Moo 🐮

A package to give reference types Copy On Write (COW) symantics using Property Wrappers and no external dependencies.

Swift Package Manager

Update your Package.swift to include this to your package dependencies:

.package(url: "https://github.com/bscothern/Moo.git", from: "1.1.0")

Usage

General

Moo works with types that conform to the Copyable protocol. You should make a class conform like this

import Moo

final class SomeType: Copyable {
    static func createCopy(of other: SomeType) -> SomeType { ... }
    ...
}

You will likely want to have your class be final in order to return a the concrete type rather than Self which is what the protocol requires. This avoids the need to use SomeNonFinalType(...) as! SomeNonFinalType as the return value and a lot of potential bugs that can come with inheritance.

Once a type is Copyable you simply applie the @COW property wrapper to the type and it will be coppied when you access the wrapped value.

struct SomeOtherType {
    @COW someType: SomeType = SomeType(...)
}

Working with NSCopying

If you have a type that conforms to NSCopying you can conform it to Copyable and it will use NSCopying‘s copy() function as the default implimentation of Copyable‘s requirements.

Bypassing Self Requirements

Because Copyable has Self requirements it can cause issues if you try to have a collection of them. If you need to create a copy of a type you can also drop into _createCopy(of:) which is a function at global scope. It will safely attempt to use the Copyable protocol on the object and return a copy or nil.

Acknowledgements

Thanks to:

• Ben Cohen’s cowed
• Doug Gregor’s property wrappers proposal COW example