Wait for a callback to be a given value in Xcode unit tests. As pure Swift objects can't be used with NSPredicate, this can help to test simple results.

XCTestExtensions.swift

//
//  XCTestExtensions.swift
//
//  Created by Antoine van der Lee on 20/05/2017.
//  Copyright © 2017 AvdLee. All rights reserved.
//

import XCTest

extension XCTestCase {
    
    /// Checks for the callback to be the expected value within the given timeout.
    ///
    /// - Parameters:
    ///   - callback: The callback which returns the value.
    ///   - expectedValue: The expected value.
    ///   - timeout: The timeout in which the value should result in the expectedValue.
    func waitFor<T: Equatable>(callback: () -> T, toBe expectedValue: T, withTimeout timeout: TimeInterval) {
        let expectation = self.expectation(description: "Wait for callback to be expectedValue")
        
        let end = Date().addingTimeInterval(timeout)
        
        while callback() != expectedValue && 0 < end.timeIntervalSinceNow {
            if RunLoop.current.run(mode: RunLoopMode.defaultRunLoopMode, before: Date(timeIntervalSinceNow: 0.002)) {
                Thread.sleep(forTimeInterval: 0.002)
            }
        }
        
        if callback() != expectedValue {
            XCTFail("Timed out waiting for callback to be true")
        } else {
            expectation.fulfill()
        }
        
        waitForExpectations(timeout: timeout, handler: nil)
    }
}