应用场景：一个页面有6个请求，请求完毕后统一刷新UI

func initData() -> Void {
        let group = DispatchGroup()
        let queue = DispatchQueue.global()
        
        queue.async(group: group, execute: {
            group.enter()
            
            queue.asyncAfter(deadline: .now() + 4) {
                
                uLog("11111111")
                group.leave()
            }
        })
        queue.async(group: group, execute: {
            group.enter()
            
            queue.asyncAfter(deadline: .now() + 1) {
                
                uLog("222")
                group.leave()
            }
        })
        queue.async(group: group, execute: {
            group.enter()
            
            queue.asyncAfter(deadline: .now() + 6) {
                
                uLog("3333")
                group.leave()
            }
        })
        queue.async(group: group, execute: {
            group.enter()
            
            queue.asyncAfter(deadline: .now() + 10) {
                
                uLog("4444")
                group.leave()
            }
        })
        queue.async(group: group, execute: {
            group.enter()
            uLog("555")
            group.leave()
        })
        
        
        group.notify(queue: queue) {
            uLog("完结咯，刷新UI")
        }
    }

打印结果：
5，2，1，3，4，完结

也可用信号量，感觉没有这样简单