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# SimpleSwiftPromises
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A simple implementation of promises in Swift. Goes with [my blog post](https://shadowfacts.net/2020/simple-swift-promises/) on the subject.
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//
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// Promise.swift
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// SimpleSwiftPromises
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//
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// Created by Shadowfacts on 2/14/20.
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// Copyright © 2020 Shadowfacts. All rights reserved.
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//
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import Foundation
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public class Promise<Result> {
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private var handlers: [(Result) -> Void] = []
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private var result: Result?
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private var catchers: [(Error) -> Void] = []
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private var error: Error?
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func resolve(_ result: Result) {
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self.result = result
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self.handlers.forEach { $0(result) }
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}
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func reject(_ error: Error) {
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self.error = error
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self.catchers.forEach { $0(error) }
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}
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func addHandler(_ handler: @escaping (Result) -> Void) {
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if let result = result {
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handler(result)
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} else {
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handlers.append(handler)
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}
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}
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func addCatcher(_ catcher: @escaping (Error) -> Void) {
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if let error = error {
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catcher(error)
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} else {
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catchers.append(catcher)
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}
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}
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}
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public extension Promise {
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static func resolve<Result>(_ value: Result) -> Promise<Result> {
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let promise = Promise<Result>()
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promise.resolve(value)
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return promise
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}
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static func reject<Result>(_ error: Error) -> Promise<Result> {
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let promise = Promise<Result>()
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promise.reject(error)
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return promise
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}
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static func all<Result>(_ promises: [Promise<Result>], queue: DispatchQueue = .main) -> Promise<[Result]> {
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let group = DispatchGroup()
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var results = [Result?](repeating: nil, count: promises.count)
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var firstError: Error?
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for (index, promise) in promises.enumerated() {
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group.enter()
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promise.then { (res) in
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queue.async {
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results[index] = res
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group.leave()
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}
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}.catch { (err) -> Void in
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if firstError == nil {
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firstError = err
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}
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group.leave()
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}
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}
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return Promise<[Result]> { (resolve, reject) in
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group.notify(queue: queue) {
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if let firstError = firstError {
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reject(firstError)
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} else {
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resolve(results.compactMap { $0 })
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}
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}
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}
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}
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convenience init(resultProvider: @escaping (_ resolve: @escaping (Result) -> Void, _ reject: @escaping (Error) -> Void) -> Void) {
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self.init()
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resultProvider(self.resolve, self.reject)
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}
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convenience init<ErrorType>(_ resultProvider: @escaping ((Swift.Result<Result, ErrorType>) -> Void) -> Void) {
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self.init { (resolve, reject) in
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resultProvider { (result) in
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switch result {
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case let .success(res):
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resolve(res)
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case let .failure(error):
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reject(error)
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}
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}
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}
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}
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@discardableResult
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func then(_ func: @escaping (Result) -> Void) -> Promise<Result> {
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addHandler(`func`)
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return self
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}
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func then<Next>(_ mapper: @escaping (Result) -> Promise<Next>) -> Promise<Next> {
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let next = Promise<Next>()
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addHandler { (parentResult) in
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let newPromise = mapper(parentResult)
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newPromise.addHandler(next.resolve)
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newPromise.addCatcher(next.reject)
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}
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addCatcher(next.reject)
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return next
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}
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func then<Next>(_ mapper: @escaping (Result) -> Next) -> Promise<Next> {
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let next = Promise<Next>()
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addHandler { (parentResult) in
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let newResult = mapper(parentResult)
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next.resolve(newResult)
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}
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addCatcher(next.reject)
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return next
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}
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@discardableResult
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func `catch`(_ catcher: @escaping (Error) -> Void) -> Promise<Result> {
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addCatcher(catcher)
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return self
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}
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func `catch`(_ catcher: @escaping (Error) -> Promise<Result>) -> Promise<Result> {
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let next = Promise<Result>()
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addHandler(next.resolve)
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addCatcher { (error) in
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let newPromise = catcher(error)
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newPromise.addHandler(next.resolve)
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newPromise.addCatcher(next.reject)
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}
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return next
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}
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func `catch`(_ catcher: @escaping (Error) -> Result) -> Promise<Result> {
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let next = Promise<Result>()
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addHandler(next.resolve)
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addCatcher { (error) in
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let newResult = catcher(error)
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next.resolve(newResult)
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}
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return next
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}
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func handle(on queue: DispatchQueue) -> Promise<Result> {
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return self.then { (result) in
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return Promise { (resolve, reject) in
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queue.async {
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resolve(result)
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}
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}
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}
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}
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}
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//
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// PromiseTests.swift
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// SimpleSwiftPromisesTests
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//
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// Created by Shadowfacts on 2/14/20.
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// Copyright © 2020 Shadowfacts. All rights reserved.
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//
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import XCTest
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@testable import SimpleSwiftPromises
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class PromiseTests: XCTestCase {
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override func setUpWithError() throws {
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// Put setup code here. This method is called before the invocation of each test method in the class.
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}
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override func tearDownWithError() throws {
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// Put teardown code here. This method is called after the invocation of each test method in the class.
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}
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func assertResultEqual<Result: Equatable>(_ promise: Promise<Result>, _ value: Result, message: String? = nil) {
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let expectation = self.expectation(description: message ?? "promise result assertion")
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promise.then {
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XCTAssertEqual($0, value)
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expectation.fulfill()
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}
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self.waitForExpectations(timeout: 2) { (error) in
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if let error = error {
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XCTFail("didn't resolve promise: \(error)")
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}
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}
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}
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func testResolveImmediate() {
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assertResultEqual(Promise<String>.resolve("blah"), "blah")
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}
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func testResolveImmediateMapped() {
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let promise = Promise<String>.resolve("foo").then {
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"test \($0)"
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}.then {
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Promise<String>.resolve("\($0) bar")
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}
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assertResultEqual(promise, "test foo bar")
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}
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func testContinueAfterReject() {
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let promise = Promise<String>.reject(TestError()).then { (res) in
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XCTFail("then on rejected promise is unreachable")
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}.catch { (error) -> String in
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XCTAssertTrue(error is TestError)
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return "caught"
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}.then {
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"\($0) error"
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}
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assertResultEqual(promise, "caught error")
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}
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func testResolveDelayed() {
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let promise = Promise<String> { (resolve, reject) in
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DispatchQueue.main.asyncAfter(deadline: .now() + 1) {
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resolve("blah")
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}
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}
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assertResultEqual(promise, "blah")
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}
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func testResolveMappedDelayed() {
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let promise = Promise<String> { (resolve, reject) in
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DispatchQueue.main.asyncAfter(deadline: .now() + 1) {
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resolve("foo")
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}
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}.then {
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"\($0) bar"
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}.then { (result) in
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Promise<String> { (resolve, reject) in
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DispatchQueue.main.asyncAfter(deadline: .now() + 0.5) {
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resolve("\(result) baz")
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}
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}
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}
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assertResultEqual(promise, "foo bar baz")
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}
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func testResolveAll() {
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let promise = Promise<[String]>.all([
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Promise<String>.resolve("a"),
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Promise<String>.resolve("b"),
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Promise<String>.resolve("c"),
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])
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assertResultEqual(promise, ["a", "b", "c"])
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}
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func testIntermediateReject() {
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let promise = Promise<String>.resolve("foo").then { (_) -> Promise<String> in
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Promise<String>.reject(TestError())
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}.catch { (error) -> String in
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XCTAssertTrue(error is TestError)
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return "caught"
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}.then { (result) -> String in
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"\(result) error"
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}
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assertResultEqual(promise, "caught error")
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}
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func testResultHelper() {
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let success = Promise<String> { (handler) in
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handler(Result<String, Never>.success("asdf"))
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}
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assertResultEqual(success, "asdf")
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let failure = Promise<String> { (handler) in
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handler(Result<String, TestError>.failure(TestError()))
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}.catch { (error) -> String in
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"blah"
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}
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assertResultEqual(failure, "blah")
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}
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}
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struct TestError: Error {
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var localizedDescription: String {
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"test error"
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}
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}
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