Handling result with less completion calls

main.swift

//
//  main.swift
//  Completions
//
//  Created by szotp on 30/06/2019.
//  Copyright © 2019 szotp. All rights reserved.
//

import Foundation

typealias CompletionBlock = Optional<() -> Void>

func apiRequest(input: Int, onSuccess: (String) -> Void, onFailure: (Error) -> Void) {
    onSuccess("")
}

func apiRequestWrapped(completion: CompletionBlock) {
    let cmpl = Completor<String>(completion) { result in
        let data = try result.get()
        print(data)
    }
    
    
    if (arc4random() % 2) == 0 {
        apiRequest(input: 5, onSuccess: cmpl.onSuccess, onFailure: cmpl.onFailure)
    } else {
        // cmpl.deinit will be called and complete automatically
    }
}

class Completor<T> {
    typealias ResultCallback = (Result<T, Error>) throws -> Void
    
    private var didComplete = false
    private var completion: CompletionBlock
    private var onResult: ResultCallback
    
    init(_ completion: CompletionBlock, onResult: @escaping ResultCallback) {
        self.completion = completion
        self.onResult = onResult
    }
    
    func onSuccess(value: T) {
        complete(.success(value))
    }
    
    func onFailure(error: Error) {
        print(error)
        complete(.failure(error))
    }
    
    func complete(_ result: Result<T, Error>?) {
        assert(!didComplete)
        if let result = result {
            do {
                try onResult(result)
            } catch let error {
                // apply default behavior here, show toast or something, depends on the app
                print(error)
            }
        }
        
        let completion = self.completion
        DispatchQueue.main.async {
            let desc = result.map { "\($0)"} ?? "nil"
            print("completion with \(desc)")
            completion?()
        }
        didComplete = true
    }
    
    
    
    deinit {
        if !didComplete {
            complete(nil)
        }
    }
}

apiRequestWrapped {
    print("ok")
}
RunLoop.main.run()