Hands-On Design Patterns with Swift

The issue with callbacks

After our thorough review of closures, it is time to take a step back and reconsider our original goal for this chapter: showing effective ways to handle concurrency in Swift. We have learned that callbacks provide an architectural building block to write concurrent code, and that closures are a great way to implement callbacks. In this section, we should ask ourselves how callbacks perform in more complex, realistic scenarios.

We have already met one shortcoming of using callbacks when we implement them through closures, namely, the risk of retain cycles. But there are more fundamental issues with callbacks in general, as we will shortly see.

Let's consider this scenario:

• You want to download a list of items using ...