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I understand what a delegate does in iOS, and I've looked at sample code, but I'm just wondering about the advantages of this type of encapsulation (as opposed to including delegate methods in the primary object).
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In Swift, a delegate is a controller object with a defined interface that can be used to control or modify the behavior of another object.
Overview. Your app delegate object manages your app's shared behaviors. The app delegate is effectively the root object of your app, and it works in conjunction with UIApplication to manage some interactions with the system.
The core purpose of the delegate pattern is to allow an object to communicate back to its owner in a decoupled way. By not requiring an object to know the concrete type of its owner, we can write code that is much easier to reuse and maintain.
In Swift, declaring a delegate property is just like declaring any other property and you specify the protocol name as the type of the property. You may notice the question mark syntax which indicates that it's a property with an optional value (there may or may not be an object assigned to it).
The advantage of the delegate design pattern is loose coupling. It enables class A (the delegate) to depend on class B (the delegating class) without class B having to have any knowledge of class A. This ensures that the dependency relationship is one-way only, rather than being circular.
It also forms the foundation (lower case "f") of Apple's frameworks because it allows them to invoke your code as appropriate when functionality specific to your application is required. For example, responding to a button tap or telling a table view how many sections there should be.
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Delegation is a design pattern not only used in iOS but many other languages. It enables you to hand values and messages over in your class hierarchy.
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