Concurrency¶

If you want to get a refresher of basic concurrency concepts please read through the os\architecture\concurrency.md file. As this specifically discusses on Swift iOS / Apple SDK implementation code snippets.

Threads Concurrency¶

GCD is a low-level C-based API.¶

NSOperation and NSOperationQueue are Objective-C classes. NSOperationQueue is objective C wrapper over GCD. If you are using NSOperation, then you are implicitly using Grand Central Dispatch.

GCD advantage over NSOperation: i. implementation For GCD implementation is very light-weight NSOperationQueue is complex and heavy-weight

Main thread

DispatchQueue.main.async  { 
        self.tableView.reloadData()
}

DispatchQueue.global(qos: .background).async {

}

NSOperation Operation Queue¶

NSOperation advantages over GCD:

i. Control On Operation you can Pause, Cancel, Resume an NSOperation

ii. Dependencies you can set up a dependency between two NSOperations operation will not started until all of its dependencies return true for finished.

iii. State of Operation can monitor the state of an operation or operation queue. ready ,executing or finished

iv. Max Number of Operation you can specify the maximum number of queued operations that can run simultaneously

When to Go for GCD or NSOperation when you want more control over queue (all above mentioned) use NSOperation and for simple cases where you want less overhead (you just want to do some work "into the background" with very little additional work) use GCD

ref: https://cocoacasts.com/choosing-between-nsoperation-and-grand-central-dispatch/ http://iosinfopot.blogspot.in/2015/08/nsthread-vs-gcd-vs-nsoperationqueue.html http://nshipster.com/nsoperation/

SO

Detailed code

Operation Queue¶

Under the hood, it is implemented under GCD.

Heavy lifting Abstraction on top of Dispatch Queue

let mainQueue = OperationQueue.main

let customQueue = OperationQueue()
customQueue.maxConcurrentOperationCount = 30

Dispatch Group¶

let group = DispatchGroup()

group.enter() group.leave()

group.notify(queue: .main, execute: {

})

Concurrency in iOS

Race Condition¶

SO

Semaphores¶

DispatchSemaphore

Learn¶

More about race conditions and avoiding critical reads / writes sections in the memory at once. Multithreading - checks for whether certain things needed to be checked for thread safety overall. The interview basically asked me all the multithreaded questions which I didn't know an answer for.

Good sources¶

Free code camp Concurrency

https://www.avanderlee.com/swift/async-await/

https://www.advancedswift.com/async-await/

https://medium.com/swlh/an-in-depth-guide-to-operation-and-operationqueue-45658a22ee37

https://www.appsloveworld.com/swift/100/174/using-dispatch-async-to-load-images-in-background