2. Using dispatch groups with synchronous wait() method

// 1
DispatchQueue.global(qos: .userInitiated).async {
var storedError: NSError?

// 2
let dispatchGroup = DispatchGroup()

for address in [
   "https://www.ayavn.us/images/8a2d61ef0eeaef65eb5cf5d82d87cc63603dcdea038f925c95d39b59e221611e.jpg",
   "https://www.ayavn.us/images/8a2d61ef0eeaef65eb5cf5d82d87cc63603dcdea038f925c95d39b59e221611e.jpg",
   "https://www.ayavn.us/images/8a2d61ef0eeaef65eb5cf5d82d87cc63603dcdea038f925c95d39b59e221611e.jpg",
] {
   guard let url = URL(string: address) else { return }

   // 3
   dispatchGroup.enter()

   let photo = DownloadRemoteImage(url: url) { _, error in
      storedError = error

      // 4
      dispatchGroup.leave()
   }
}   

// 5      
dispatchGroup.wait()

// 6
DispatchQueue.main.async {
   completion?(storedError)
}   
}

The synchronous wait method blocks the current thread. Thus, you need to use DispatchQueue.main.async to place the entire method into a background queue. This ensures you don’t block the main thread.

Create a new dispatch group.

Call enter() to manually notify the group that a task has started. You must balance out the number of enter() calls with the number of leave() calls, or your app will crash.

Notify the group that this work is done.

Call wait() to block the current thread while waiting for tasks’ completion. This waits forever — which is fine because the photos creation task always completes. You can use wait(timeout:) to specify a timeout and bail out on waiting after a specified time.

At this point, you know that all image tasks have either completed or timed out. You then make a call back to the main queue to run your completion closure.

3. Using dispatch groups with asynchronous method (notify(queue:work:))

var storedError: NSError?

// 2
let dispatchGroup = DispatchGroup()

for address in [
   "https://www.ayavn.us/images/8a2d61ef0eeaef65eb5cf5d82d87cc63603dcdea038f925c95d39b59e221611e.jpg",
   "https://www.ayavn.us/images/8a2d61ef0eeaef65eb5cf5d82d87cc63603dcdea038f925c95d39b59e221611e.jpg",
   "https://www.ayavn.us/images/8a2d61ef0eeaef65eb5cf5d82d87cc63603dcdea038f925c95d39b59e221611e.jpg",
] {
   guard let url = URL(string: address) else { return }

   dispatchGroup.enter()

   let photo = DownloadRemoteImage(url: url) { _, error in
      storedError = error
      dispatchGroup.leave()
   }
}  

// 2    
downloadGroup.notify(queue: DispatchQueue.main) {
   completion?(storedError)
}

You don’t need to put the method in an async call since you’re not blocking the main thread.

notify(queue:work:) serves as the asynchronous completion closure. It runs when there are no more items left in the group.

iOS Swift - Grand Central Dispatch (1)

1. What is Task ?

Is code block
Operates based on FIFO principle

2. Queue

Queue includes many Tasks
Task can be executed sequentially (Serial Queue) or concurrently (Concurrent Queue)

3. DispatchQueue

#### 3.1 Main Queue - For update user interface

DispatchQueue.main.async() {
   //Code Run On Main Thread
}

#### 3.2 Global …

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iOS Swift - Grand Central Dispatch (1)

1. What is Task ?

2. Queue

3. DispatchQueue

Other articles

1. What is Task ?

2. Queue

3. DispatchQueue

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