Escaping Closures 两点：本质是生命周期标示符

1、block需要（拷贝）保存；

2、block引用的环境变量需要处理。

相当于oc中的copy block。

Escaping Closures

A closure is said to escape a function when the closure is passed as an argument to the function, but is called after the function returns. When you declare a function that takes a closure as one of its parameters, you can write @escaping before the parameter’s type to indicate that the closure is allowed to escape.

One way that a closure can escape is by being stored in a variable that is defined outside the function. As an example, many functions that start an asynchronous operation take a closure argument as a completion handler. The function returns after it starts the operation, but the closure isn’t called until the operation is completed—the closure needs to escape, to be called later. For example:

1. var completionHandlers: [() -> Void] = []
2. func someFunctionWithEscapingClosure(completionHandler: @escaping () -> Void) {
3. completionHandlers.append(completionHandler)
4. }

The someFunctionWithEscapingClosure(_:) function takes a closure as its argument and adds it to an array that’s declared outside the function. If you didn’t mark the parameter of this function with @escaping, you would get a compile-time error.

Marking a closure with @escaping means you have to refer to self explicitly within the closure. For example, in the code below, the closure passed to someFunctionWithEscapingClosure(_:) is an escaping closure, which means it needs to refer to self explicitly. In contrast, the closure passed to someFunctionWithNonescapingClosure(_:) is a nonescaping closure, which means it can refer to self implicitly.

1. func someFunctionWithNonescapingClosure(closure: () -> Void) {
2. closure()
3. }
4. class SomeClass {
5. var x = 10
6. func doSomething() {
7. someFunctionWithEscapingClosure { self.x = 100 }
8. someFunctionWithNonescapingClosure { x = 200 }
9. }
10. }
11. let instance = SomeClass()
12. instance.doSomething()
13. print(instance.x)
14. // Prints "200"
15. completionHandlers.first?()
16. print(instance.x)
17. // Prints "100"