Is copying an NSMutableArray thread-safe?

Question

Let's say I have a mutable array containing some objects in the variable array.

Let's say from thread A I do:

NSArray *arrayCopy = [array copy];

and from thread B:

[array addObject:NSNull.null];

Is it safe? Specifically, can this code crash because of a potential modification of array while it is being copied?

Answer 1

No. Mutating an NSMutableArray is not a thread safe operation.

Answer 2

Apple says that NSMutableArray is not thread-safe. However if you add objects in one thread and copy in other, it won't crash. If you add object from different threads, it would crash.

import Foundation
import XCPlayground

var array = NSMutableArray()
var queues = [NSOperationQueue]()

let syncQueue = NSOperationQueue()
syncQueue.maxConcurrentOperationCount = 1

for i in 0..<100 {
    let queue = NSOperationQueue()
    queue.maxConcurrentOperationCount = 1
    queues.append(queue)

    queue.addOperationWithBlock({
        for j in 0..<100000 {
            if j == 0 {
                sleep(1)
            } else {
                usleep(j < 5 ? 100 : rand() % 10 == 0 ? 1 : 0)
            }

            if i % 2 == 0 {
                syncQueue.addOperationWithBlock({
                    array.addObject(NSNumber(int: rand()))
                })
            } else {
                var copy = array.copy()
            }
        }
    })
}

XCPSetExecutionShouldContinueIndefinitely()

However, I didn't find documentation, where it will be described (that copy function is thread safe on NSMutableArray). If someone provides a link, I'll appreciate.