I am reading a iOS book which says that "dispatch_sync function blocks the concurrent queue to which the block is submitted i.e it makes the queue wait". Based on that concept I created my own example which is as follows.The following snippet is written in the "viewDidLoad" method
dispatch_queue_t concQueue1 = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
void (^secondBlock)(void) =^( void)
{
//Second Block
for (int count=0; count<1000; count++)
{
if( [NSThread currentThread] == [NSThread mainThread ] )
{
NSLog(@"2 block main Thread") ;
}
else
{
NSLog(@"2 block other THREAD") ;
}
}
};
void (^firstBlock)(void) =^( void)
{
//First Block
for (int count=0; count<100; count++)
{
if( [NSThread currentThread] == [NSThread mainThread ] )
{
NSLog(@"1 block main Thread") ;
}
else
{
NSLog(@"1 block other THREAD") ;
}
}
dispatch_sync(concQueue1, secondBlock) ;
};
dispatch_async(concQueue1,firstBlock);
//Making the main thread sleep for some time
[NSThread sleepForTimeInterval:0.1] ;
dispatch_async( concQueue1, ^(void) {
//Third Block
for (int count=0; count<1000; count++)
{
if( [NSThread currentThread] == [NSThread mainThread ] )
{
NSLog(@"3 block main Thread") ;
}
else
{
NSLog(@"3 block other THREAD") ;
}
}
});
I am making the main thread sleep for some time so that dispatch_sync function in the "first block " gets executed. The output i am getting is this. I am showing the part of the output.
GCDExamples[2459:554259] 2 block other THREAD
.
.
.
GCDExamples[2459:554259] 2 block other THREAD
GCDExamples[2459:554256] 3 block other THREAD
GCDExamples[2459:554256] 3 block other THREAD
GCDExamples[2459:554259] 2 block other THREAD //Point first
GCDExamples[2459:554256] 3 block other THREAD
Some points about the output : The output shown in "3 block other THREAD" and "2 block other THREAD" are the first occurences of that output lines
MY QUESTION: According to the concept because of dispatch_sync function once the second block starts it should make the queue wait rather than allowing "Third block" to start. But as shown in the earlier output "2 block other THREAD" follows "3 block other THREAD" statement at "//Point first" .This shows that the dispatch_sync function did'nt make the queue wait. How is that possible?? .Please ask me any another other infomration if needed.
EDIT 1 : I am putting the text of the well known book here to explain my point . The book is "iOS 7 Programming cookbook". Text follows:- "For any task that doesn’t involve the UI, you can use global concurrent queues in GCD. These allow either synchronous or asynchronous execution. But synchronous execution does not mean your program waits for the code to finish before continuing. It simply means that the concurrent queue will wait until your task has finished before it continues to the next block of code on the queue. When you put a block object on a concurrent queue, your own program always continues right away without waiting for the queue to execute the code. This is because concurrent queues, as their name implies, run their code on threads other than the main thread."
As the bold text says that the concurrent queue would wait UNTIL my task is finished before continuing with the next block. My block printing "2 block other THREAD" should be allowed to finish before "3 block other THREAD" starts, but that is not the case my " 2 block other THREAD" is printed again intermingling with "3 block other THREAD" statement when in fact my all "2 block other THREAD" should be allowed to get completed and then "3 block other THREAD" should follow. Comment if more info is required.
dispatch_get_global_queuereturns a well-known globalconcurrentqueue. Which runs multiple blocks – ryancrunchidispatch_sync(concQueue1, secondBlock) ;until block2 finished. As block1 is async, it waits async. And your block3 runs async too. – ryancrunchi