ios - 为什么这个 View Controller 需要初始化器？

Question

import UIKit
import MediaPlayer

我在下面收到一条错误消息，说类“playMusicViewController”没有初始化器，我不确定为什么我需要在 View Controller 类中使用初始化器，因为我的其他 View Controller 没有 View 初始化器

class playMusicViewController: UIViewController {

    var play : MPMusicPlayerController

    var selectedSong : MPMediaItemCollection

    @IBAction func play (sender: AnyObject){

        func playMusic() -> MPMusicPlayerController {

            var musicPlayer: MPMusicPlayerController = MPMusicPlayerController()

            return musicPlayer
        }

        playMusic()
    }

    override func viewDidLoad() {   
        super.viewDidLoad()

        func prepareToPlay() -> Bool{

            let myplayer = MPMusicPlayerController.applicationMusicPlayer()

            myplayer.setQueueWithItemCollection(selectedSong)

            play = myplayer

            myplayer.play()

            return true
        }}


    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
    }

}

Answer 1

因为每个 var 都应该在 Swift 中初始化，否则它会报错。如果您不想初始化变量，可以将变量设为可选，并且 nil 将最初分配给它们。

var play : MPMusicPlayerController?
var selectedSong : MPMediaItemCollection?

来自 Swift's Doc

Classes and structures must set all of their stored properties to an appropriate initial value by the time an instance of that class or structure is created. Stored properties cannot be left in an indeterminate state.

You can set an initial value for a stored property within an initializer, or by assigning a default property value as part of the property’s definition