global_queue with qos_class_user_interactive

问题

I try to understand GCD and wrote this code to find out run priority:

  override func viewDidLoad() {
    super.viewDidLoad()

    fetchImage()
    print(1)

    dispatch_async(dispatch_get_global_queue(QOS_CLASS_USER_INITIATED, 0)) {
      print(2)
    }

    dispatch_async(dispatch_get_main_queue()) {
      print(3)
    }
    dispatch_async(dispatch_get_global_queue(QOS_CLASS_USER_INTERACTIVE, 0)) { 
      print(5)
    }
  }

I got next result in the console:

1

2

5

3

So the question is:

Part 1: Why 3 is after 5 (main_queue has highest priority?)

Part 2: And why 2 is higher that 3 and 5 as well?

Thank you guys!

回答1:

Bear in mind: this is multi-threading, on a multi-core device, writing output to a log that you don't know the thread safety and internal management of...

That said:

1 is first because it's synchronous
2 is second because it's also synchronous
3 is not next because it's pushed into the queue of things waiting to run on the main thread run loop and you don't know what else is already in that queue
5 is before 3 because it's (basically) the same priority but it's running on a queue that probably does't have anything else waiting (QOS_CLASS_USER_INTERACTIVE ~= main thread priority)

Note, I say ~= because I haven't checked the exact values and it may differ slightly though I expect the priority values to match, otherwise 'interactive' wouldn't mean much...

来源：https://stackoverflow.com/questions/36735946/global-queue-with-qos-class-user-interactive

标签

ios

swift

grand-central-dispatch