Implement a Job Scheduler using Dispatch Queues

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);