grand central dispatch

Question

ShinehahGnolaum OP

Created Jun ’16

Replies 3

Boosts 0

Participants 3

I was given this code but I don't see the results of the NSLog statement in the debug log window:

dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);

dispatch_async(queue, ^{

NSLog(@"Block for asynchronous execution");

});

I find this code in the very last section of this page:

https://developer.apple.com/library/ios/documentation/Cocoa/Conceptual/ProgrammingWithObjectiveC/WorkingwithBlocks/WorkingwithBlocks.html

Boost

Answer 1

Ken Thomases OP

Jun ’16

In what context are you running the above code? In particular, is this in an app? Or is it in a command-line tool? If it's in a command-line tool, it's common to unthinkingly allow the program to exit (by returning from main()) and thus prevent background threads from completing their work.

0

Answer 2

ShinehahGnolaum OP

Jun ’16

I run that code in main() of a command line tool project for OS X.

What should I do to be able to see the code work?

0

Answer 3

DTS Engineer OP

Apple

Jun ’16

You will need to block the main thread of the command line tool until the async work you’re started via

dispatch_async

is complete. The best approach depends on your specific situation. For example:

You could just a dispatch group (
```
dispatch_group_create
```
), enter the group when you start work (
```
dispatch_group_enter
```
), leave the group when the work is done (
```
dispatch_group_leave
```
), and have the main thread wait for the group to be done (
```
dispatch_group_wait
```
).
You could ‘park’ the main thread in
```
dispatch_main
```
and then have the work, when it’s done, explicitly exit the tool by calling
```
exit
```
.
You could combine these techniques by using a notifying dispatch group (
```
dispatch_group_notify
```
).

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0