ios - DispatchGroup doesn't work as expected -

i have made dispatchgroup , run 2 async tasks. 1 on main , other 1 on global().

as long understand, dispatchgroup.notify's block should invoked once tasks done doesn't work thought.

class que {     let group = dispatchgroup()      init() {         group.notify(queue: .main) {         print("group done")         }     }      func run() {         doc()         dod()     }      fileprivate func doc() {         group.enter()         dispatchqueue.main.async(group: group) {             var rst = 0             idx in 0 ..< 500 {                 rst += idx             }             print("work item c done")             self.group.leave()         }     }      fileprivate func dod() {         group.enter()         dispatchqueue.global().async(group: group) {             var rst = 0             idx in 0 ..< 50 {                 rst += idx             }             print("work item d done")             self.group.leave()         }     } } 

the result is

work item d done group done work item c done 

i want know why not

work item d done work item c done group done 

if made c , d task run on global() queue, worked.

you put call notify in wrong place , called far early.

most want call notify @ end of run method.

init() { }  func run() {     doc()     dod()      group.notify(queue: .main) {         print("group done")     } } 

note says in documentation notify method:

schedules work item submitted queue when group of previously submitted block objects have completed.

the bold mine.