Can you explain what the term Dispatch Barrier means in the context of multithreading and thread synchronization?

Dispatch Barrier is a mechanism in multithreading used in GCD (Grand Central Dispatch) on iOS/macOS for synchronizing access to shared resources.

When you submit a task with a barrier (dispatch_barrier_async), it guarantees that all previously queued tasks will complete before it executes, and subsequent tasks will only start after its completion. This allows safe execution of write or modify operations on data, preventing race conditions and ensuring order.

Example of usage:

let queue = DispatchQueue(label: "com.example.myqueue", attributes: .concurrent)

// Reading
queue.async {
    print("Reading data")
}

// Writing with barrier
queue.async(flags: .barrier) {
    print("Writing data — executed exclusively")
}

// Reading
queue.async {
    print("Reading data")
}

Thus, Dispatch Barrier enables safe access to resources in a concurrent environment.