Swift tricks-Nonmutating

Swift tricks系列收集Swift牛逼的patterns和让你代码更加Swifty的tricks，持续更新中……

开始今天的话题前，我们先来看几行代码：

let unsafemutablepointer : UnsafeMutablePointer
unsafemutablepointer = UnsafeMutablePointer.alloc(3)
unsafemutablepointer[0] = 1
unsafemutablepointer[1] = 2
unsafemutablepointer[2] = 3
///////////
let intArray : Array = [1,2,3]
intArray[1] = 3

****上面代码有什么问题？****

可能你已经想到了，编译器会报错，因为我们在尝试修改一个常量数组。

“If you create an instance of a structure and assign that instance to a constant, you cannot modify the instance’s properties, even if they were declared as variable properties”
“This behavior is due to structures being value types. When an instance of a value type is marked as a constant, so are all of its properties.”

error!.png

那么，另外一个问题来了：同样是用下标访问，同样是值类型(Value Type)，为什么UnsafeMutablePointer不会报错？

UnsafeMutablePointer和Array都是struct，存储的类型也一样都是Int，额……

---

Array:

public subscript (index: Int) -> Element

UnsafeMutablePointer:

public subscript (i: Int) -> Memory { get nonmutating set }

nonmutating是什么东西？！

---

nonmutating

nonmutating是swift的一个关键字，它和mutating一样用来修饰方法，只不过nonmutating一般用来修饰setter方法。
当用nonmutating修饰setter方法的时候，swift编译器会知道该方法并不会改变常量变量，因此不会报错。
使用nonmutating有一定的危险性，因为它跳过了swift编译器的安全检查机制。不过我们可以在合适的情况下使用它，写出更加灵活的代码

假设我们想通过下标访问NSUserDefaults，我们会这么做：

extension NSUserDefaults {
    subscript(key: String) -> AnyObject? {
        get { return objectForKey(key) }
        set { setObject(newValue, forKey: key) }
    }
}

var defaults: NSUserDefaults = NSUserDefaults.standardUserDefaults()
defaults["key"] = "value"

不幸的是，defaults是mutable的，这就意味着它可以被别人重新赋值，进而引发问题。使用nonmutating，我们就可以这样做：

protocol NSUserDefaultsSubscipt {
    subscript(key: String) -> AnyObject? { get nonmutating set }
}
extension NSUserDefaults:NSUserDefaultsSubscipt {
    subscript(key: String) -> AnyObject? {
        get { return objectForKey(key) }
        set { setObject(newValue, forKey: key) }
    }
}
let defaults: NSUserDefaults = NSUserDefaults.standardUserDefaults()
defaults["key"] = "value"

现在再看defaults是不是有点class的感觉？

参考:The why of nonmutating