Question

Let’s say that I want to keep things nice and speedy in the main UI, so I break off slow parts into queues (using the global concurrent queues). Assume that selectedUser in this case remains static throughout.

In one View Controller I have something like this:

- (IBAction)buttonPressed:(id)sender {

    User *selectedUser = [self getSelectedUser];

    dispatch_queue_t queue;
    queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
    dispatch_async(queue, ^{
        @autoreleasepool {

            [userManager doSomething:selectedUser withForceOption:YES];

        }

    });

}

And in another class I have a singleton defined (userManager), with a method like this:

- (void)doSomething:(User*)user withForceOption:(BOOL)force {

    SAppDelegate *delegate = (SAppDelegate *)[UIApplication sharedApplication].delegate;
    extlib_main_queue = delegate.extlib_main_queue;

    dispatch_async(extlib_main_queue, ^{
        @autoreleasepool {
            extlib_status_t status;
            user.opIsStarting = YES;

            extlib_call_id callId = -1;

            // this is the part that worries me:
            extlib_str_t uri = extlib_str((char *) [[NSString stringWithFormat:@"http:%@@%s", user.account,DOMAIN] UTF8String]);
            status = extlib_call_make_call(0, &uri, 0, NULL, NULL, &callId);
        }

    });
}

My question is: is it safe to do this, or do I need to do something else to make sure that the passed User instance’s parameters remain accessible to both blocks?

Answer 1

You have nothing to worry about, because blocks retain the variables that they refer to.

The block in buttonPressed: retains selectedUser since the block refers to it, and the block in doSomething:withForceOption: retains user because the block in there refers to it as well.

Read this section of Blocks Programming Topics for more details on how this works.