What does a module mean in swift?

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A module is a single unit of code distribution—a framework or application that is built and shipped as a single unit and that can be imported by another module with Swift’s import keyword.

Each build target (such as an app bundle or framework) in Xcode is treated as a separate module in Swift. If you group together aspects of your app’s code as a stand-alone framework—perhaps to encapsulate and reuse that code across multiple applications—then everything you define within that framework will be part of a separate module when it’s imported and used within an app, or when it’s used within another framework.

As the docs indicate, the module is an application or a framework (library). If you create a project with classes A and B, they are part of the same module. Any other class in the same project can inherit from those classes. If you however import that project to another project, classes from that another project won’t be able to subclass A nor B. For that you would have to add open indicator before their declarations.

Basically, if you work on a single app then you are working in one single module and unless declared as private or fileprivate, the classes can subclass each other.

EDIT

Let us have following class in module (project) Module1:

class A {
}

Since this class is not open, it can be subclassed only within the same module. That means that following class:

class B: A {
}

Can be written only in the same project, in Module1.

If you add Module1 as a dependency to project Module2, and try to do this:

import Module1

class C: A {
}

It will not compile. That’s because class A is not open (in other words it has access public or less) and it does not belong to the same module as C. A belongs to Module1, C belongs to Module2.

Note

import keyword imports a dependency module to your current module. If you write import UIKit in your project, you are telling the compiler that you want to use module UIKit in your module. import does not define current module. Current module is the current project.

Adding import UIKit at the beginning of the file does not change nor define to which module the file belongs. It just tells the compiler that in that file you want to use code from UIKit module.

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