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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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19. Mediator Pattern
Written by Joshua Greene

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The mediator pattern is a behavioral design pattern that encapsulates how objects communicate with one another. It involves four types:

  1. The colleagues are the objects that want to communicate with each other. They implement the colleague protocol.

  2. The colleague protocol defines methods and properties that each colleague must implement.

  3. The mediator is the object that controls the communication of the colleagues. It implements the mediator protocol.

  4. The mediator protocol defines methods and properties that the mediator must implement.

Each colleague contains a reference to the mediator, via the mediator protocol. In lieu of interacting with other colleagues directly, each colleague communicates through the mediator.

The mediator facilitates colleague-to-colleague interaction: Colleagues may both send and receive messages from the mediator.

When should you use it?

This mediator pattern is useful to separate interactions between colleagues into an object, the mediator.

This pattern is especially useful when you need one or more colleagues to act upon events initiated by another colleague, and, in turn, have this colleague generate further events that affect other colleagues.

Playground example

Open AdvancedDesignPattern.xcworkspace in the Starter directory, or continue from your own playground workspace you’ve been continuing to work on throughout the book, and then open the Mediator page from the File hierarchy.

What should you be careful about?

This pattern is very useful in decoupling colleagues. Instead of colleagues interacting directly, each colleague communicates through the mediator.

Tutorial project

In this chapter, you’ll add functionality to an app called YetiDate. This app will help users plan a date that involves three different locations: a bar, restaurant and movie theater. It uses CocoaPods to pull in YelpAPI, a helper library for searching Yelp for said venues.

Registering for a Yelp API key

If you worked through CoffeeQuest in the Intermediate Section, you’ve already created a Yelp API key. You would have done this in Chapter 10, “Model-View-ViewModel Pattern”. Copy your existing key and paste it where indicated within APIKeys.swift, then skip the rest of this section and head to the “Creating required protocols” section.

Creating required protocols

Since the app shows nearby restaurants, bars and movie theaters, it works best for areas with many businesses nearby. So the app’s default location has been set to San Francisco, California.

Key points

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

Where to go from here?

You also created Yeti Dates in this chapter! This is a neat app, but there’s a lot more you can do with it:

18. Flyweight Pattern
20. Composite Pattern
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