Swift readonly external, readwrite internal property
In Swift, what is the conventional way to define the common pattern where a property is to be externally readonly, but modifiable internally by the class (and subclasses) that own it.
In Objective-C, there are the following options:
- Declare the property as readonly in the interface and use a class extension to access the property internally. This is message-based access, hence it works nicely with KVO, atomicity, etc.
- Declare the property as readonly in the interface, but access the backing ivar internally. As the default access for an ivar is protected, this works nicely in a class hierarchy, where subclasses will also be able to modify the value, but the field is otherwise readonly.
In Java the convention is:
- Declare a protected field, and implement a public, read-only getter (method).
What is the idiom for Swift?
Answer
Given a class property, you can specify a different access level by prefixing the property declaration with the access modifier followed by get or set between parenthesis. For example, a class property with a public getter and a private setter will be declared as:
private(set) public var readonlyProperty: Int
Suggested reading: Getters and Setters
Martin's considerations about accessibility level are still valid - i.e. there's no protected modifier, internal restricts access to the module only, private to the current file only, and public with no restrictions.
Swift 3 notes
2 new access modifiers, fileprivate and open have been added to the language, while private and public have been slightly modified:
openapplies to class and class members only: it's used to allow a class to be subclassed or a member to be overridden outside of the module where they are defined.publicinstead makes the class or the member publicly accessible, but not inheritable or overridableprivatenow makes a member visible and accessible from the enclosing declaration only, whereasfileprivateto the entire file where it is contained
More details here.