testGCD_6.swift

        let group: DispatchGroup = DispatchGroup()
        
        let queue1 = DispatchQueue(label: "queue1", attributes: .concurrent)
        queue1.async(group: group) {
            // 事件A
            for i in 1 ... 100 {
                print("i: \(i)")
            }
        }
        let queue2 = DispatchQueue(label: "queue2", attributes: .concurrent)
        queue2.async(group: group) {
            // 事件B
            for j in 101 ... 200 {
                print("j: \(j)")
            }
        }
        
        group.notify(queue: DispatchQueue.main) { 
            // 已處理完事件A和事件B
            print("處理完成事件A和事件B...")
        }