函数已创建,但从未调用,但仍在执行

我对iOS开发和Swift非常陌生,如果这是一个微不足道的问题,我很抱歉。我有以下代码

import UIKit

class ViewController: UIViewController, UINavigationControllerDelegate, UIImagePickerControllerDelegate {

    var imagePicker: UIImagePickerController!

    @IBOutlet weak var imageView: UIImageView!

    @IBAction func openGalery(_ sender: UIButton) {
        if UIImagePickerController.isSourceTypeAvailable(.photoLibrary) {
            imagePicker = UIImagePickerController()
            imagePicker.delegate = self
            imagePicker.sourceType = .photoLibrary;
            imagePicker.allowsEditing = true
            present(imagePicker, animated: true, completion: nil)
        }
    }

    func imagePickerController(_ picker: UIImagePickerController, didFinishPickingMediaWithInfo info: [String : Any]) {
        imagePicker.dismiss(animated: true, completion: nil)
        imageView.image = info[UIImagePickerControllerOriginalImage] as? UIImage
    }

    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view, typically from a nib.
    }

    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
        // Dispose of any resources that can be recreated.
    }
}

在代码中,我创建了 func imagePickerController . imagePickerController 负责在 imageView . 它起作用了。我打开盖里,我选择一个图像,然后有一个转换回图像集的视图。但是,为什么它能工作?我从不调用函数 图像采集控制器 . 将它与我以前使用的语言进行比较,我应该在 PhotoLibrary . 像一个事件或什么的。但在这里,它是以某种方式自动调用的,只有通过定义函数。为什么?