[swift-corelibs-dev] DispatchQueue and Threads

Scott Lessans scottl021 at mac.com
Fri Dec 22 17:20:37 CST 2017

Those GCD examples make sense to me on the surface actually...

print(Thread.isMainThread) // true (actually on the main thread)
DispatchQueue.main.async {
    print(Thread.isMainThread) // false (on some libdispatch/system owned thread)

The following makes sense as the main DispatchQueue is not necessarily being executed on the main thread — it is likely being executed on a pre-allocated thread or thread pool managed by libdispatch (or the thread dispatchMain is called on I believe).

However, when you move into sync, what you are actually doing is momentarily blocking the existing thread and preventing the queue from executing anymore until your block is over. So in:

print(Thread.isMainThread) // true
DispatchQueue.main.sync {
    print(Thread.isMainThread) // true

Since you are on the main thread when you call sync, dispatch attempts to block the thread until execution of the block is finished. Dispatch will attempt to optimize sync calls by running them on the thread they are called from when possible (since there is no point moving to a different thread and tying up the calling thread to do nothing but wait) (see bottom of https://developer.apple.com/documentation/dispatch/dispatchqueue/1452870-sync <https://developer.apple.com/documentation/dispatch/dispatchqueue/1452870-sync>).

I believe both of these come down to where dispatchMain() is called (but somebody with deeper expertise on libdispatch feel free to jump in an correct anything erroneous).


> On Dec 22, 2017, at 12:57 PM, swift-corelibs-dev-request at swift.org wrote:
> DispatchQueue and Threads

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