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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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22. Chain-of-Responsibility Pattern
Written by Joshua Greene

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The chain-of-responsibility pattern is a behavioral design pattern that allows an event to be processed by one of many handlers. It involves three types:

  1. The client accepts and passes events to an instance of a handler protocol. Events may be simple, property-only structs or complex objects, such as intricate user actions.

  2. The handler protocol defines required properties and methods that concrete handlers must implement. This may be substituted for an abstract, base class instead allowing for stored properties on it. Even then, it’s still not meant to be instantiated directly. Rather, it only defines requirements that concrete handlers must fulfill.

  3. The first concrete handler implements the handler protocol, and it’s stored directly by the client. Upon receiving an event, it first attempts to handle it. If it’s not able to do so, it passes the event on to its next handler.

Thereby, the client can treat all of the concrete handlers as if they were a single instance. Under the hood, each concrete handler determines whether or not to handle an event passed to it or pass it on to the next handler. This happens without the client needing to know anything about the process!

If there aren’t any concrete handlers capable of handling the event, the last handler simply returns nil, does nothing or throws an error depending on your requirements.

When should you use it?

Use this pattern whenever you have a group of related objects that handle similar events but vary based on event type, attributes or anything else related to the event.

Concrete handlers may be different classes entirely or they may be the same class type but different instances and configurations.

For example, you can use this pattern to implement a VendingMachine that accepts coins:

  • The VendingMachine itself would be the client and would accept coin input events.
  • The handler protocol would require a handleCoinValidation(_:) method and a next property.
  • The concrete handlers would be coin validators. They would determine whether an unknown coin was valid based on certain criteria, such as a coin’s weight and diameter, and use this to create a known coin type, such as a Penny.

Playground example

Open AdvancedDesignPatterns.xcworkspace in the Starter directory, and then open the ChainOfResponsibility page.

What should you be careful about?

The chain-of-responsibility pattern works best for handlers that can determine very quickly whether or not to handle an event. Be careful about creating one or more handlers that are slow to pass an event to the next handler.

Tutorial project

You’ll build an app called RWSecret in this chapter. This app allows users to decrypt secret messages by attempting several known passwords provided by the user.

Key points

You learned about the chain-of-responsibility pattern in this chapter. Here are its key points:

Where to go from here?

Using the chain-of-responsibility pattern, you created a secret message app that decrypts messages using passwords provided by the user. There’s still a lot of functionality that you can add to RWSecret:

21. Command Pattern
23. Coordinator Pattern
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