Weakify

Weakify is a µframework providing some commonly useful variations of the weakify() function. weakify() is primarily a way to be able to use a method on a class as a "closure" value that would be managed by some other component, but in a way that prevents memory leaks from occurring.

If you were to define a (admittedly contrived) class like this:

class Thing {
    func doSomething() {
        print("Something!")
    }

    var callback: () -> Void = {}

    func registerCallback() {
        callback = self.doSomething
    }
}

let thing = Thing()
thing.registerCallback()

You would be creating a retain cycle, and thing would never be deallocated. Whenever you reference a method on an object without calling it, the instance of the class that the method is bound to is captured by the method for the lifetime of the method reference. This is because in Swift instance methods are effectively curried functions: the actual methods you write on classes and instances close over references to self (strongly) so that those references are guaranteed to live for the lifetime of the method.

You can get around this by doing the following in the registerCallback method:

func registerCallback() {
	callback = { [weak self] in
		self?.doSomething()
	}
}

which breaks the retain cycle. However, having to create a new closure whenever you want to do this is a little bit cumbersome if the method you're calling has the same signature, which is where weakify() comes in. Using it, you can rewrite this method like so:

func registerCallback() {
	callback = weakify(self, type(of: self).doSomething)
}

weakify() separates the instance of the object from the method using static method references (you can reference the doSomething method statically with Thing.doSomething or type(of: self).doSomething, which has a type of (Thing) -> () -> ()). In this example weakify weakly applies self to the curried function's first argument, returning a closure that has the type () -> () which, when called, will execute the doSomething method only if self has not been deallocated (much like the manual closure that weakly captures self defined earlier).

There are a few variants of weakify available in this library for you to use:

func weakify <T: AnyObject, U>(_ owner: T, _ f: (T) -> () -> ()) -> (U) -> ()
func weakify <T: AnyObject, U>(_ owner: T, _ f: (T) -> () throws ->()) -> (U) throws -> ()

may be applied to any method that takes no arguments and returns none. The resulting closure can accept an argument which will simply be ignored (useful in cases like NSNotificationCenter when you don't care about the notification argument), or the type may also represent Void, meaning no input arguments are necessary.

func weakify <T: AnyObject, U>(_ owner: T, _ f: (T) -> (U) -> ()) -> (U) -> ()
func weakify <T: AnyObject, U>(_ owner: T, _ f: (T) -> (U) throws ->()) -> (U) throws -> ()

may be applied to a method that accepts an argument and returns none, which the resulting closure mirrors.

func weakify <T: AnyObject, U, V>(_ owner: T, _ f: (T) -> (U) -> V) -> (U) -> V?
func weakify <T: AnyObject, U, V>(_ owner: T, _ f: (T) -> (U) throws -> V) -> (U) throws -> V?

may be applied to a function that accepts and returns something; effectively a union of the two previous cases.

func weakify <T: AnyObject, U, V>(_ owner: T, _ f: (T) -> (U?) -> ()) -> (V) -> ()
func weakify <T: AnyObject, U, V>(_ owner: T, _ f: (T) -> (U?) throws -> ()) -> (V) throws -> ()

may be applied to a function that accepts an optional value. The resulting closure can have a completely different type for the input argument. If owner is not nil at call time, the argument to the resulting closure is conditionally cast from V to U with the as? operator, and the result of that is passed to the original function (which is why it must accept an optional, in case the cast fails).

• 0.4.0 is supported on Xcode 8/Swift 3
• 0.3.0 is supported on Xcode 8/Swift 3, or on Xcode 7.3/Swift 2.2+
• 0.2.3 is supported on Xcode 8/Swift 2.3, or on Xcode 7/Swift 2.x
• 0.1.3 is supported on Xcode 6.3+/Swift 1.2
• iOS 8+/OS X 10.9+/watchOS 2+/tvOS 9+

Weakify is available through CocoaPods. To install it, simply add the following line to your Podfile:

# Swift 3.x:
pod "Weakify", "~> 0.4.0"

# Swift 2.x:
pod "Weakify", "~> 0.2.3"

# Swift 1.2:
pod "Weakify", "~> 0.1.3"

Weakify can be integrated with Carthage. Add the following to your Cartfile to use it:

# Swift 3:
github "klundberg/Weakify" ~> 0.4.0

# Swift 2:
github "klundberg/Weakify" ~> 0.2.3

# Swift 1.2:
github "klundberg/Weakify" ~> 0.1.3

Add the following line to your dependencies list in your Package.swift file (altering the version as appropriate for your target swift version):

.Package(url: "https://github.com/klundberg/weakify.git", versions:Version(0,4,0)..<Version(0,5,0)),

If you cannot use CocoaPods (e.g. if you still need to target iOS 7 at a minimum for instance), the recommended way to install this is to simply manually copy weakify.swift from the repo into your project. You may also opt to reference this repo as a git submodule, which is an exercise I leave to you.

Kevin Lundberg, kevin at klundberg dot com

If you have additional variants of Weakify you'd like to see, feel free to submit a pull request! Please include unit tests with any changes.

Weakify is available under the MIT license. See the LICENSE file for more info.