SimpleSwiftPromises/SimpleSwiftPromises/Promise.swift

//
//  Promise.swift
//  SimpleSwiftPromises
//
//  Created by Shadowfacts on 2/14/20.
//  Copyright © 2020 Shadowfacts. All rights reserved.
//

import Foundation

public class Promise<Result> {
    private var handlers: [(Result) -> Void] = []
    private var result: Result?
    private var catchers: [(Error) -> Void] = []
    private var error: Error?

    func resolve(_ result: Result) {
        self.result = result
        self.handlers.forEach { $0(result) }
    }
    
    func reject(_ error: Error) {
        self.error = error
        self.catchers.forEach { $0(error) }
    }
    
    func addHandler(_ handler: @escaping (Result) -> Void) {
        if let result = result {
            handler(result)
        } else {
            handlers.append(handler)
        }
    }
    
    func addCatcher(_ catcher: @escaping (Error) -> Void) {
        if let error = error {
            catcher(error)
        } else {
            catchers.append(catcher)
        }
    }
}

public extension Promise {
    static func resolve<Result>(_ value: Result) -> Promise<Result> {
        let promise = Promise<Result>()
        promise.resolve(value)
        return promise
    }
    
    static func reject<Result>(_ error: Error) -> Promise<Result> {
        let promise = Promise<Result>()
        promise.reject(error)
        return promise
    }
    
    static func all<Result>(_ promises: [Promise<Result>], queue: DispatchQueue = .main) -> Promise<[Result]> {
        let group = DispatchGroup()
        
        var results = [Result?](repeating: nil, count: promises.count)
        var firstError: Error?

        for (index, promise) in promises.enumerated() {
            group.enter()
            promise.then { (res) in
                queue.async {
                    results[index] = res
                    group.leave()
                }
            }.catch { (err) -> Void in
                if firstError == nil {
                    firstError = err
                }
                group.leave()
            }
        }
        
        return Promise<[Result]> { (resolve, reject) in
            group.notify(queue: queue) {
                if let firstError = firstError {
                    reject(firstError)
                } else {
                    resolve(results.compactMap { $0 })
                }
            }
        }
    }
    
    convenience init(resultProvider: @escaping (_ resolve: @escaping (Result) -> Void, _ reject: @escaping (Error) -> Void) -> Void) {
        self.init()
        resultProvider(self.resolve, self.reject)
    }
    
    convenience init<ErrorType>(_ resultProvider: @escaping ((Swift.Result<Result, ErrorType>) -> Void) -> Void) {
        self.init { (resolve, reject) in
            resultProvider { (result) in
                switch result {
                case let .success(res):
                    resolve(res)
                case let .failure(error):
                    reject(error)
                }
            }
        }
    }

    @discardableResult
    func then(_ func: @escaping (Result) -> Void) -> Promise<Result> {
        addHandler(`func`)
        return self
    }
    
    func then<Next>(_ mapper: @escaping (Result) -> Promise<Next>) -> Promise<Next> {
        let next = Promise<Next>()
        addHandler { (parentResult) in
            let newPromise = mapper(parentResult)
            newPromise.addHandler(next.resolve)
            newPromise.addCatcher(next.reject)
        }
        addCatcher(next.reject)
        return next
    }
    
    func then<Next>(_ mapper: @escaping (Result) -> Next) -> Promise<Next> {
        let next = Promise<Next>()
        addHandler { (parentResult) in
            let newResult = mapper(parentResult)
            next.resolve(newResult)
        }
        addCatcher(next.reject)
        return next
    }
    
    @discardableResult
    func `catch`(_ catcher: @escaping (Error) -> Void) -> Promise<Result> {
        addCatcher(catcher)
        return self
    }
    
    func `catch`(_ catcher: @escaping (Error) -> Promise<Result>) -> Promise<Result> {
        let next = Promise<Result>()
        addHandler(next.resolve)
        addCatcher { (error) in
            let newPromise = catcher(error)
            newPromise.addHandler(next.resolve)
            newPromise.addCatcher(next.reject)
        }
        return next
    }

    func `catch`(_ catcher: @escaping (Error) -> Result) -> Promise<Result> {
        let next = Promise<Result>()
        addHandler(next.resolve)
        addCatcher { (error) in
            let newResult = catcher(error)
            next.resolve(newResult)
        }
        return next
    }
    
    func handle(on queue: DispatchQueue) -> Promise<Result> {
        return self.then { (result) in
            return Promise { (resolve, reject) in
                queue.async {
                    resolve(result)
                }
            }
        }
    }
}