Swift tricks系列收集Swift牛逼的patterns和讓你代碼更加Swifty的tricks派近，持續(xù)更新中……
開始今天的話題前算芯，我們先來看幾行代碼：
let unsafemutablepointer : UnsafeMutablePointer<Int>
unsafemutablepointer = UnsafeMutablePointer.alloc(3)
unsafemutablepointer[0] = 1
unsafemutablepointer[1] = 2
unsafemutablepointer[2] = 3
///////////
let intArray : Array<Int> = [1,2,3]
intArray[1] = 3
****上面代碼有什么問題抖剿？****
可能你已經(jīng)想到了灼捂，編譯器會報錯渤涌，因為我們在嘗試修改一個常量數(shù)組片吊。
“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.”
那么绽昏，另外一個問題來了：同樣是用下標(biāo)訪問，同樣是值類型(Value Type)俏脊，為什么UnsafeMutablePointer不會報錯全谤？
UnsafeMutablePointer和Array都是struct，存儲的類型也一樣都是Int联予，額……
Array:
public subscript (index: Int) -> Element
UnsafeMutablePointer:
public subscript (i: Int) -> Memory { get nonmutating set }
nonmutating是什么東西啼县？材原！
nonmutating
nonmutating是swift的一個關(guān)鍵字，它和mutating一樣用來修飾方法季眷，只不過nonmutating一般用來修飾setter方法余蟹。
當(dāng)用nonmutating修飾setter方法的時候，swift編譯器會知道該方法并不會改變常量變量子刮，因此不會報錯威酒。
使用nonmutating有一定的危險性，因為它跳過了swift編譯器的安全檢查機(jī)制挺峡。不過我們可以在合適的情況下使用它葵孤，寫出更加靈活的代碼
假設(shè)我們想通過下標(biāo)訪問NSUserDefaults，我們會這么做：
extension NSUserDefaults {
subscript(key: String) -> AnyObject? {
get { return objectForKey(key) }
set { setObject(newValue, forKey: key) }
}
}
var defaults: NSUserDefaults = NSUserDefaults.standardUserDefaults()
defaults["key"] = "value"
不幸的是橱赠，defaults是mutable的尤仍，這就意味著它可以被別人重新賦值，進(jìn)而引發(fā)問題狭姨。使用nonmutating宰啦，我們就可以這樣做：
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"
現(xiàn)在再看defaults是不是有點class的感覺？
