Waiting for multiple asynchronous download tasks

Question

I want to download some files, for example 100 files, at the same time. So I decided to add my download threads to a dispatch queue, and GCD will adjust how many threads will run at the same time.

The problem here is: the block in dispatch_async will be completed immediately, because task will run on another thread. So, if urls's length is 100, it will create 100 threads immediately.

var queueDownloadTask = dispatch_queue_create("downloadQueue", nil)

for url in urls {
    dispatch_async(queueDownloadTask) {

        let config = NSURLSessionConfiguration.defaultSessionConfiguration()
        let fileTransferSession = NSURLSession(configuration: config)

        let task = fileTransferSession.downloadTaskWithURL(url, completionHandler: { (responseUrl, response, error) -> Void in
            println("completed")
        })
        task.resume()
    }
}

How can I configure the block in dispatch_async to wait for the download task to complete? I don't want use dispatch_semaphore, because it only allow run one download task at the same time.

Answer 1

In Swift 4,

func executeMultiTask() {
     //1. Create group
     let taskGroup = DispatchGroup()

     //2. Enter group
     taskGroup.enter()
     myTask1.execute(completeHandler: {
         // ...
         //3. Leave group
         taskGroup.leave() //< balance with taskGroup.enter()
     })

     /* Add more tasks ...
     //2. Enter group
     taskGroup.enter()
     myTask2.execute(completeHandler: {
         //3. Leave group
         defer {
            // Use `defer` to make sure, `leave()` calls are balanced with `enter()`.
            taskGroup.leave() 
         }
         // ... more
     })
     */

     //4. Notify when all task completed at main thread queue.
     taskGroup.notify(queue: .main) { 
         // All tasks are done.
         // ...   
     }

}