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Writer's pictureHui Wang

109. iOS: The SOLID Principles in iOS Development

Introduction:


In the realm of software engineering, we find ourselves encountering five fundamental principles, collectively known as the SOLID principles. These principles are designed to foster better software design—enhancing maintainability, understandability, and flexibility. For those of us venturing into the world of iOS development, mastering and applying these principles is of paramount importance. They serve as the cornerstone for building high-quality applications. Today, let's embark on a journey to explore my understanding of these principles, supplemented by concrete iOS development examples to illustrate their practical application.


Single Responsibility Principle (SRP):


The Single Responsibility Principle advises that a class should be tasked with a single responsibility. Within the context of iOS development, adhering to this principle aids in averting the creation of unwieldy classes, advocating instead for each class to focus on a singular functionality.


For instance, consider a podcast episode controller named EpisodeDetailViewController, whose primary role is to display details about a podcast episode. This controller should not be burdened with the responsibility of fetching these details from the network. Segregating these responsibilities enhances the code's maintainability and testability.



Should your page include code pertinent to network requests, it may be prudent to consider the establishment of a separate class, such as EpisodeDataService. By doing so, EpisodeDetailViewController adheres to the Single Responsibility Principle, rendering it more coherent and manageable.


Open-Closed Principle (OCP):


The Open-Closed Principle accentuates that software entities (such as classes, modules, functions, etc.) should be open for extension but closed for modification.


To put it simply, once a class is designed and developed, it should be capable of performing its duties independently without the need for any modifications to its codebase. If there's a desire to augment its functionality, one should resort to extensions, inheritance, or implementing interfaces instead.


This principle is equally prevalent in iOS development. Let's imagine designing a protocol for music effects, named MusicEffect, where, upon its completion, the protocol remains unchanged. Suppose this protocol contains a singular method apply(to:), which takes a song and returns it with an effect applied.



Now, if we wish to introduce a reverb effect, we can simply create a new class that adheres to this protocol and implements the apply(to:) method:



By adopting this approach, any new music effect can be integrated by implementing the MusicEffect protocol, thereby adhering to the Open-Closed Principle.


Liskov Substitution Principle (LSP):


The Liskov Substitution Principle primarily addresses inheritance, stipulating that subclasses should be able to replace their base classes without compromising the correctness of the program. This implies that subclasses ought to fully implement the behavior of their parent classes.


This principle is generally upheld across modern programming languages, including in the realm of iOS development. To illustrate with a straightforward example, suppose we have a Podcast class and a DownloadablePodcast subclass. According to LSP, any instance of DownloadablePodcast should seamlessly substitute an instance of Podcast.



By ensuring that DownloadablePodcast adheres to LSP, we enhance the flexibility and reliability of our code, allowing for a more robust and interchangeable system of classes within our iOS applications.


Interface Segregation Principle (ISP):


The Interface Segregation Principle advocates that clients should not be forced to depend on interfaces they do not use. This principle shares some similarity with the Single Responsibility Principle in that a large interface, encompassing numerous functionalities, may not always be necessary for its inheritors. Thus, it's often beneficial to break down a large interface into smaller, more specific ones.


For instance, imagine we have a class for a media player that can play podcasts, stream music, and download episodes. We could decompose these functionalities into distinct protocols, allowing each type of media player to depend only on the functionalities it requires.



By implementing these protocols, classes only need to concern themselves with the functionalities they genuinely require, thereby rendering the code more concise and flexible.


Dependency Inversion Principle (DIP):


The Dependency Inversion Principle underscores that high-level modules should not depend on low-level modules; both should rely on abstractions.


To demystify this concept, let's consider an example involving a podcast app. Suppose we have a class for fetching podcast episodes from an online source, OnlineEpisodeFetcher, and another for retrieving them from local storage, LocalEpisodeStorage. If a class, say EpisodeProvider, directly depends on these two classes for episode data, it contravenes the Dependency Inversion Principle.



The appropriate approach involves abstracting OnlineEpisodeFetcher and LocalEpisodeStorage behind a new interface that EpisodeProvider will depend on. We could define an EpisodeFetcher protocol and have both OnlineEpisodeFetcher and LocalEpisodeStorage implement this protocol:


With this setup, EpisodeProvider depends on the EpisodeFetcher protocol rather than any specific episode fetching mechanism. This approach enhances the code's flexibility and testability by adhering to the Dependency Inversion Principle.


Conclusion:


Adhering to the SOLID principles is pivotal in crafting robust, maintainable, and flexible iOS applications. Through practical examples, we've glimpsed how these principles can significantly refine our design and architecture. While some, especially novices, might find these principles daunting or challenging at first, accruing experience gradually unveils their importance. Embracing these programming philosophies is not just about enhancing your skillset; it's a crucial journey towards becoming a seasoned developer. The journey of mastering these principles is indeed a testament to the ongoing learning and adaptation required in the dynamic realm of software development.


 

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