swift 随笔

1、使用===和!===比较两个对象是否相等

2、可选值与三目运算配合使用减少代码量：

var a: Int? 
let b: Int = 2
let c = a ?? b

当 a 不为空时，c=a, 当 a 为空时， c=b

等同于

let a:Int? = 2
let b:Int = 2
let c = (a != nil) ? a : b

3、String在被传递给方法或者被赋值给其他变量或者常量的时候会被复制，然后将复制的那个string 传递过去，而不是原来那个数值,不必担心因为某个值改变了而影响初始值

4、计算String长度：

let stringCount = someSTring.character.count

5、string下标：

var originString = "apple"
let startIdx = originString.startIndex // 0
let endIndex = originString.endIndex// 5,不是一个合法的下标
let succeesor = startIdx.successor()   //1
let predecessor = endIndex.predecessor() //4

originString[succeesor]    //p
originString[predecessor] //e
//开始下标往后两个的字母
originString[startIdx.advancedBy(2)] //p

移除子字符串

let startRange = originString.startIndex
let endRange  = originString.startIndex.advancedBy(2)
let range = startRange ..< endRange
originString.removeRange(range)  //prints ""

6 、使用下标修改数组元素

使用下标修改数组元素时，可以同时修改一个范围的元素，如，修改数组0~2的元素:

let arr = ["a","b","c","d","e","f"]
arr[0...2] = ["z","y","x"]

允许修改的元素个数与所给定的范围不一致：

let arr = ["a","b","c","d","e","f"]
arr[0...1] = ["z","y","x"]

//这个时候  arr = ["z","y","x","c","d","e","f"],相当于用z,y,x替代了a,b

7、用key值访问字典的value，将这个value设置为nil时，这个key-value会从字典中移除：

var stringDic = ["a":1,"b":2,"c":3];
stringDic["a"] = nil

//现在 stringDic = ["b":2,"c":3]

8、单独获取字典的所有key值或者value值，并用数组表示：

var stringDic:[String:AnyObject] = ["a":1,"b":2,"c":3];
let keys = [String](stringDic.keys)
let values = [AnyObject](stringDic.values)

字典是无序的，所以得到的keys和values也是无序的，如果需要特定排序，调用sort()方法：

let keys = [String](stringDic.keys.sort())

9、使用fallthrough使得switch中的case都往下执行操作，不会自动跳出：

let integerToDescribe = 5
var description = "The number \(integerToDescribe) is"
switch integerToDescribe {
case 2, 3, 5, 7, 11, 13, 17, 19:
    description += " a prime number, and also"
    fallthrough
default:
    description += " an integer."
}
// description = "The number 5 is a prime number, and also an integer."

如果再加一个case：

let integerToDescribe = 4
var description = "The number \(integerToDescribe) is"

// description = "The number 4 is a even number, and also a prime number, and also an integer."
switch integerToDescribe {
case  4,6,8:
    description += " a even number, and also"
    fallthrough
case 2, 3, 5, 7, 11, 13, 17, 19:
    description += " a prime number, and also"
    fallthrough
default:
    description += " an integer."
}

10、使用元组表示switch的判断条件：

用坐标系为例，

let point:(Double,Double) = (1.0,1.0)
let description:String?
switch point {
case (0..2,0..2):
    description = "第一象限"
case (-2..<0,0...2):
    description = "第二象限"
case (-2..<0,-2..<0):
    description = "第三象限"
case (0..2,-2..<0):
    description = "第四象限"
default:
    desc = "out of range"
    break
}

11、switch的值绑定（value binding）

switch只判断元组中的某一个判断条件

let descp:String?
let p = (1,1)

// p = (1,0):   descp = "on the x-axis with an value of 1."
// p = (1,1)：descp = "p is at x: 1, y: 1"
switch p {
case (let x, 0):
    descp = "on the x-axis with an value of \(x)."
case (0, let y):
    descp = "on the x-axis with an value of \(y)."
case (0,0):
    descp = "p is in origin point"
case (let x, let y):
    descp = "p is at x: \(x), y \(y)"
}

12、 switch中的where

在switch中用where添加额外的判断：

let descp:String?
let p = (1,1)

// p = (1,-1):   descp = "x is equal to -y."
// p = (1,1)：   descp = "x is equal to y."
// p = (3,1)：   descp = "x is more than y."
switch p {
case (let x, let y) where x == y:
    descp = "x is equal to y."
case (let x, let y) where x == -y:
    descp = "x is equal to -y."
case (let x, let y) where x > y:
    descp = "x is more than y."

13、 系统版本判断

if #available(*platform name* *version*, *...*, *) {

      //statements to execute if the APIs are available

} else {

      //fallback statements to execute if the APIs are unavailable

}