Gestalt is an unintrusive and light-weight framework for application theming with support for animated theme switching.

Let's say you want to theme a view controller with a single label:

import Gestalt

struct Theme: Gestalt.Theme {
    let view: ViewTheme = .init()

    static let light: Theme = .init(view: .light)
    static let dark: Theme = .init(view: .dark)
}

struct ViewTheme: Gestalt.Theme {
    let font = UIFont.preferredFont(forTextStyle: .headline)
    let color: UIColor
    let backgroundColor: UIColor

    static let light: Theme = .init(
        color: UIColor.black
        backgroundColor: UIColor.white
    )

    static let dark: Theme = .init(
        color: UIColor.white
        backgroundColor: UIColor.black
    )
}

// In `AppDelegate.application(_:didFinishLaunchingWithOptions:)`
// assign a default theme (or user's choice from user defaults):
ThemeManager.default.theme = Theme.light

class ViewController: UIViewController {
    @IBOutlet var label: UILabel!

    override func viewDidLoad() {
        super.viewDidLoad()
        
        self.observe(theme: \Theme.view)
    }
}

extension ViewController: Themeable {

    typealias Theme = ViewTheme

    func apply(theme: Theme) {
        self.view.backgroundColor = theme.backgroundColor
        self.label.textColor = theme.color
        self.label.font = theme.font
    }
}

The call self.observe(theme: \Theme.view) registers the receiver for theme observation on ThemeManager.default for future theme changes and then calls it once immediately. The initial call is not animated, any further changes however are animated.

To change the current theme (even while the app is running) simply assign a different theme to your given ThemeManager in use:

ThemeManager.default.theme = Theme.dark

This will cause all previously registered closures on the given ThemeManager to be called again.

See the GestaltDemo target for a more realistic/elaborate usage example.

1. It is generally sufficient to use ThemeManager.default. It is however possible to create dedicated ThemeManagers via let manager = ThemeManager().

The use appearance proxies after a view has already been loaded this library uses a hack that removes and re-adds the root view of the application from the main window to activate the proxies. This is not possible in app extensions, such as a today widget, because the extension safe API restricts access to the main window. So to use this library in app extensions you need to manually trigger the reload of the root view by adding something like this to your root view controller after you set up your themes.

ThemeManager.default.observe(theme: Theme.self) { [weak self] _ in
        if let strongSelf = self, let superview = strongSelf.view.superview {
            strongSelf.view.removeFromSuperview()
            superview.addSubview(strongSelf.view)
        }
    }

1. The body of func apply(theme: Theme) should be idempotent to avoid unwanted side-effects on repeated calls.

The recommended way to add Gestalt to your project is via Carthage:

github 'regexident/Gestalt' ~> 2.0.0

or via Cocoapods:

pod 'Gestalt', '~> 2.0.0'

or via Swift Package Manager:

let package = Package(
    name: "GestaltDemo",
    dependencies: [
        .package(url: "https://github.com/regexident/Gestalt.git", from: "2.0.0")
    ],
    targets: [
        .target(name: "GestaltDemo", dependencies: [ "Gestalt" ])
    ]
)

Gestalt is available under the MPL-2.0 license. See the LICENSE file for more info.