Wait for all Operations in queue to finish before performing task

I use the next solution:

private let queue = OperationQueue()

private func addOperations(_ operations: [Operation], completionHandler: @escaping () -> ()) {
    DispatchQueue.global().async { [unowned self] in
        self.queue.addOperations(operations, waitUntilFinished: true)
        DispatchQueue.main.async(execute: completionHandler)
    }
}

A suitable solution is KVO

First before the loop add the observer (assuming queue is the OperationQueue instance)

queue.addObserver(self, forKeyPath:"operations", options:.new, context:nil)

Then implement

override func observeValue(forKeyPath keyPath: String?, of object: Any?, change: [NSKeyValueChangeKey : Any]?, context: UnsafeMutableRawPointer?) {
    if object as? OperationQueue == queue && keyPath == "operations" {
        if queue.operations.isEmpty {
            // Do something here when your queue has completed
            self.queue.removeObserver(self, forKeyPath:"operations")
        }
    } else {
        super.observeValue(forKeyPath: keyPath, of: object, change: change, context: context)
    }
}

Edit:

In Swift 4 it's much easier

Declare a property:

var observation : NSKeyValueObservation?

and create the observer

observation = queue.observe(\.operationCount, options: [.new]) { [unowned self] (queue, change) in
    if change.newValue! == 0 {
        // Do something here when your queue has completed
        self.observation = nil
    }
}

Since iOS13 and macOS15 operationCount is deprecated. The replacement is to observe progress.completedUnitCount.

Another modern way is to use the KVO publisher of Combine

var cancellable: AnyCancellable?

cancellable = queue.publisher(for: \.progress.completedUnitCount)
    .filter{$0 == queue.progress.totalUnitCount}
    .sink() { _ in 
       print("queue finished") 
       self.cancellable = nil           
    }

Set the maximum number of concurrent operations to 1

operationQueue.maxConcurrentOperationCount = 1

then each operation will be executed in order (as if each was dependent on the previous one) and your completion operation will execute at the end.

You can use operation dependencies to initiate some operation upon the completion of a series of other operations:

let queue = OperationQueue()

let completionOperation = BlockOperation {
    // all done
}

for object in objects {
    let operation = ...
    completionOperation.addDependency(operation)
    queue.addOperation(operation)
}

OperationQueue.main.addOperation(completionOperation)  // or, if you don't need it on main queue, just `queue.addOperation(completionOperation)`

Or, in iOS 13 and later, you can use barriers:

let queue = OperationQueue()

for object in objects {
    queue.addOperation(...)
}

queue.addBarrierBlock {
    DispatchQueue.main.async {
        // all done
    }
}