Question

I am looking for an equivalent pattern in multi-threading in iOS as there is in .NET for pulse and wait. Essentially, I want a background thread to lay dormant until a flag is set, which essentially “kicks” the thread into action.

It is an alternative to a loop+thread.sleep(). The flag can be set on a different thread than the actual background thread doing the processing. Thanks!

Answer 1

There are a variety of different mix-and-match thread APIs available on iOS and OS X. What are you using to create your thread?

The simplest suggestion is to use a Grand Central Dispatch (GCD) semaphore.

Setup code:

dispatch_semaphore_t semaphore = dispatch_semaphore_create(0);
// Then make sure your thread has access to this semaphore

Thread code:

dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
// Will block forever until the semaphore is triggered

Trigger code:

dispatch_semaphore_signal(semaphore);

An even better suggestion: GCD already manages its own thread pool, so just take advantage of it instead of spinning up your own thread. It’s very easy to use dispatch_async to run some code in a background thread.