ConditionTester.swift

import Foundation

// Swift translation of https://pspdfkit.com/blog/2016/running-ui-tests-with-ludicrous-speed/
class ConditionTester {
    typealias TestCondition = () -> (Bool)
    
    private let condition: TestCondition
    private var fulfilled: Bool { return condition() }
    private var currentRunLoop: CFRunLoop {
        return RunLoop.current.getCFRunLoop()
    }
    
    init(condition: @escaping TestCondition) {
        self.condition = condition
    }
    
    func wait(for timeout: TimeInterval) -> Bool {
        let timer = getRepeatingTimer()
        timer.resume()
        
        let observer = getRunLoopObserver(checkingCondition: condition)
        CFRunLoopAddObserver(currentRunLoop, observer, CFRunLoopMode.defaultMode)
        CFRunLoopRunInMode(.defaultMode, timeout, false)
        CFRunLoopRemoveObserver(currentRunLoop, observer, CFRunLoopMode.defaultMode)
        
        timer.cancel()
        
        return fulfilled
    }
    
    private func getRepeatingTimer() -> DispatchSourceTimer {
        let timer: DispatchSourceTimer = DispatchSource.makeTimerSource(queue: getCurrentQueue())
        timer.scheduleRepeating(
            deadline: .now(),
            interval: .milliseconds(50),
            leeway: .milliseconds(50)
        )
        timer.setEventHandler { /* noop */ }
        return timer
    }
    
    private func getCurrentQueue() -> DispatchQueue {
        let currentQueueName = String(validatingUTF8: __dispatch_queue_get_label(nil)) ?? ""
        return DispatchQueue(label: currentQueueName)
    }
    
    private func getRunLoopObserver(checkingCondition condition: @escaping TestCondition) -> CFRunLoopObserver {
        let observer = CFRunLoopObserverCreateWithHandler(kCFAllocatorDefault, CFRunLoopActivity.beforeWaiting.rawValue, true, 0) { _ in
            if self.fulfilled {
                CFRunLoopStop(self.currentRunLoop)
            }
        }
        return observer!
    }

}