSuccess()) {
           return Json.parse(serializer, it.readText())
       } else {
           throw BadResponseStatusException(it.status, it)
       }
   }
}

This function accept a serializer and the request configuration. It will perform a request "manually" using the request configuration and then check whether the request was successful, if so it will parse the body using the provided serializer, if not it will throw a BadResponseStatusException (which is the default behaviour for Ktor).