Nested dispatch queues not firing inner dispatch queue-CodePudding

I have a pair of nested dispatch queues. For some reason the code inside the second one never gets fired:

dispatchGroupOne.notify(queue: .main) { [weak self] in
  // Gets here
  self?.dispatchGroupTwo.notify(queue: .main) {
    // Doesn't get here
  }
}

Even if I don't enter/leave dispatchGroupTwo, it just never will get fired. Why is this? It works when it's not nested:

dispatchGroupOne.notify(queue: .main) {
  // Gets here
}
dispatchGroupTwo.notify(queue: .main) {
  // Gets here
}

However, I want to specifically perform code only after both of the dispatch groups have been fired.

CodePudding user response：

Without nesting you register the listen separately so every group will act according to it's submitted tasks ,while when you nest them , then the notify of the inner group will depend on the outer one plus whether or not the it's (the inner group) tasks ended/working when it's notify is added upon triggering the outer notify

    let dispatchGroupOne = DispatchGroup()
     
    let dispatchGroupTwo = DispatchGroup()
    
    let dispatchGroupThird = DispatchGroup()
     
    dispatchGroupThird.enter() 
    dispatchGroupOne.notify(queue: .main) {
       // Gets here
        dispatchGroupThird.leave()
    }
    
    dispatchGroupThird.enter()
    dispatchGroupTwo.notify(queue: .main) {
      // Gets here
        dispatchGroupThird.leave() 
    }
    
    dispatchGroupThird.notify(queue: .main) {
       // All groups are done
    }