Asynchronous Operations in iOS with Grand Central Dispatch

Some times while you read the news from any news app the images related to that articles downloads slowly but before that you can read the news articles, this can be possible by using multiple thread operation using GCD

Grand Central Dispatch, or GCD for short, is a C API that makes it exceptionally easy to perform asynchronous operations in iOS. Asynchronous operations are a fundamental part of every iOS app when you want to perform long operations without freezing or blocking the user interface. You can image that if your entire app froze without warning then your users would get quite irritated.

GCD queues can be concurrent or serial but serial queues (where there is one queue and each item is executed one after the other in order) are easier to understand so we’ll look at those.
The dispatch_get_current_queue function will return the current queue from which the block is dispatched and the dispatch_get_main_queue function will return the main queue where your UI is running.

The dispatch_get_main_queue function is very useful for updating the iOS app’s UI as UIKit methods are not thread safe (with a few exceptions) so any calls you make to update UI elements must always be done from the main queue.

A typical GCD call would look something like this:

// Doing something on the main thread

dispatch_queue_t myQueue = dispatch_queue_create("My Queue",NULL);
dispatch_async(myQueue, ^{
// Perform long running process

dispatch_async(dispatch_get_main_queue(), ^{
// Update the UI

});
});

GCD relies on block so it has a really nice, readable syntax. It becomes clear what happes in the background thread and the main thread. For example, here’s how you might load a few images:

NSArray *images = @[@"http://mycodetips.com/sampleimage1.png",
@"http://mycodetips.com/sampleimage2.png",
@"http://mycodetips.com/sampleimage3.png",
@"http://mycodetips.com/sampleimage4.png"];

dispatch_queue_t imageQueue = dispatch_queue_create("Image Queue",NULL);

for (NSString *urlString in images) {
dispatch_async(imageQueue, ^{

NSURL *url = [NSURL URLWithString:urlString];
NSData *imageData = [NSData dataWithContentsOfURL:url];
UIImage *image = [UIImage imageWithData:imageData];

NSUInteger imageIndex = [images indexOfObject:urlString];
UIImageView *imageVIew = (UIImageView *)[self.view viewWithTag:imageIndex];

if (!imageView) return;

dispatch_async(dispatch_get_main_queue(), ^{
// Update the UI
[imageVIew setImage:image];
});

}); 
}

DISPATCH_QUEUE_PRIORITY_HIGH Items dispatched to the queue will run at high priority, i.e. the queue will be scheduled for execution before any default priority or low priority queue.

DISPATCH_QUEUE_PRIORITY_DEFAULT Items dispatched to the queue will run at the default priority, i.e. the queue will be scheduled for execution after all high priority queues have been scheduled, but before any low priority queues have been scheduled.

DISPATCH_QUEUE_PRIORITY_LOW Items dispatched to the queue will run at low priority, i.e. the queue will be scheduled for execution after all default priority and high priority queues have been scheduled.

DISPATCH_QUEUE_PRIORITY_BACKGROUND Items dispatched to the queue will run at background priority, i.e. the queue will be scheduled for execution after all higher priority queues have been scheduled and the system will run items on this queue on a thread with background status as per setpriority(2) (i.e. disk I/O is throttled and the thread’s scheduling priority is set to lowest value).