example-dispatch-group.swift

import UIKit


// Dispatch Groups

func collectAllReviewIds() {
    
    let group = DispatchGroup()
    var reviewIds: [Int] = []
    
    group.enter()
    getReviewIdsFromGoogle { ids in
        reviewIds.append(contentsOf: ids)
        group.leave()
    }
    
    group.enter()
    getReviewIdsFromYelp { ids in
        reviewIds.append(contentsOf: ids)
        group.leave()
    }
    
    group.enter()
    getReviewIdsFromNextDoor { ids in
        reviewIds.append(contentsOf: ids)
        group.leave()
    }
    
    // example of a function where the function is responsible for calling
    // the enter and leave events
    getReviewsFromAmazon(group: group) { ids in
        reviewIds.append(contentsOf: ids)
    }
   
    group.notify(queue: DispatchQueue.global()) {
        // handle reviewIds
        print(reviewIds)
    }
}

func getReviewsFromAmazon(group: DispatchGroup, completion: @escaping ([Int]) -> Void) {
    group.enter()
    DispatchQueue.main.asyncAfter(deadline: .now() + 2.0) {
        let result = Array(repeating: 0, count: 5).map { _ in
            return Int.random(in: 1...20)
        }
        completion(result)
        group.leave()
    }
}

func getReviewIdsFromGoogle(completion: @escaping ([Int]) -> Void) {
    DispatchQueue.main.asyncAfter(deadline: .now() + 2.0) {
        let result = Array(repeating: 0, count: 5).map { _ in
            return Int.random(in: 1...20)
        }
        completion(result)
    }
}

func getReviewIdsFromYelp(completion: @escaping ([Int]) -> Void) {
    DispatchQueue.main.asyncAfter(deadline: .now() + 2.0) {
        let result = Array(repeating: 0, count: 5).map { _ in
            return Int.random(in: 1...20)
        }
        completion(result)
    }
}

func getReviewIdsFromNextDoor(completion: @escaping ([Int]) -> Void) {
    DispatchQueue.main.asyncAfter(deadline: .now() + 2.0) {
        let result = Array(repeating: 0, count: 5).map { _ in
            return Int.random(in: 1...20)
        }
        completion(result)
    }
}

collectAllReviewIds()