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The 5 Access Levels in Swift

The Default

The default is internal, which means access is restricted to within a module.

What is a module? An app is 1 module.

Hence, the default for an app is everything is accessible within the app.

Which to use?

The good practise is to start being extremely restrictive.

Start with private, and expose more, only if necessary.

Application vs Framework Development

For regular application development, you will use only (1) to (3).

For developers working on framework/library/SDK, they will use (4) and (5), because their “module” is exposed to other developers. The difference between the last 2 levels is that public does not allow the type/func to be subclassed/overriden, while open let you do whatever you want.

Implicit

If a type has a certain access level, the properties within will have the same level, implicitly.

private class X {
    int i   // implicitly private
}

Specify explicitly for top-level definitions

A good practise is to specify the access level explicitly for the top-level types and functions.

Don’t leave it to the default (internal). Think hard if you need other part of your app to access it.

Testing

@testable import MyApp

In your unit tests, you can import with @testable attribute, which is a superpower to change access levels in the module/app, so that in your tests you can access them.

For example, an internal class is not accessible to test target, because a test target is an external module. With @testable, the access level is increased to open, and you can now access it (and may even subclass it)!

Final

Finally, another good practise is to lock down your definitions with final (:

This attribute provides an additional restriction – prevent others from subclassing and overriding it.


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@samwize

¯\_(ツ)_/¯

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