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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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16. Multicast Delegate Pattern
Written by Joshua Greene

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The multicast delegate pattern is a behavioral pattern that’s a variation on the delegate pattern. It allows you to create one-to-many delegate relationships, instead of one-to-one relationships in a simple delegate. It involves four types:

  1. An object needing a delegate, also known as the delegating object, is the object that has one or more delegates.

  2. The delegate protocol defines the methods a delegate may or should implement.

  3. The delegate(s) are objects that implement the delegate protocol.

  4. The multicast delegate is a helper class that holds onto delegates and allows you to notify each whenever a delegate-worthy event happens.

The main difference between the multicast delegate pattern and the delegate pattern is the presence of a multicast delegate helper class. Swift doesn’t provide you this class by default. However, you can easily create your own, which you’ll do in this chapter.

Note: Apple introduced a new Multicast type in the Combine framework in Swift 5.1. This is different than the MulticastDelegate introduced in this chapter. It allows you to handle multiple Publisher events. In such, this could be used as an alternative to the multicast delegate pattern as part of a reactive achitecture.

Multicast is an advanced topic in the Combine framework, and it’s beyond the scope of this chapter. If you’d like to learn more about Combine, check out our book about it, Combine: Asynchronous Programming with Swift (http://bit.ly/swift-combine).

When should you use it?

Use this pattern to create one-to-many delegate relationships.

For example, you can use this pattern to inform multiple objects whenever a change has happened to another object. Each delegate can then update its own state or perform relevant actions in response.

Playground example

Open IntermediateDesignPattern.xcworkspace in the Starter directory, or continue from your own playground workspace from the last chapter, and then open the MulticastDelegate page from the File hierarchy.

What should you be careful about?

This pattern works best for “information only” delegate calls.

Tutorial project

You’ll continue the Mirror Pad app from the previous chapter.

Key points

You learned about the multicast delegate pattern in this chapter. Here are its key points:

15. State Pattern
17. Facade Pattern
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