指南：析构过程（Deinitialization）

• 析构器只适用于类类型，当一个类的实例被释放之前，析构器会被立即调用。析构器用关键字deinit来标示。

析构过程原理（How Deinitialization Works）

• Swift 会自动释放不再需要的实例以释放资源。

• 在类的定义中，每个类最多只能有一个析构器，而且析构器不带任何参数。

• 析构器是在实例释放发生前被自动调用。

• 不能主动调用析构器。

• 子类继承了父类的析构器，并且在子类析构器实现的最后，父类的析构器会被自动调用。

• 因为直到实例的析构器被调用后，实例才会被释放，所以析构器可以访问实例的所有属性，并且可以根据那些属性可以修改它的行为（比如查找一个需要被关闭的文件）。

class Bank {
    static var coinsInBank = 10_000
    static func vendCoins(numberOfCoinsRequested: Int) -> Int {
        let numberOfCoinsToVend = min(numberOfCoinsRequested, coinsInBank)
        coinsInBank -= numberOfCoinsToVend
        return numberOfCoinsToVend
    }
    static func receiveCoins(coins: Int) {
        coinsInBank += coins
    }
}

class Player {
    var coinsInPurse: Int
    init(coins: Int) {
        coinsInPurse = Bank.vendCoins(coins)
    }
    func winCoins(coins: Int) {
        coinsInPurse += Bank.vendCoins(coins)
    }
    deinit {
        Bank.receiveCoins(coinsInPurse)
    }
}

var playerOne: Player? = Player(coins: 100)
print("A new player has joined the game with \(playerOne!.coinsInPurse) coins")
// 打印 "A new player has joined the game with 100 coins"
print("There are now \(Bank.coinsInBank) coins left in the bank")
// 打印 "There are now 9900 coins left in the bank"

playerOne!.winCoins(2_000)
print("PlayerOne won 2000 coins & now has \(playerOne!.coinsInPurse) coins")
// 输出 "PlayerOne won 2000 coins & now has 2100 coins"
print("The bank now only has \(Bank.coinsInBank) coins left")
// 输出 "The bank now only has 7900 coins left"

playerOne = nil
print("PlayerOne has left the game")
// 打印 "PlayerOne has left the game"
print("The bank now has \(Bank.coinsInBank) coins")
// 打印 "The bank now has 10000 coins"