What does the dollar sign do in Swift / SwiftUI?

Question

This tutorial by Apple about SwiftUI uses a dollar sign to bind data, and I‘m having trouble finding more information about this data binding in SwiftUI.

Toggle(isOn: $showFavoritesOnly) {

You use the $ prefix to access a binding to a state variable, or one of its properties.

Is this some sort of inout type parameter? That uses the ampersand to pass it on.

Faceharden

Answer 1

The $ is used in conjunction with property wrappers (previously known as "property delegates").

It's not an operator, but a prefix (thanks @matt!).

For more about property delegates, see this Swift Evolution document.

e.g. in @State var aState = false, State is a property wrapper.

This means that if we write:

• aState we're accessing a Bool value
• $aState we're accessing a Binding<Bool> value

Different property delegates will generate different values, called "projected values".

Chappell

You make it sound like some kind of operator. It isn't. The dollar sign is merely the first letter of the variable name. It doesn't "make" a binding. $aState is the binding. – Grantee

The Binding value should be projectedValue, right? – Arta

Also docs has a nice example in Projecting a Value From a Property Wrapper at docs.swift.org/swift-book/LanguageGuide/Properties.html – Apostil

@Arta yes, projectedValue of @State property wrapper has type Binding – Apostil

I am now seeing _aState for @State and @Binding variables, what does it mean? As far as I can see, the underscore does the same as the dollar sign. – Faceharden

@Grantee developer.apple.com/documentation/combine/published/… "The projectedValue is the property accessed with the $ operator." What's the "operator"? – Arette

Answer 2

This is very well explained in WWDC 2019 video 415. You are merely looking at one special case of a broad language feature, namely property wrappers.

A property wrapper (such as @State) is actually a way of referring to an instance of a type (usually a struct or enum) with the same name (such as State). The latter provides instructions for turning this instance property into a computed property whose getter and setter are the getter and setter for a certain computed property of itself (its wrappedValue). It also typically holds private storage backing that computed property.

Thus, after the declaration

@State var showFavoritesOnly = true

...showFavoritesOnly is turned into a computed property, with its getter and setter supplied by the State struct. When you set showFavoritesOnly to true, that is routed through the State struct's setter and ends up in a stored property of the State instance.

All of this implies that somewhere there is a State instance associated with your showFavoritesOnly. And there is, but it's hidden from view. Its name, in case you'd like to see that State instance, is _showFavoritesOnly.

Okay, but when you say $showFavoritesOnly, you do not get a State struct; you get a Binding struct. Why? That's because a property wrapper has a mechanism for specifying what the returned value from the $ name should be. In the case of State, it specifies that this value should be its own binding property, which is a Binding (see the docs: https://developer.apple.com/documentation/swiftui/state).

By an amazing coincidence, Toggle's isOn initializer takes a Binding (again, see the docs, https://developer.apple.com/documentation/swiftui/toggle/3232112-init). You could not have set the Toggle's isOn to showFavoritesOnly even if you wanted to! Instead, you set it to the Binding<Bool> supplied by the State instance, so that the Toggle has automatic two-way communication with the State object. The SwiftUI framework enforces its own correct usage; a Toggle can exist only in relation to some binding that acts as the underlying source of truth for its on/off state. And because it's a binding, not a mere Bool, communication works in both directions: when the user taps the switch in the Toggle, the change in value flows "up" to the State variable by way of the binding.

Grantee

Neat! The referenced video is here: developer.apple.com/videos/play/wwdc2019/415 – Retrogressive

Specifically, the property wrapper part of the talk starts at 23:30 ^ – Retrogressive