Execute just once function (SeeAlso: http://qiita.com/YusukeHosonuma/items/95315add4004b59e5f00)

ExecuteJustOnce.swift

typealias ExecuteOnce = () -> Void

func executeOnce(f: () -> Void) -> ExecuteOnce {
    var first = true
    return {
        if first {
            first = false
            f()
        }
    }
}

/// Ex01
final class ViewController: UIViewController {
    private var someInitilize: ExecuteOnce = {}

    override func viewDidLoad() {
        someInitilize = executeOnce { [unowned self] in
            // implement.
        }
    }

    override func viewWillAppear(animated: Bool) {
        someInitilize()
    }
}

/// Ex02
final class UseCase {
    private var executeOnce: ExecuteOnce? = {
        // Implement.
        print("first execute.")
    }

    func executeJustOnce() {
        executeOnce?()
        executeOnce = nil
    }
}

let uc = UseCase()
uc.executeJustOnce() // first execute.
uc.executeJustOnce()

/// Ex03
func executeOnce(inout execute: ExecuteOnce?) {
    if let f = execute {
        f()
        execute = nil
    }
}

class UseCase2 {
    var exec: ExecuteOnce? = {
        print("first execute2.")
    }

    func executeJustOnce() {
        executeOnce(&exec)
    }
}

let uc2 = UseCase2()
uc2.executeJustOnce() // first execute2.
uc2.executeJustOnce()

/// Ex4
// http://qiita.com/codelynx/items/f0243d631f2448e89026

class UseCase3 {
    private lazy var executeOnce: ExecuteOnce? = {
        print("first execute3.")
        self.executeOnce = nil
    }

    private lazy var executeOnce2: ExecuteOnce? = {
        print("first execute3'.")
        return nil
    }()

    func executeJustOnce() {
        executeOnce?()
    }

    func executeJustOnce2() {
        executeOnce2?()
    }
}

let uc3 = UseCase3()
uc3.executeJustOnce() // first execute3.
uc3.executeJustOnce()
uc3.executeJustOnce2() // first execute3'.
uc3.executeJustOnce2()