Tag: EnvironmentKey

Instead of initializing SwiftUI views with dependencies, SwiftUI also offers other ways for injecting dependencies. This time let’s take a look on EnvironmentKey which defines a key for inserting objects to environment and vise-versa. We need to create a EnvironmentKey, adding an object to environment and then getting the object in SwiftUI view.

Creating EnvironmentKey and extending EnvironmentValues

Example object we use is DependencyManager what in real app can contains loads of dependencies. EnvironmentKey is a protocol in SwiftUI what requires to define associated type and default value. Default value is used when we have not explicitly inserted an instance of DependencyManager to the environment, more about it a little bit later. EnvironmentValues is a struct containing a collection of environment objects. We’ll add a property to EnvironmentValues which later will be used by @Environment property wrapper and also when setting an instance of the object to the environment.

	import Foundation
	import SwiftUI
	struct DependencyManager {
	let identifier: String
	let urlSession = URLSession.shared
	}
	struct DependencyManagerKey: EnvironmentKey {
	typealias Value = DependencyManager
	static var defaultValue = DependencyManager(identifier: "Default created by environment")
	}
	extension EnvironmentValues {
	var dependencyManager: DependencyManager {
	get {
	return self[DependencyManagerKey.self]
	}
	set {
	self[DependencyManagerKey.self] = newValue
	}
	}
	}

view raw DependencyManager.swift hosted with ❤ by GitHub

Inserting objects to environment

If we would like to insert a DependencyManager to the Environment, we can use func environment(_ keyPath: WritableKeyPath, _ value: V) -> some View using our DependencyManagerKey and an instance created by us. If we do not insert our own, SwiftUI will use the instance returned by defaultValue when the key is first time accessed.

	// Setting non-default instance of DependencyManager, otherwise default instance is used created in DependencyManagerKey
	let dependencyManager = DependencyManager(identifier: "Scene delegate created")
	let contentView = ContentView().environment(\.dependencyManager, dependencyManager)

view raw SceneDelegate.swift hosted with ❤ by GitHub

Getting objects from environment

Objects can be read from the environment using @Environment property wrapper and specifing the EnvironmentKey.

	import SwiftUI
	struct ContentView: View {
	@Environment(\.dependencyManager) var dependencyManager: DependencyManager
	var body: some View {
	Text(dependencyManager.identifier)
	}
	}

view raw ContentView.swift hosted with ❤ by GitHub

Summary

We created an environment key and inserted an object into environment. We looked into how SwiftUI handles default values for environment objects and used @Environment property wrapper for getting the instance from the environment using the created key.

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