Swift的闭包（Closure）与ObjC中的block很像，但Swift闭包语法更简洁，也新增了许多新特性。本文是我学习@autoclosure的记录。
当前是morning时，下述方法会打印出Good morning欢迎语：
func goodMorning(morning: Bool, whom: String) {
if morning {
print("Good Morning, \(whom)")
}
}
goodMorning(morning: true, whom: "Pavel")
goodMorning(morning: false, whom: "John")
打印内容为：
Good Morning, Pavel
对于John，第一个参数为false，不会打印欢迎语。
下面新增一个欢迎提示：
func giveAname() -> String {
print("giveAname() is called")
return "Robert"
}
goodMorning(morning: true, whom: giveAname())
这一次goodMorning(morning:whom:)方法的第二个参数是另一个函数giveAname()，giveAname函数返回值类型为String。
运行后输出如下：
giveAname() is called
Good Morning, Robert
goodMorning(morning:whom:)第一个参数传false，如下所示：
func giveAname() -> String {
print("giveAname() is called")
return "Robert"
}
goodMorning(morning: false, whom: giveAname())
控制台打印如下：
giveAname() is called
当goodMorning(morning:whom:)第一个参数为false时，第二个参数不会被使用，但giveAname()函数仍然进行了调用。如果giveAname()函数不希望被调用，可以使用闭包（closure）：
func goodMorning(morning: Bool, whom: () -> String) {
if morning {
print("Good morning, \(whom())")
}
}
func giveAname() -> String {
print("giveAname() is called")
return "Robert"
}
goodMorning(morning: false, whom: giveAname)
现在，闭包作为第二个参数，giveAname不会在goodMorning(morning:whom:)前调用，且只有在用到时才会调用。
现在第二个参数是闭包类型，不接受String常量和变量。@autoclosure可以解决这一问题。
自动闭包（autoclosure）是自动创建的闭包，用于包装作为参数传递给函数的表达式，其不接受参数，调用时返回表达式的值。autoclosure的语法便利性让开发者直接编写普通表达式即可，而非显式闭包，也可以省略参数周围的大括号。
使用自动闭包后如下：
func goodMorning(morning: Bool, whom: @autoclosure () -> String) {
if morning {
print("Good morning, \(whom())")
}
}
func giveAname() -> String {
print("giveAname() is called")
return "Robert"
}
goodMorning(morning: true, whom: giveAname())
goodMorning(morning: false, whom: giveAname())
goodMorning(morning: true, whom: "Pavel")
goodMorning(morning: false, whom: "Jhon")
控制台打印如下：
giveAname() is called
Good morning, Robert
Good morning, Pavel
goodMorning(morning:whom:)的第二个参数是自动闭包，传入的函数、变量、常量会被自动转换为闭包，且只有在调用到的时候才执行。
通常，调用函数参数可能是自动闭包类型，开发者编写方法不太常设置为自动闭包类型。例如，assert(condition:message:file:line:)的condition、message参数都是auto closure，condition只在playground、非优化（即非release模式）模式生效，message只在condition为false时触发。
/// Performs a traditional C-style assert with an optional message.
///
/// Use this function for internal sanity checks that are active during testing
/// but do not impact performance of shipping code. To check for invalid usage
/// in Release builds, see `precondition(_:_:file:line:)`.
///
/// * In playgrounds and `-Onone` builds (the default for Xcode's Debug
/// configuration): If `condition` evaluates to `false`, stop program
/// execution in a debuggable state after printing `message`.
///
/// * In `-O` builds (the default for Xcode's Release configuration),
/// `condition` is not evaluated, and there are no effects.
///
/// * In `-Ounchecked` builds, `condition` is not evaluated, but the optimizer
/// may assume that it *always* evaluates to `true`. Failure to satisfy that
/// assumption is a serious programming error.
///
/// - Parameters:
/// - condition: The condition to test. `condition` is only evaluated in
/// playgrounds and `-Onone` builds.
/// - message: A string to print if `condition` is evaluated to `false`. The
/// default is an empty string.
/// - file: The file name to print with `message` if the assertion fails. The
/// default is the file where `assert(_:_:file:line:)` is called.
/// - line: The line number to print along with `message` if the assertion
/// fails. The default is the line number where `assert(_:_:file:line:)`
/// is called.
public func assert(_ condition: @autoclosure () -> Bool, _ message: @autoclosure () -> String = String(), file: StaticString = #file, line: UInt = #line)
autoclosure可以延迟执行，只有在调用时才执行。对于有副作用或比较昂贵的操作，延迟执行会很有用。
滥用autoclosure可能导致代码难以阅读，上下文环境或函数名应标记出延迟执行。
函数参数可以使用返回值类型相同的无参闭包替换。这种类型的闭包可以使用@autoclosure标记。使用@autoclosure标记后，闭包前后不需要添加大括号，自动闭包会自动添加大括号。使用@autoclosure标记后，传入的函数不会立即执行，会延迟到函数体内调用才执行。
Demo名称：autoclosure
源码地址：https://github.com/pro648/BasicDemos-iOS/tree/master/Autoclosure
参考资料：