swift语言点评四-Closure

总结：整个Closure的作用在于简化语言表述形式。

一、闭包的简化

Closure expression syntax has the following general form:

• { () -> in
•     }

• reversedNames = names.sorted(by: { (s1: String, s2: String) -> Bool in
•     return s1 > s2
• })

Because the sorting closure is passed as an argument to a method, Swift can infer the types of its parameters and the type of the value it returns. 

reversedNames = names.sorted(by: { s1, s2 in return s1 > s2 } )

Implicit Returns from Single-Expression Closures

reversedNames = names.sorted(by: { s1, s2 in s1 > s2 } )

Shorthand Argument Names

Swift automatically provides shorthand argument names to inline closures, which can be used to refer to the values of the closure’s arguments by the names $0, $1, $2, and so on.

reversedNames = names.sorted(by: { $0 > $1 } )

Operator Methods

reversedNames = names.sorted(by: >)

二、拖尾变换

• func someFunctionThatTakesAClosure(closure: () -> Void) {
•     // function body goes here
• }
•  
• // Here‘s how you call this function without using a trailing closure:
•  
• someFunctionThatTakesAClosure(closure: {
•     // closure‘s body goes here
• })
•  
• // Here‘s how you call this function with a trailing closure instead:
•  
• someFunctionThatTakesAClosure() {
•     // trailing closure‘s body goes here
• }

闭包的实现在函数参数列表外；

再次精简

reversedNames = names.sorted() { $0 > $1 }

reversedNames = names.sorted { $0 > $1 }

完全移出

• let strings = numbers.map { (number) -> String in
•     var number = number
•     var output = ""
•     repeat {
•         output = digitNames[number % 10]! + output
•         number /= 10
•     } while number > 0
•     return output
• }

Closures Are Reference Types

Whenever you assign a function or a closure to a constant or a variable, you are actually setting that constant or variable to be a reference to the function or closure.

Escaping Closures

异步解决方案

Autoclosures

表达式语句

• // customersInLine is ["Ewa", "Barry", "Daniella"]
• func serve(customer customerProvider: @autoclosure () -> String) {
•     print("Now serving \(customerProvider())!")
• }
• serve(customer: customersInLine.remove(at: 0))
• // Prints "Now serving Ewa!"