Difference between synchronous and asynchronous calls in Objective-C

Ans : 

Synchronous : This call means task will be executed in order.
Asynchronous : This call means task may or may not be executed in order.

When call is called synchronously, then thread that initiated that operation will be wait to current task to be finished.
When call is called asynchronously, then it will not wait.

If we want to do some task without harassing UI, we can do those tasks in background thread. This goal is to keep free main thread, so it continuously respond UI event. So we can dispatch our task in background state asynchronously.

So for do task in background thread, we will divide in 2 parts.

1. GCD - Grand Central Dispatch. By using GCD, you have to grab one of global background queue or create your own background queue.

// one of the global concurrent background queues
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);

// or you could create your own serial background queue:
// dispatch_queue_t queue = dispatch_queue_create("com.iosiqa.app.queuename", 0);

2. Dispatch your task to that queue asynchronously

dispatch_async(queue, ^{
    // task that to be done in background and it may be slow
});

The pattern for operation queues is very similar. Create an operation queue and add operations to that queue.

Lets see example :

Asynchronous call with Multithreading :

// Methods gets called in different thread and does not block the current thread.
[NSURLConnection sendAsynchronousRequest:request
                                   queue:queue
                       completionHandler:
    ^(NSURLResponse *response, NSData *data, NSError *error) {
}];

Synchronous call with Multithreading (not so useful):

//Do something
dispatch_sync(queue, ^{
    //Do something else // work in another queue or thread
});

//Do some more task