Why Dispatch_group_notify works different in different environment? - ios

Question

The first situation is that I create a Command Line Tool Application,and run this code.
NSLog(#"Main:%#", [NSThread currentThread]);
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_group_t group = dispatch_group_create();
dispatch_group_async(group, queue, ^{
NSLog(#"Task1:%#", [NSThread currentThread]);
});
dispatch_group_async(group, queue, ^{
NSLog(#"Task2:%#", [NSThread currentThread]);
});
dispatch_group_notify(group, dispatch_get_main_queue(), ^{
NSLog(#"Finish:%#", [NSThread currentThread]);
});
The log in terminal is
Main:<NSThread: 0x1028033b0>{number = 1, name = main}
Task2:<NSThread: 0x10040f0f0>{number = 2, name = (null)}
Task1:<NSThread: 0x1006008d0>{number = 3, name = (null)}
If I want to show last log in queue and replace the main queue
dispatch_group_notify(group, dispatch_get_main_queue(), ^{
NSLog(#"Finish:%#", [NSThread currentThread]);
});
with queue
dispatch_group_notify(group, queue, ^{
NSLog(#"Finish:%#", [NSThread currentThread]);
});
The terminal print the last log.But why it can't revoke in Main queue?
When i copy this code to simple iOS Application.All works well:
Main:<NSThread: 0x600000070ac0>{number = 1, name = main}
Task2:<NSThread: 0x6000002633c0>{number = 3, name = (null)}
Task1:<NSThread: 0x600000263480>{number = 4, name = (null)}
MainFinish:<NSThread: 0x600000070ac0>{number = 1, name = main}
And I try to add sleep(1) over Task1 in 'Command Tool Line', but it seems block the queue and only print log:Task2.... But this all works well in simple iOS Application.
Why lead to these different?

Answer 1

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Unlike other queues that active on created,you should call dispatch_main() method to execute blocks submitted to main thread
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The reason for the same code runs well in iOS Application is that the application start a default runloop which execute the task submitted to the main queue according to a certain rule like UI update notification.
The reference as follow:
swift-corelibs-libdispatch
Concurrency Programming Guide