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A collection of Swift Property Wrappers (formerly "Property Delegates")
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🌯🌯 Burritos

Bitrise Build Status Swift Package Manager Platform

A collection of well tested Swift Property Wrappers.

🚧 Beta Software: 🚧

Property Wrappers have a final shape 🎉. See the accepted proposal Expecting bug fixes in incoming betas but no changes.

👉 Did you know: Property Wrappers were announced by Apple during WWDC 2019. They are a fundamental component in SwiftUI syntax sugar hence Apple pushed them into the initial Swift 5.1 beta, skipping the normal Swift Evolution process. This process continued after WWDC and it took 3 reviews to reach their final form on Xcode 11 beta 4.

Requirements

Xcode 11.0 Beta 4 & Swift 5.1

Installation

Swift Package Manager

Xcode 11.0+ integration

  1. Open MenuBarFileSwift PackagesAdd Package Dependency...
  2. Paste the package repository url https://github.com/guillermomuntaner/Burritos and hit Next.
  3. Select your rules. Since this package is in pre-release development, I suggest you specify a concrete tag to avoid pulling breaking changes.

Package.swift

If you already have a Package.swift or you are building your own package simply add a new dependency:

dependencies: [
    .package(url: "https://github.com/guillermomuntaner/Burritos", from: "0.0.2")
]

Cocoapods

Add Burritos to your Podfile:

pod 'Burritos', '~> 0.0.2'

Each wrapper is a submodule, so you add just the one(s) you want

pod 'Burritos/Copying', '~> 0.0.2'
pod 'Burritos/UndoRedo', '~> 0.0.2'
pod 'Burritos/UserDefault', '~> 0.0.2'

@AtomicWrite

A property wrapper granting atomic write access to the wrapped property. Reading access is not atomic but is exclusive with write & mutate operations. Atomic mutation (read-modify-write) can be done using the wrapper mutate method.

@Atomic var count = 0

// You can atomically write (non-derived) values directly:
count = 99

// To mutate (read-modify-write) always use the wrapper method:
DispatchQueue.concurrentPerform(iterations: 1000) { index in
    _count.mutate { $0 += 1 }
}

print(count) // 1099

@Clamping

A property wrapper that automatically clamps its wrapped value in a range.

@Clamping(range: 0...1)
var alpha: Double = 0.0

alpha = 2.5
print(alpha) // 1.0

alpha = -1.0
print(alpha) // 0.0

@Copying

A property wrapper arround NSCopying that copies the value both on initialization and reassignment. If you are tired of calling .copy() as! X you will love this one.

@Copying var path: UIBezierPath = .someInitialValue

public func updatePath(_ path: UIBezierPath) {
    self.path = path
    // You don't need to worry whoever called this method mutates the passed by reference path.
    // Your stored self.path contains a copy.
}

@DefaultValue

A property wrapper arround an implicitly unwrapped optional value which fallbacks to a given default value.

@DefaultValue(default: 0)
var count
count = 100
// or
@DefaultValue(default: 0)
var count = 100

// Assigning nil resets to the default value
print(count) // 100
count = nil
print(count) // 0

@DynamicUIColor

A property wrapper arround UIColor to support dark mode.

By default in iOS >= 13 it uses the new system wide user interface style trait and dynamic UIColor constructor to support dark mode without any extra effort. On prior iOS versions it defaults to light.

@DynamicUIColor(light: .white, dark: .black)
var backgroundColor: UIColor

// The color will automatically update when traits change
view.backgroundColor = backgroundColor

To support older iOS versions and custom logics (e.g. a switch in your app settings) the constructor can take an extra style closure that dynamically dictates which color to use. Returning a nil value results in the prior default behaviour. This logic allows easier backwards compatiblity by doing:

let color = DynamicUIColor(light: .white, dark: .black) {
    if #available(iOS 13.0, *) { return nil }
    else { return Settings.isDarkMode ? .dark : .light }
}

view.backgroundColor = color.value

// On iOS <13 you might need to manually observe your custom dark
// mode settings & re-bind your colors on changes:
if #available(iOS 13.0, *) {} else {
    Settings.onDarkModeChange { [weak self] in
        self?.view.backgroundColor = self?.color.value
    }
}

Original idea courtesy of @bardonadam

@EnvironmentVariable

A property wrapper to set and get system environment variables values.

@EnvironmentVariable(name: "PATH")
var path: String?

// You can set the environment variable directly:
path = "~/opt/bin:" + path!

@Expirable

A property wrapper arround a value that can expire. Getting the value after given duration or expiration date will return nil.

@Expirable(duration: 60)
var apiToken: String?

// New values will be valid for 60s
apiToken = "123456abcd"
print(apiToken) // "123456abcd"
sleep(61)
print(apiToken) // nil

// You can also construct an expirable with an initial value and expiration date:
@Expirable(initialValue: "zyx987", expirationDate: date, duration: 60)
var apiToken: String?
// or just update an existing one:
_apiToken.set("zyx987", expirationDate: date)

Courtesy of @v_pradeilles

@LateInit

A reimplementation of Swift Implicitly Unwrapped Optional using a property wrapper.

var text: String!
// or 
@LateInit var text: String

// Note: Accessing it before initializing will result in a fatal error:
// print(text) // -> fatalError("Trying to access LateInit.value before setting it.")

// Later in your code:
text = "Hello, World!"

@Lazy

A property wrapper which delays instantiation until first read access. It is a reimplementation of Swift lazy modifier using a property wrapper.

@Lazy var result = expensiveOperation()
...
print(result) // expensiveOperation() is executed at this point

As an extra on top of lazy it offers reseting the wrapper to its "uninitialized" state.

@LazyConstant

Same as @Lazy + prevents changing or mutating its wrapped value.

@LazyConstant var result = expensiveOperation()
...
print(result) // expensiveOperation() is executed at this point

result = newResult // Compiler error

Note: This wrapper prevents reassigning the wrapped property value but NOT the wrapper itself. Reassigning the wrapper _value = LazyConstant(initialValue: "Hola!") is possible and since wrappers themselves need to be declared variable there is no way to prevent it.

@Trimmed

A wrapper that automatically trims strings both on initialization and reassignment.

@Trimmed
var text = " \n Hello, World! \n\n    "

print(text) // "Hello, World!"

// By default trims white spaces and new lines, but it also supports any character set
@Trimmed(characterSet: .whitespaces)
var text = " \n Hello, World! \n\n    "
print(text) // "\n Hello, World! \n\n"

@UndoRedo

A property wrapper that automatically stores history and supports undo and redo operations.

@UndoRedo var text = ""

text = "Hello"
text = "Hello, World!"

_text.canUndo // true
_text.undo() // text == "Hello"

_text.canRedo // true
_text.redo() // text == "Hello, World!"

You can check at any time if there is an undo or a redo stack using canUndo & canRedo properties, which might be particularly usefull to enable/disable user interface buttons.

Original idea by @JeffHurray

@UserDefault

Type safe access to UserDefaults with support for default values.

@UserDefault("test", defaultValue: "Hello, World!")
var test: String

By default it uses the standard user defauls. You can pass any other instance of UserDefaults you want to use via its constructor, e.g. when you use app groups:

let userDefaults = UserDefaults(suiteName: "your.app.group")
@UserDefault("test", defaultValue: "Hello, World!", userDefaults: userDefaults)
var test: String

@Cached

TODO

@Dependency (Service locator pattern)

TODO

Thread safety

TODO

Command line parameters

TODO

Property observer -> willSet, didSet !

TODO: Reimplement

Print/Log

TODO: A property wrapper that prints/logs any value set.

About Property Wrappers

A property wrapper, or property delegate, is a pattern to abstract custom read and writte behaviours. In Swift we can use a generic struct exposing a computed property whose get & set methods we can implement however we want. This allows us to reuse some get/set logic with any type:

struct SomeWrapper<T> {
    ...
    var value: T {
        get {
            // Your custom get logic
            ...
        }
        set {
            // Your custom set logic
            ...
        }
    }
}

In plain old Swift you would have used this as follows:

// Instantiate your property wrapper
var wrappedProperty = SomeWrapper<String>("Hello, World!")

// Access its value
wrappedProperty.value

// In order to avoid having to unwrap wrappedProperty.value all the time, you can use a computed property
var value: String {
    get { return wrappedProperty.value }
    set { wrappedProperty.value = newValue }
}

// So now accessing the value is much easier
value

Swift 5.1 leverages annotations and the compiler to generate this code for you. It also bridges the assignment operator to the wrapper constructor which leads to a really nice syntax. So now, we can simply use:

@SomeWrapper var value = "Hello, World!"

// Access the wrapped value:
value

// You can also access the wrapper by using _
_value // <- This is the SomeWrapper<String> instance

Interesting reads:

Equivalents in other languages:

License

Burritos is released under the MIT license.

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