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Design Patterns by Tutorials

Third Edition · iOS 13 · Swift 5 · Xcode 11

Before You Begin

Section 0: 4 chapters
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Section I: Hello, Design Patterns!

Section 1: 2 chapters
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Section II: Fundamental Design Patterns

Section 2: 7 chapters
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Section III: Intermediate Design Patterns

Section 3: 8 chapters
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Section IV: Advanced Design Patterns

Section 4: 7 chapters
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4. Delegation Pattern
Written by Joshua Greene

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The delegation pattern enables an object to use another “helper” object to provide data or perform a task rather than do the task itself. This pattern has three parts:

  • An object needing a delegate, also known as the delegating object. It’s the object that has a delegate. The delegate is usually held as a weak property to avoid a retain cycle where the delegating object retains the delegate, which retains the delegating object.

  • A delegate protocol, which defines the methods a delegate may or should implement.

  • A delegate, which is the helper object that implements the delegate protocol.

By relying on a delegate protocol instead of a concrete object, the implementation is much more flexible: any object that implements the protocol can be used as the delegate!

When should you use it?

Use this pattern to break up large classes or create generic, reusable components. Delegate relationships are common throughout Apple frameworks, especially UIKit. Both DataSource- and Delegate-named objects actually follow the delegation pattern, as each involves one object asking another to provide data or do something.

Why isn’t there just one protocol, instead of two, in Apple frameworks?

Apple frameworks commonly use the term DataSource to group delegate methods that provide data. For example, UITableViewDataSource is expected to provide UITableViewCells to display.

Apple frameworks typically use protocols named Delegate to group methods that receive data or events. For example, UITableViewDelegate is notified whenever a row is selected.

It’s common for the dataSource and delegate to be set to the same object, such as the view controller that owns a UITableView. However, they don’t have to be, and it can be very beneficial at times to have them set to different objects.

Playground example

Let’s take a look at some code!

What should you be careful about?

Delegates are extremely useful, but they can be overused. Be careful about creating too many delegates for an object.

Tutorial project

The playground example has given you a small taste for what it looks like to implement the delegation pattern. It’s now time to take that theory and make use of it in an app. You’ll continue the RabbleWabble app from the previous chapter, and add a menu controller to select the group of questions.

Setting up the views

Open Main.storyboard, select the Object library button and enter UIViewController into the search field in the new window that appears.

Displaying selected question groups

Open Main.storyboard again and select the SelectQuestionGroupViewController scene. Press the Editor menu button, and then Embed In ▸ Navigation Controller.

Creating a custom delegate

The app is starting to come along, but there’s still a few things missing:

Key points

You learned about the delegation pattern in this chapter, including how to use Apple-provided delegates and how to create your own delegates as well. Here are the key points you learned:

3. Model-View-Controller Pattern
5. Strategy Pattern
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