Swift Closures Tips

闭包表达式语法的一般形式：

{ (parameters) -> returnType in
    statements
}```

####根据上下文推断类型
通过内联闭包表达式构造的闭包作为参数传递给函数或方法时，总是能够推断出闭包的参数和返回值类型。这意味着闭包作为函数或者方法的参数时，你几乎不需要利用完整格式构造内联闭包。
####单表达式闭包隐式返回

let names = ["Chris", "Alex", "Ewa", "Barry", "Daniella"]
reversedNames = names.sorted(by: { s1, s2 in s1 > s2 } )

#####参数名称缩写

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

#####闭包是引用类型
无论你将函数或闭包赋值给一个常量还是变量，你实际上都是将常量或变量的值设置为对应函数或闭包的引用。

var myClosure = oneClosure
let thisClosure = twoClosure

#####自动闭包
自动闭包是一种自动创建的闭包，用于包装传递给函数作为参数的表达式。这种闭包不接受任何参数，当它被调用的时候，会返回被包装在其中的表达式的值。

var customersInLine = ["Chris", "Alex", "Ewa", "Barry", "Daniella"]
print(customersInLine.count)
// 打印出 "5"

let customerProvider = { customersInLine.remove(at: 0) /* 这时不会执行*/}
print(customersInLine.count)
// 打印出 "5"

print("Now serving (customerProvider())!")
// Prints "Now serving Chris!"
print(customersInLine.count)
// 打印出 "4"

// customersInLine is ["Alex", "Ewa", "Barry", "Daniella"]
func serve(customer customerProvider: () -> String) {
print("Now serving (customerProvider())!")
}
serve(customer: { customersInLine.remove(at: 0) } )
// 打印出 "Now serving Alex!"

使用**@autoclosure**关键字让你能够省略闭包的花括号，用一个普通的表达式来代替显式的闭包

// customersInLine is ["Alex", "Ewa", "Barry", "Daniella"]
func serve(customer customerProvider: () -> String) {
print("Now serving (customerProvider())!")
}
serve(customer: { customersInLine.remove(at: 0) } )
// 打印出 "Now serving Alex!"

如果你想让一个自动闭包可以“逃逸”，则应该同时使用 @autoclosure 和 @escaping 属性。

// customersInLine i= ["Barry", "Daniella"]
var customerProviders: [() -> String] = []
func collectCustomerProviders(_ customerProvider: @autoclosure @escaping () -> String) {
customerProviders.append(customerProvider)
}
collectCustomerProviders(customersInLine.remove(at: 0))
collectCustomerProviders(customersInLine.remove(at: 0))

print("Collected (customerProviders.count) closures.")
// 打印 "Collected 2 closures."
for customerProvider in customerProviders {
print("Now serving (customerProvider())!")
}
// 打印 "Now serving Barry!"
// 打印 "Now serving Daniella!"