How to make one operation wait for another to complete using addDependency()

Swift version: 5.6

Paul Hudson @twostraws October 29th 2019

When working with multiple instances of Operation, you’ll often want to queue up work that needs to be performed sequentially rather than all at once. If you want one operation to wait for another to complete before it starts, regardless of which operation queue either one is running on, you should use addDependency() to make the sequence clear to the system.

As an example, we could create two instances of BlockOperation that each print messages and pause a little:

let operation1 = BlockOperation {
    print("Operation 1 is starting")
    Thread.sleep(forTimeInterval: 1)
    print("Operation 1 is finishing")
}

let operation2 = BlockOperation {
    print("Operation 2 is starting")
    Thread.sleep(forTimeInterval: 1)
    print("Operation 2 is finishing")
}

If we added those directly to an operation queue, they would both start running immediately. However, we could tell operation2 that it needs to wait for operation1 to complete, like this:

operation2.addDependency(operation1)

Now if we add the operations to a queue they will execute sequentially rather than in parallel:

print("Adding operations")
let queue = OperationQueue()
queue.addOperation(operation1)
queue.addOperation(operation2)
queue.waitUntilAllOperationsAreFinished()
print("Done!")

You can add dependencies across operation queues if you need, which means you can queue up work to run in the background, then the main thread, then back to the background again without causing problems. So, we could rewrite the above code to run the operations on separate operation queues and we’d still get the same end result:

print("Adding operations")
let queue1 = OperationQueue()
let queue2 = OperationQueue()
queue1.addOperation(operation1)
queue2.addOperation(operation2)
queue2.waitUntilAllOperationsAreFinished()
print("Done!")