UIButton addTarget如何自行工作？

Question

I try figure out why self point to the GameViewController instead of Answer

GameViewController.swift

class GameViewController: UIViewController {
    var gameplay = QuestionsController(colors: colors)

    override func viewDidLoad() {
        super.viewDidLoad()
        self.view.addSubview(gameplay.answersController.answers[0].button)
    }

    func didPressAnswerButton(sender: UIButton!) {
        sender.setTitle("Im from GameViewController class", forState: .Normal)
    }
}

QuestionsController.swift

class QuestionsController {
    var question: Question
    var answersController: AnswersController
}

AnswersController.swift

class AnswersController {
    var answers = [Answer]()

    func prepareAnswers() {
        let answer = Answer()
        answers.append(answer)
    }
}

Answer.swift

class Answer{
    let button: UIButton

    func prepareButton() {
        let answerButton = AnswerButton(type: .System)
        answerButton.addTarget(self, action: "didPressAnswerButton:", forControlEvents: .TouchUpInside)
        button = answerButton
    }

    func didPressAnswerButton(sender: UIButton!) {
        sender.setTitle("Im from Answer class", forState: .Normal)
    }
}

Answer 1

addTarget:action:forControlEvents: tells the control ( answerButton in this case) what method to call, and what object to call it on, when the user taps the button. Looking at your code in more detail:

 answerButton.addTarget(self, action: "didPressAnswerButton:", forControlEvents: .TouchUpInside)

• When the user taps a button, the TouchUpInside event fires on the answerButton , and when that happens we want to invoke a method didPressAnswerButton: on an Answer object
• So, we need to tell answerButton what do do when this TouchUpEvent fires. You do this calling the addTarget:action:forControlEvents method on the answerButton
• The self argument tells the answerButton what object to notify about the event: it is the target . In this context, self is an Answer object.
• The "didPressAnswerButton:" argument indicates what method the answerButton should call in response to the tap event: this is the action

This is the target-action mechanism of Objective-C/Cocoa. It's a very common pattern, it's worth it to read the linked documentation to learn a bit more about how it works. The key is that this is based on Objective-C* message passing: in the code above, "didPressAnswerButton:" indicates a selector , which when paired with a target ( self ), tells the answerButton how to send a "message" to the target when the user taps the button.

Also, note that when you are editing a storyboard and ctrl-drag from a button to your view controller and select a method, you are also setting up a target/action using this same mechanism. You select the target object by dragging to the view controller icon (or some other icon), and then you pick the action/selector when clicking on a method name in the popup.

* Target-Action was originally designed for Objective-C, but for the common case of implementing a view controller, you can assume Swift works the same way. Just note when reading documentation that Swift uses simple strings for actions, whereas Objective-C uses @selector(...) .