Objective-c – How to wait for an asynchronously dispatched block to finish

grand-central-dispatchobjective cunit testing

I am testing some code that does asynchronous processing using Grand Central Dispatch. The testing code looks like this:

[object runSomeLongOperationAndDo:^{
    STAssert…
}];

The tests have to wait for the operation to finish. My current solution looks like this:

__block BOOL finished = NO;
[object runSomeLongOperationAndDo:^{
    STAssert…
    finished = YES;
}];
while (!finished);

Which looks a bit crude, do you know a better way? I could expose the queue and then block by calling dispatch_sync:

[object runSomeLongOperationAndDo:^{
    STAssert…
}];
dispatch_sync(object.queue, ^{});

…but that’s maybe exposing too much on the object.

Best Answer

Trying to use a dispatch_semaphore. It should look something like this:

dispatch_semaphore_t sema = dispatch_semaphore_create(0);

[object runSomeLongOperationAndDo:^{
    STAssert…

    dispatch_semaphore_signal(sema);
}];

if (![NSThread isMainThread]) {
    dispatch_semaphore_wait(sema, DISPATCH_TIME_FOREVER);
} else {
    while (dispatch_semaphore_wait(sema, DISPATCH_TIME_NOW)) { 
        [[NSRunLoop currentRunLoop] runMode:NSDefaultRunLoopMode beforeDate:[NSDate dateWithTimeIntervalSinceNow:0]]; 
    }
}

This should behave correctly even if runSomeLongOperationAndDo: decides that the operation isn't actually long enough to merit threading and runs synchronously instead.

Related Solutions

Ios – How to trigger a block after a delay, like -performSelector:withObject:afterDelay:

I think you're looking for dispatch_after(). It requires your block to accept no parameters, but you can just let the block capture those variables from your local scope instead.

int parameter1 = 12;
float parameter2 = 144.1;

// Delay execution of my block for 10 seconds.
dispatch_after(dispatch_time(DISPATCH_TIME_NOW, 10 * NSEC_PER_SEC), dispatch_get_main_queue(), ^{
    NSLog(@"parameter1: %d parameter2: %f", parameter1, parameter2);
});

More: https://developer.apple.com/documentation/dispatch/1452876-dispatch_after

Objective-c – Do you need to create an NSAutoreleasePool within a block in GCD

Does the same rule apply to a block that is placed within a Grand Central Dispatch queue and will be run on a non-main thread? That is, do you need to create an NSAutoreleasePool within each block you dispatch to anything other than the main queue?

Grand central dispatch will manage an autorelease pool per queue automatically. However, there are no guarantees as to when the pool will be drained; it may be after one block is processed, it may be after hundreds (but probably won't be).

So, if you are only allocating a few objects, don't worry about it. However, if you are allocating any significant number of objects (and since you are targeting a memory constrained environment), then you should be creating and draining pools.

The documentation has been updated.

See https://developer.apple.com/library/content/documentation/General/Conceptual/ConcurrencyProgrammingGuide/OperationQueues/OperationQueues.html#//apple_ref/doc/uid/TP40008091-CH102-SW1

If your block creates more than a few Objective-C objects, you might want to enclose parts of your block’s code in an @autorelease block to handle the memory management for those objects. Although GCD dispatch queues have their own autorelease pools, they make no guarantees as to when those pools are drained. If your application is memory constrained, creating your own autorelease pool allows you to free up the memory for autoreleased objects at more regular intervals.

Best Answer

Related Solutions

Ios – How to trigger a block after a delay, like -performSelector:withObject:afterDelay:

Objective-c – Do you need to create an NSAutoreleasePool within a block in GCD

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