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Private Properties in Swift Protocols

How to make types that confirm to Swift Protocols have private fields

This Git repo contains 3 sources to aid you

  1. SLIDES
  2. This Readme
  3. Working Xcode Workspace

What

In Swift, the types which can confirm to a Protocol are:

Class Struct Enum as well as another Protocol which is called Protocol Inheritance.

Protocols can contain

  • Properties
  • Methods
  • Class properties and Methods: For Protocols which apply to only a class
  • Static properties and methods: For all types
  • Initializers: Required and Optional
  • @Attributes : eg. @objc @DiscardableResult @State @Inlinable @IBAction, etc

Once you define these fields, you can only choose them to be either of internal public or open.

However, the Swift compiler does not allow to mark a field in Protocol as private or fileprivate.

But by using concepts explained here, you will be able to use private / fileprivate modifier in a Protocol.

Code will be roughly equivalent to:

protocol ContainsAPrivateTableView {
  private var tableView: UITableView!
}

Solution:

Step 1: Mark the fields you want Private as Public (or Internal)

Step 2: Write the Protocol Extension on it's Methods, utilizing those fields which you made public in Step 1

Protocol Extensions are default implementation for Types confirming to that Protocol

Step 3: Program a meta programming tool to copy-paste this implementation to every conforming type

Here, I am using Sourcery as Meta programming tool, and it's setup requires a Configuration file, and a Template file.

The Config file: <ProjectRoot>/.sourcery.yml

This file contains the commands to pass to Sourcery

The Template file: <ProjectRoot>/<ProjectName>/SourceryTemplate/LabelSettable.stencil

This file contains behaviour for the templating tool. Here is where the instruction is to copy particular piece of code to every type implementing / confirming to a Protocol

Optional: Build configuration in Xcode

This command calls sourcery --config, as added in this commit

Step 4: Revert steps 1 and 2:

Remove the field requirement altogether from the Protocol

Since the field does not exist in the protocol, the protocol extension will complain of undefined field. Hence to satisfy the compiler, remove that protocol extension.

Mark that field in it's implementing type as private

You have successfully implemented Private properties inside of protocols. If ever you change the field name, there will be compile time errors, which you will be forced to fix.

How to use this project:

  1. clone this repo on a Mac
  2. In terminal, at root of this directory, run pod install && xed . to open workspace in Xcode
  3. The workspace comes preconfigured, and injects the required behavior on every compilation. So compile the project and watch the git diffs.

Practical Use Cases

Code sharing across multiple UIViews and UIViewControllers

To have shared code to be at single place, I had to either:

  • Make fields public
  • Have a derived class with those implementation and use that derived class

The 2nd option had a limitation, It was not composable.

The 1st option had a limitation, since there were code reviews in the project I was working on, I was not allowed to expose UIViews outside of a class. Hence I had to come up with scalable and intuitive approach. I quickly figured out that Extensions of a Type have same access control where the Extension is declared in. I could not access the properties outside of the file. This led to me coming up with a good use of Sourcery to Copy Paste the implementation in every file confirming to that Protocol.

Thank You

  • Yogesh Singh, for mentoring and providing a platform to showcase my ideas
  • Pointfree.co, for improving my understanding of Programming, Composition, Functional Programming, Thinking about in terms of Software.
  • Krzysztof Zablocki, for building sourcery, a library to automate swift code generation and encouraging Meta-programming

For the reader

Want me to explain you or your coworkers on benefits of using this approach, you can always drop me an email or tweet me.

Open for Speaking opportunities and Mentoring upcoming Mobile App developers🌟.

I have given a community talk on this topic, more details -> here

Have an idea for an iOS App? I can make you a prototype. Let's connect via email