Swift-interceptor

Introduction

This Interceptor “concept” comes from the Interceptor implementation in the library Okhttp for Android (see Okhttp interceptor wiki). This mechanism is very powerful when you want to modify/do some actions on an input object before and after the process it was given for. See the interceptors as a middleware between the source of your input object and its final destination.

In this Swift implementation, two more things has been added:

• Generic input.
• Asynchronous process: the interceptor does not return the output synchronously but call a closure when it decides the process is done.

With Interceptors, you have:

• clean and robust solution to handle:
  • network monitoring
  • adding parameters for all your requests in a single place
  • authentication (get/refresh a token). Signed your request before sending it.
  • retry failed request
  • and more.. ;)

• clear distribution of roles:
  • each interceptor has a clear purpose
  • do only one thing on the input object

• an easy way to improve test coverage with unit tests on each of your interceptors

To finish, as it’s generic, you can apply this mechanism in another context. Be creative ! :)

Usage

Interceptor

First thing to do, it’s to create the Interceptor(s) you need. Remember, Interceptor is designed to be used to intercept the input object. Group your Interceptors by concern and avoid duplicated ones

To do so, you just have to conform to the protocol Interceptor:

protocol Interceptor {
    associatedtype Input

    func intercept(chain: InterceptorChain<Input>, completion: (Input) -> Void) -> Void
}

Example

Intercept the input

struct MyInterceptor: Interceptor {
    func intercept(chain: InterceptorChain<URLRequest>, completion: (URLRequest) -> Void) -> Void {
        // 1) Retrieve the input object (the request)
        var request = chain.input

        // 2) Do things with/on the request

        // 3) Give it back to the chain
        chain.input = request

        // 4) Continue the chaining to others interceptors. /!\ Don't forget to call this, if you don't, you will get stuck in this interceptor.
        chain.proceed(completion: completion)
    }
}

Interceptor chain

When you have your interceptors implemented, it’s finished !

To launch the process:

// 1) I create an instance of interceptor chain. You can add many interceptor you want.
let chain = InterceptorChain()
    .add(interceptor: AnyInterceptor(base: MyInterceptor()))
chain.input = request

// 2) Launch the process
chain.proceed { (request) in
    // 3) I get my request intercepted by all the interceptors
}

Additional informations

For more explanations about the interceptor mechanism, don’t hesitate to read the documentation in the okhttp wiki. For RxSwift fans, there is the same implementation with RxSwift, see SwiftRxInterceptor.