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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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23. Coordinator Pattern
Written by Joshua Greene

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The coordinator pattern is a structural design pattern for organizing flow logic between view controllers. It involves the following components:

  1. The coordinator is a protocol that defines the methods and properties all concrete coordinators must implement. Specifically, it defines relationship properties, children and router. It also defines presentation methods, present and dismiss.

    By holding onto coordinator protocols, instead of onto concrete coordinators directly, you can decouple a parent coordinator and its child coordinators. This enables a parent coordinator to hold onto various concrete child coordinators in a single property, children.

    Likewise, by holding onto a router protocol instead of a concrete router directly, you can decouple the coordinator and its router.

  2. The concrete coordinator implements the coordinator protocol. It knows how to create concrete view controllers and the order in which view controllers should be displayed.

  3. The router is a protocol that defines methods all concrete routers must implement. Specifically, it defines present and dismiss methods for showing and dismissing view controllers.

  4. The concrete router knows how to present view controllers, but it doesn’t know exactly what is being presented or which view controller will be presented next. Instead, the coordinator tells the router which view controller to present.

  5. The concrete view controllers are typical UIViewController subclasses found in MVC. However, they don’t know about other view controllers. Instead, they delegate to the coordinator whenever a transition needs to performed.

This pattern can be adopted for only part of an app, or it can be used as an “architectural pattern” to define the structure of an entire app.

You’ll see both of these at work in this chapter: In the Playground example, you’ll call a coordinator from an existing view controller, and in the Tutorial Project, you’ll adopt this pattern across the entire app.

When should you use it?

Use this pattern to decouple view controllers from one another. The only component that knows about view controllers directly is the coordinator.

Consequently, view controllers are much more reusable: If you want to create a new flow within your app, you simply create a new coordinator!

Playground example

Open AdvancedDesignPatterns.xcworkspace in the starter directory, and then open the Coordinator page.

Creating the Router Protocol

First, open Router.swift. This is where you’ll implement the Router protocol.

Creating the Concrete Router

You next need to implement the Concrete Router. Open NavigationRouter.swift, and add the following code to it:

Creating the Coordinator

Your next task is to create the Coordinator protocol. Open Coordinator.swift and add the following to it:

Creating the concrete coordinator

The last type you need to create is the Concrete Coordinator. Open HowToCodeCoordinator.swift and add the following code, ignoring any compiler errors you get for now:

Trying out the playground example

You’ve created all of the components, and you’re ready to put them into action!

What should you be careful about?

Make sure you handle going-back functionality when using this pattern. Specifically, make sure you provide any required teardown code passed into onDismiss on the coordinator’s present(animated:onDismiss:).

Tutorial project

You’ll build an app called RayPets in this chapter. This is a “pet” project by Ray: an exclusive pets-only clinic for savvy iOS users.

Creating AppDelegateRouter

You’ll first implement a new concrete router. Within the Routers group, create a new file called AppDelegateRouter.swift, and replace its contents with the following:

Creating HomeCoordinator

You next need to create a coordinator to instantiate and display the HomeViewController. Within the Coordinators group, create a new file named HomeCoordinator.swift. Replace its contents with the following, ignoring the compiler error for now:

Using HomeCoordinator

You also need to actually use HomeCoordinator. To do so, open AppDelegate.Swift and replace its contents with the following:

Creating PetAppointmentBuilderCoordinator

Open Models ▸ PetAppointment.swift, and you’ll see a model and related builder has already been defined: PetAppointment and PetAppointmentBuilder.

Creating ModalNavigationRouter

You may be wondering, “Couldn’t I just use NavigationRouter from the playground example?”

Using the PetAppointmentBuilderCoordinator

Fantastic! You’ve created all of the necessary pieces to display the schedule-a-visit flow. You now just need to put them all together.

Key points

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

Where to go from here?

The coordinator pattern is a great pattern for organizing long-term or very complex apps. It was first introduced to the iOS community by Soroush Khanlou. You can learn more about this pattern’s roots in his blog post about it here:

22. Chain-of-Responsibility Pattern
24. Conclusion
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