An Exploration of Capture Semantics. Read the blog post: http://www.chibicode.org/?p=28

ForgotCaptureSemantics.swift

func attemptLogin(user: String, password: String) {
    self.loginRequest = APIClient.sharedClient().createLoginRequest(user:user, password:password) {
        result in
        switch result {
            case .Success(let data):
                parseLoginData(data) // Compiler error: implicit reference to self
            case .Failure(let error):
                errorHandlingMethod(error) // Compiler error: implicit reference to self
        }
    }
}

StrongCaptureSemantics.swift

func attemptLogin(user: String, password: String) {
    self.loginRequest = APIClient.sharedClient().createLoginRequest(user:user, password:password) {
        result in
        switch result {
            case .Success(let data):
                self.parseLoginData(data)
            case .Failure(let error):
                self.errorHandlingMethod(error)
        }
    }
}

UnownedCaptureSemantics.swift

func attemptLogin(user: String, password: String) {
    self.loginRequest = APIClient.sharedClient().createLoginRequest(user:user, password:password) {
        [unowned self] result in
        switch result {
            case .Success(let data):
                self.parseLoginData(data)
            case .Failure(let error):
                self.errorHandlingMethod(error)
        }
    }
}

WeakCaptureSemantics.swift

func attemptLogin(user: String, password: String) {
    self.loginRequest = APIClient.sharedClient().createLoginRequest(user:user, password:password) {
        [weak self] result in
        guard let strongSelf = self else { return }
        switch result {
            case .Success(let data):
                strongSelf.parseLoginData(data)
            case .Failure(let error):
                strongSelf.errorHandlingMethod(error)
        }
    }
}