27.3 · Queue ordering, deadlocks and work items

Queue puzzles are about which lines are ordered by the queue and which merely look ordered. Async never runs inline, a serial queue is first in first out, and a stored work item can quietly own your object.

Mental model. Draw the queue. Async appends and returns; sync waits for its turn; a serial queue runs one block at a time, in order; and only what the queue orders is guaranteed.

Order, blocking and ownership

Nested main-queue async, serial and barrier ordering, self-sync traps and work-item cycles.

1. Blocks are scheduled on the main queue from the main thread, and one of them schedules another. In what order do the prints appear? 27.3.1 · L2
   DispatchQueue.main.async never runs its block inline: it appends the block to the main queue and returns at once. The main queue is first in first out and runs…
2. On a serial queue, which prints are guaranteed to be ordered, what does a barrier guarantee, and what happens if a block calls sync on its own queue? 27.3.2 · L3
   A serial queue runs its blocks one at a time in the order they were submitted, so every print inside a block on that queue is ordered relative to the others…
3. A screen stores a DispatchWorkItem whose closure uses self. Does cancelling the item fix the leak, and what does? 27.3.3 · L3
   The object owns the item (a stored property) and the item’s closure owns the object (it uses self strongly): a retain cycle, and dropping your last outside…

More in Puzzles

• 27.1 · Objects, closures and weak references
• 27.2 · defer: what runs, when, and what it can change
• 27.4 · Actor reentrancy, task ordering, SwiftUI identity, and how to answer
• 27.5 · Values, optionals, patterns, order and isolation: ten more checked puzzles