NSBlockOperation and the start method

Question

In reviewing my code, I've been seeing that in many places I have been making the assumption that calling [NSBlockOperationInstance start]; will start this operation on the main thread. I don't know why I thought this, but I shouldn't have been so sure any way. I checked the documentation but couldn't find any explicit mention of the thread the block would run on. However, asserting assert([NSThread isMainThread]); in the main body of the block does pass every time using start, so I'm not sure if this is a coincidence. Any one have more solid understanding of how this would work?

I forgot to mention that [op start] is being called on the main thread.

Answer 1

OK, it all depends on where you call start(). While NSBlockOperation will farm out blocks to other threads, start() is synchronous, and will not return until all the blocks that have been given to NSBlockOperation have completed.

While NSBlockOperation will concurrently execute the blocks it is given, NSBlockOperation itself is NOT concurrent (i.e., isConcurrent is false). Thus, according to the documentation, start() will execute in its entirety in the thread of the caller to start().

Since the thread that calls start() will not return until all the blocks have executed, it makes sense to let the calling thread be involved in the thread pool that is executing the concurrent blocks. That is why you will see some blocks executing in the thread that called start().

If you are seeing a block execute in the main thread, then you must have called it from the main thread.

On a related note, if your NSBlockOperation contains a single block, than that block will always execute in the calling thread.

Remember, if you want a NSOperation to be fully concurrent, you must implement the appropriate functionality in a subclass.

Barring that, you can give any NSOperation to a NSOperationQueue, and it will execute concurrently, because the NSOperation is given to a queue, and the thread running the operation calls start().

Personally, I do not see any advantage in using NSBlockOperation over dispatch_async() unless I need to use its features. If you are only executing one block, just call

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{ });

If you want to utilize the features of NSBlockOperation, but you do not want to wait for them to complete in the current calling thread, it still makes sense to do this...

// Add lots of concurrent blocks
[op addExecutionBlock:^{ /*whatever*/ }];
// Execute the blocks asynchronously
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
    [op start];
    // Now do what you want after all the concurrent blocks have completed...
    // Maybe even tell the UI
    dispatch_async(dispatch_get_main_queue(), ^{
        // Update the UI now that all my concurrent blocks have finished.
    });
});

EDIT To address your comment to tc's answer...

If you call

op = [NSBlockOperation blockOperationWithBlock:^{assert([NSThread isMainThread])}];
[op start];

from the main thread, then there are some guarantees, and some high probabilities.

First, you are guaranteed that [op start] will run to completion in the calling thread. That's because NSBlockOperation does not override the default behavior of NSOperation that specifies it is NOT a concurrent operation.

Next, you have a very high probability that if the NSBlockOperation only has one block, that it will run in the calling thread. You have almost the same probability that the first block will run in the calling thread.

However, the above "probabilities" are not guarantees (only because the documentation does not say it). I guess, some engineer may find some reason to spin that single block to one of the concurrent queues, and just have the calling thread join on the operation that is executing in another thread... but I highly doubt that.

Anyway, maybe your confusion comes from the fact that the documentation for NSBlockOperation says it executes block concurrently, which it does. However, the operation itself is not concurrent, so the initial operation is synchronous. It will wait for all blocks to execute, and it may (or may not) execute some of them on the calling thread.

While there is no guarantee, I find it highly unlikely that a NSBlockOperation with only one block will do anything other than execute on the calling thread.