Implement a Job Scheduler using Dispatch Queues - objective-c

Question

Question: Implement a job scheduler which takes in function f and int n and calls f after n seconds.
Since im using Obj C to answer this question, I should probably use dispatch queues instead of selectors or function pointers.
Here's my attempt to implement the answer, but it's not returning a time stamp:
void jobSch(dispatch_queue_t queue, int64_t n, dispatch_block_t f)
{
NSLog(#"%d\n", n);
dispatch_after(dispatch_time(DISPATCH_TIME_NOW, n * NSEC_PER_SEC), dispatch_get_main_queue(), f);
}
void (^aBlock)(void) = ^(){
NSDateFormatter *dateFormatter=[[NSDateFormatter alloc] init];
[dateFormatter setDateFormat:#"yyyy-MM-dd HH:mm:ss"];
NSLog(#"%#",[dateFormatter stringFromDate:[NSDate date]]);
};
int main(int argc, const char * argv[])
{
#autoreleasepool {
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, ^{
jobSch(queue, 3, aBlock);
});
dispatch_group_wait(group, DISPATCH_TIME_FOREVER);
}
return 0;
}
What could I be doing wrong?
Also my intended implementation would be able to call jobSch(queue, 10, aBlock) at runtime 0 and jobSch(queue, 3, aBlock) at runtime 2, so that would mean the second call would print before the first.

Answer 1

The problem is that main is exiting long before your enqueued block is run. And that is because your call to dispatch_group_wait isn't waiting. And that is because your dispatch group is empty.
In order for a dispatch group and a call to dispatch_group_wait to be useful, you need to make paired calls to dispatch_group_enter and dispatch_group_leave.
You need to call dispatch_group_enter before you start any async call and you need to call dispatch_group_leave after the block has finished (aBlock in this case).
Your code is not setup to make this easy. You would need to make group a global variable. Then you can call dispatch_group_enter and dispatch_group_leave where needed.
Something like this should work:
dispatch_group_t group;
void jobSch(dispatch_queue_t queue, int64_t n, dispatch_block_t f)
{
NSLog(#"%d\n", n);
dispatch_group_enter(group);
dispatch_after(dispatch_time(DISPATCH_TIME_NOW, n * NSEC_PER_SEC), queue, f);
}
void (^aBlock)(void) = ^(){
NSDateFormatter *dateFormatter=[[NSDateFormatter alloc] init];
[dateFormatter setDateFormat:#"yyyy-MM-dd HH:mm:ss"];
NSLog(#"%#",[dateFormatter stringFromDate:[NSDate date]]);
dispatch_group_leave(group);
};
int main(int argc, const char * argv[])
{
#autoreleasepool {
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
group = dispatch_group_create();
// no need for dispatch_group_async here
jobSch(queue, 10, aBlock);
jobSch(queue, 3, aBlock);
dispatch_group_wait(group, DISPATCH_TIME_FOREVER);
}
return 0;
}
Also note that you never actually make use of queue inside jobSch.
You need to change:
dispatch_after(dispatch_time(DISPATCH_TIME_NOW, n * NSEC_PER_SEC), dispatch_get_main_queue(), f);
to:
dispatch_after(dispatch_time(DISPATCH_TIME_NOW, n * NSEC_PER_SEC), queue, f);