Swift 3 multithreading using which queue? - ios

Question

I'm going through Stanford CP 193P, looking at a Twitter client.
When a network is called, I assumed it would always be called on the main queue unless invoked on another queue. However without dispatch back onto the main queue (as below) the App does not work as expected - meaning we must not be on the main queue. How?
When tweets are fetched the following closure is used - and to update the UI means that the work needs to be done on the main thread (DispatchQueue.main.async)
request.fetchTweets { [weak self] (newTweets) in
DispatchQueue.main.async {
if request == self?.lastTwitterRequest {
self?.tweets.insert(newTweets, at: 0)
self?.tableView.insertSections([0], with: .fade)
}
}
}
This calls a convenience function that is commented as "handler is not necessarily invoked on the main queue". I can't find anywhere that declares which queue it is invoked on, so I assume it is on the main queue?
// convenience "fetch" for when self is a request that returns Tweet(s)
// handler is not necessarily invoked on the main queue
open func fetchTweets(_ handler: #escaping ([Tweet]) -> Void) {
fetch { results in
var tweets = [Tweet]()
var tweetArray: NSArray?
if let dictionary = results as? NSDictionary {
if let tweets = dictionary[TwitterKey.Tweets] as? NSArray {
tweetArray = tweets
} else if let tweet = Tweet(data: dictionary) {
tweets = [tweet]
}
} else if let array = results as? NSArray {
tweetArray = array
}
if tweetArray != nil {
for tweetData in tweetArray! {
if let tweet = Tweet(data: tweetData as? NSDictionary) {
tweets.append(tweet)
}
}
}
handler(tweets)
}
}
I did not write the Twitter framework, and it appears to have been authored by the Stanford instructor.

Answer 1

You ask:
This calls a convenience function that is commented as "handler is not necessarily invoked on the main queue". I can't find anywhere that declares which queue it is invoked on, so I assume it is on the main queue?
No, you cannot assume it is on the main queue. In fact, it sounds like it's explicitly warning you that it isn't. The only time you can be assured it's on the main queue, is if it explicitly says so.
For example, if the underlying framework is using URLSession, it, by default, does not use the main queue for its completion handlers. The init(configuration:​delegate:​delegate​Queue:​) documentation warns us that the queue parameter is as follows:
An operation queue for scheduling the delegate calls and completion handlers. The queue should be a serial queue, in order to ensure the correct ordering of callbacks. If nil, the session creates a serial operation queue for performing all delegate method calls and completion handler calls.
And for a given framework, it may be completely unrelated to URLSession queue behavior. It might also be using its own queues for completion handlers.
Bottom line, if the framework doesn't explicitly assure you that the closure always runs on the main queue, you should never assume it does. So, yes, in the absence of any assurances to this effect, you'd want to dispatch any UI stuff to the main queue and do the appropriate synchronization for any model objects.
You can, if you have code that must run on a particular thread and you want to make sure this is the case, you can add a dispatchPrecondition to test if it's on the main thread. The behavior of this changes between debug builds and release builds, but it's a quick way of quickly testing if it's using the queue you think it is:
dispatchPrecondition(condition: .onQueue(.main))