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To ensure safe access to an array in a multithreaded environment in iOS (Swift), synchronization mechanisms are typically used to prevent race conditions and data corruption.

Main approaches:

1. Serializing access via DispatchQueue

let queue = DispatchQueue(label: "com.example.arrayQueue")
var array: [Int] = []

func appendElement(_ element: Int) {
    queue.sync {
        array.append(element)
    }
}

func getElement(at index: Int) -> Int? {
    return queue.sync {
        guard array.indices.contains(index) else { return nil }
        return array[index]
    }
}

2. Using NSLock or other locking primitives

let lock = NSLock()
var array: [Int] = []

func appendElement(_ element: Int) {
    lock.lock()
    array.append(element)
    lock.unlock()
}

3. Using a concurrent queue with barriers (Dispatch barriers) for reading/writing

This allows parallel reading, but writing blocks the queue.

The key is to control access to the array via synchronization to prevent simultaneous modifications from different threads.