What is the @Environment property wrapper?

Paul Hudson @twostraws October 10th 2019

Fully updated for Xcode 11 GM

SwiftUI gives us both @Environment and @EnvironmentObject property wrappers, but they are subtly different: whereas @EnvironmentObject allows us to inject arbitrary values into the environment, @Environment is specifically there to work with pre-defined keys.

For example, @Environment is great for reading out things like a Core Data managed object context, whether the device is in dark mode or light mode, what size class your view is being rendered with, and more – fixed properties that come from the system. In code, it looks like this:

@Environment(\.horizontalSizeClass) var horizontalSizeClass
@Environment(\.managedObjectContext) var managedObjectContext

On the other hand, @EnvironmentObject is designed for arbitrary objects to be read from the environment, like this:

@EnvironmentObject var order: Order

That difference might sound small, but it’s important because of the way @EnvironmentObject is implemented. When we say that order is of type Order SwiftUI will look through its environment to find an object of that type and attach it to the order property. However, when using @Environment the same behavior isn’t possible, because many things might share the same data type.

For example:

@Environment(\.accessibilityReduceMotion) var reduceMotion
@Environment(\.accessibilityReduceTransparency) var reduceMotion
@Environment(\.accessibilityEnabled) var accessibilityEnabled

All three of those environment keys return a Boolean, so without specifying exactly which key we mean it would be impossible to read them correctly.