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十四、析构器 Deinitialization

1. 当一个实例被释放(deallocated)时,析构器会立即被调用。使用关键字 deinit 来定义析构器。只有类类型才有析构器。

2. 析构器工作原理 How Deinitialization Works

当一个实例的自动引用计数 Automatic Reference Counting 为0时,Swift会释放这个实例。一般情况下,当一个实例被dealloc时,释放资源的工作都有Swift完成,但是某些时候,你需要自己进行一些释放资源的工作,比如你创建一个类执行打开文件操作,并写数据到文件中,在实例被dealloc时,你需要关闭文件。

    deinit {      // perform the deinitialization    }

析构器是编译器自动调用的,你不能手动调用。父类的析构器会被子类继承,当子类的析构器被调用时,父类的析构器会自动被调用。即使子类没有提供自己的析构器,父类的析构器也会被调用。
析构器可以访问调用它的实例的所有属性和方法。

例子:

    struct Bank {       static var coinsInBank = 10_000 //剩下的硬币      static func vendCoins(var numberOfCoinsToVend: Int) -> Int {//出售的硬币        numberOfCoinsToVend = min(numberOfCoinsToVend, coinsInBank)        coinsInBank -= numberOfCoinsToVend        return numberOfCoinsToVend      }      static func receiveCoins(coins: Int) { //收到的硬币        coinsInBank += coins      }    }

在这个例子中,我们只需要一个银行,所有它的属性和方法都设为静态类型。

    class Player {      var coinsInPurse: Int //玩家拥有的硬币      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)    println("A new player has joined the game with \(playerOne!.coinsInPurse) coins")    // prints "A new player has joined the game with 100 coins"    println("There are now \(Bank.coinsInBank) coins left in the bank")    // prints "There are now 9900 coins left in the bank"
    playerOne!.winCoins(2_000)    println("PlayerOne won 2000 coins & now has \(playerOne!.coinsInPurse) coins")    // prints "PlayerOne won 2000 coins & now has 2100 coins"    println("The bank now only has \(Bank.coinsInBank) coins left")    // prints "The bank now only has 7900 coins left"
    playerOne = nil    println("PlayerOne has left the game")    // prints "PlayerOne has left the game"    println("The bank now has \(Bank.coinsInBank) coins")    // prints "The bank now has 10000 coins"

参考:https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Deinitialization.html#//apple_ref/doc/uid/TP40014097-CH19-ID142

十四、析构器 Deinitialization