Groovy for Closure

原文地址:http://groovy.codehaus.org/Closures

What is a Closure?

什么是闭包?

Groovy闭包就像是”代码块”或是方法指示块(method pointer).这些代码块被定义在一起,然后在最后执行

Simple Example

def clos = { println "hello!" }

println "Executing the closure:"

clos()                           //prints "hello!"

注意到上面的例子,当闭包被呼叫时”hello”才打印出来,并不是在定义时.

当定义闭包时,它也能”界定”局部变量:

def localMethod() {

 def localVariable = new java.util.Date()

 return { println localVariable }

}

 

def clos = localMethod()

 

println "Executing the closure:"

clos()

Parameters( 参数 )

闭包的参数要在前面加 ->标记,就像这样:

def clos = { a, b -> print a+b }

clos( 5, 7 )                        //prints "12"

如果你的闭包中定义的参数少于两个可以省略掉 ->标记.

Implicit variables( 隐式变量 )

在闭包内,很多变量定义有其特殊含义:

 

It

 

如果你的闭包有一个单变元(single argument),你可以在闭包中省略参数定义,像这样:

def clos = { print it }

clos( "hi there" )               //prints "hi there"

 

this, owner, and delegate

 

this:正如在Java中,this代表了在闭包中定义的封装类(enclosing class)

 

owner:封闭地对象(the enclosing object)(this或者它周围的闭包)

 

delegate:默认情况下,与owner一样,只是在builder或ExpandoMetaClass是可改变的

 

例如 :

class Class1 {

 def closure = {

    println this.class.name

    println delegate.class.name

    def nestedClos = {

      println owner.class.name

    }

    nestedClos()

 }

}

 

def clos = new Class1().closure

clos.delegate = this

clos()

/* prints:

 Class1

 Script1

 Class1$_closure1 */

 

Closures as Method Arguments( 闭包的函数参数 )

当在 method中闭包作为最后一个参数，你可以定义内线闭包(closure inline):

def list = ['a','b','c','d']

def newList = []

 

list.collect( newList ) {

 it.toUpperCase()

}

println newList            // ["A", "B", "C", "D"]

在上面的例子中 ,collect方法收到一个List和一个闭包参数,下面的例子说明情况和上面例子一样(虽然比较啰嗦):

def list = ['a','b','c','d']

def newList = []

 

def clos = { it.toUpperCase() }

list.collect( newList, clos )

 

assert newList == ["A", "B", "C", "D"]

 

More Information( 更多信息 )

Groovy继承了java.lang.Object用以许多methods来作为闭包参数.参见GDK Extensions to Object中针对闭包的示例.