前言
在swift中，我们经常会用到的两种快速遍历的方法，一种是最常见也是最常用的for ... in ..., 另外一种也是不同于Objective-C的forEach。那么，两者的使用有什么区别呢？今天，让我们来聊一聊swift中遍历方法的那些事。
for in 与 forEach
同类型的泛型集合
for in
let array = ["1", "2", "3", "4", "5"]
for element in array {
print(element)
}
打印结果: 1, 2, 3, 4, 5
forEach
let array = ["1", "2", "3", "4", "5"]
array.forEach { (element) in
print(element)
}
打印结果: 1, 2, 3, 4, 5
在集合的元素类型相同（比如上面的数组是String类型）的情况下，两者遍历效果相同，方便、敏捷，我们可以随意选用。
不同类型元素的集合
for in
let array = [1, 2, 3, "cat", "rabbit"] as [Any] // as [Any]是swift 3的语法要求，因为数组中有两种不同类型的元素，分别是：Int 、String， 所以需要转化成 [Any]类型
for element in array {
print(element)
}
打印结果：1， 2， 3， cat， rabbit
forEach
let array = [1, 2, 3, "cat", "rabbit"] as [Any]
array.forEach { (element) in
print(element)
}
打印结果：1， 2， 3， cat， rabbit
在集合的元素类型不相同（比如上面的数组是Int和String类型）的情况下，两者遍历效果相同，方便、敏捷，我们可以也随意选用。
return关键字
for in
let array = ["1", "2", "3", "4", "5"]
for element in array {
if element == "3" {
return
}
print(element)
}
print("Hello World")
打印结果：1， 2
forEach
let array = ["1", "2", "3", "4", "5"]
array.forEach { (element) in
if element == "3" {
return
}
print(element)
}
print("Hello World")
打印结果：1， 2， 4， 5， Hello World
在使用return关键字的时候，很明显，for in中是当符合当前执行语句时，程序直接终止到此并返回, 比如上面的元素 "4"、"5"、"Hello World" 没有被执行；而forEach中是当符合当前执行语句时，程序跳过本次判断继续执行， 比如上面的元素"4"、"5"、"Hello World"被执行。
continue关键字
for in
let array = ["1", "2", "3", "4", "5"]
for element in array {
if element == "3" {
continue
}
print("element is \(element)")
}
print("Test \"continue\"")
打印结果：
element is 1
element is 2
element is 4
element is 5
Test "continue"
forEach
let array = ["1", "2", "3", "4", "5"]
array.forEach { (element) in
if element == "3" {
continue
}
print(element)
}
print("Test \"continue\"")
错误： 程序根本不能执行
error：continue is only allowed inside a loop
在使用continue关键字的时候，for in可以正常遍历并且执行，而且 continue的作用是跳出本次循环，不影响后面的执行； 而在 forEach中，swift是不允许这样执行的，报错的原因是说 continue只允许出现在循环语句中，也就是说不能使用在 forEach的closure中。
break关键字
for in
let array = ["1", "2", "3", "4", "5"]
for element in array {
if element == "3" {
break
}
print("element is \(element)")
}
print("Test \"continue\"")
打印结果：
element is 1
element is 2
Test "continue"
forEach
let array = ["1", "2", "3", "4", "5"]
array.forEach { (element) in
if element == "3" {
break
}
print(element)
}
print("Test \"continue\"")
错误：程序根本不能执行
error：Unlabeled 'break' is only allowed inside a loop or switch, a labeled break is required to exit an if or do
在break关键字中，对于for in来说是可以的，跳出本层循环，也就是for循环，然后继续执行后面的程序； 对于forEach来说，同continue关键字的效果一样，swift不允许这样使用，原因说的是break只能用于循环语句或switch语句，break会退出本层循环语句。
Apple官方对 forEach 的说明
下面是Apple的官方文档解释，对forEach遍历方法做了个大致的介绍，有兴趣可以看一下
/// Calls the given closure on each element in the sequence in the same order
/// as a `for`-`in` loop.
///
/// The two loops in the following example produce the same output:
///
/// let numberWords = ["one", "two", "three"]
/// for word in numberWords {
/// print(word)
/// }
/// // Prints "one"
/// // Prints "two"
/// // Prints "three"
///
/// numberWords.forEach { word in
/// print(word)
/// }
/// // Same as above
///
/// Using the `forEach` method is distinct from a `for`-`in` loop in two
/// important ways:
///
/// 1. You cannot use a `break` or `continue` statement to exit the current
/// call of the `body` closure or skip subsequent calls.
/// 2. Using the `return` statement in the `body` closure will exit only from
/// the current call to `body`, not from any outer scope, and won't skip
/// subsequent calls.
///
/// - Parameter body: A closure that takes an element of the sequence as a
/// parameter.
