Observable

A property wrapper type that creates an ObservableObject for just a single property.

This can have enormous performance benefits due to how SwiftUI recomputes views.

SwiftUI + Performance

Views get called to update each time any of an object's properties change. Even if it's irrelevant to a given view.

Consider the following common pattern of a typical custom view with one @State property.

struct BedroomLabel: View {
	@State var areTheLightsOn = false
	var numberOfPeople = 0
	
	var body: some View {
		VStack {
			Text("This bedroom has \(numberOfPeople) people in it.")
			
			Toggle("Lights On/Off", isOn: $areTheLightsOn)
		}
	}
}

This is a very common pattern in SwiftUI, and it works alright, but the BedroomLabel will update in its entirety whenever the lights get toggled. This means the CPU spends unnecessary time checking the entire body whenever this happens.

How can we do better?

One ObservableObject per Property

Here is that same example written using Observable.

struct BedroomLabel: View {
	@Observable var areTheLightsOn = false
	var numberOfPeople = 0
	
	var body: some View {
		VStack {
			Text("This bedroom has \(numberOfPeople) people in it.")
			
			Observing(bindingOf: $areTheLightsOn) { isOn in
				Toggle("Lights On/Off", isOn: isOn)
			}
		}
	}
}

@Observable writes just like @State, except it stores the value in an implicit generic ObservableObject type. Observing takes care of unwrapping that and recomputing a view closure whenever its changed.

This means BedroomLabel won't be computed on changes. Only the Toggle will be.

ObservableSample is a sample app that demonstrates, side-by-side, the performance gains of using @Observable instead of @State.

Installation

Use Swift Package Manager with this GitHub URL.

License

MIT License

Copyright © 2021-2022 Nicolas Cook Leon

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