Promise.swift

//
//  Promise.swift
//  ShellFish
//
//  Created by Anders Borum on 25/02/2019.
//  Copyright © 2019 Anders Borum. All rights reserved.
//

import Foundation

// Helper functions to go from asyncronous code to syncronous by waiting on a condition triggered by
// the completion handler.
public class Promise<Result> {
    
    private let condition = NSCondition()
    private var result: Result?
    
    public init() {}
        
    public var completionHandler: (Result) -> Void {
        return { result in
            self.condition.lock()
            
            self.result = result
            
            self.condition.signal()
            self.condition.unlock()
        }
    }
    
    // blocks until completionHandler has been called and we shouldn't
    // call this from the main thread
    public var value: Result {
        assert(!Thread.isMainThread)
        
        condition.lock()
        while result == nil {
            condition.wait()
        }
        condition.unlock()
        
        return result!
    }

    // blocks until completionHandler has been called and can be called from
    // main thread but you will get deadlocks if the work we are waiting to
    // complete will itself block the main thread
    public var valueEvenMain: Result {        
        condition.lock()
        while result == nil {
            condition.wait()
        }
        condition.unlock()
        
        return result!
    }

    // used to provide result when timeout passes, which can be used for
    // timeout errors and default values
    public func scheduleTimeout(_ timeout: TimeInterval, value: Result) {
        DispatchQueue.main.asyncAfter(deadline: .now() + timeout) { [weak self] in
            self?.completionHandler(value)
        }
    }
}

public extension Promise where Result == Error? {
    // throw if result is non-nil Error
    func attempt() throws {
        let error = value
        if error != nil {
            throw error!
        }
    }
}