什么是闭包(Groovy) flyleave ITeye技术网站

什么是闭包(Groovy)

博客分类：
Grails & Groovy
Groovy Java C C++C#
Groovy的闭包就像一个代码段或者说方法指针。它是一段被定义并在稍后的时点上运行的代码。

Simple Example

Java代码
def clos = { println "hello! }
def clos = { println "hello! }
Java代码
示例： def clos = { println "hello!}
      println "Executing the closure:"
      clos()
示例： def clos = { println "hello!}
      println "Executing the closure:"
      clos()  
注意在上面的例子中，"hello"在闭包被调用的时候才打印出来，而不是在定义的时候。

Closures may be "bound" to variables within the scope where they are defined:

闭包可以与在特定范围内定义的变量绑定起来(这句不知翻译的对错，请看原文)

Java代码
def localMethod() {
  def localVariable = new java.util.Date()
  return { println localVariable }
}

def clos = localMethod()

println "Executing the closure:"
clos()     //打印出当localVariables被创建时的时间
def localMethod() {
  def localVariable = new java.util.Date()
  return { println localVariable }
}

def clos = localMethod()

println "Executing the closure:"
clos()     //打印出当localVariables被创建时的时间
参数：

闭包的参数在->之前被列出，比如

Java代码
def clos = { a, b -> print a+b }
clos( 5, 7 ) //prints "12"
def clos = { a, b -> print a+b }
clos( 5, 7 )                       //prints "12"
-> 这个符号是可选的，当你定义的闭包的参数小于两个时就可将其省略

隐含的变量

在闭包中，有几个变量有着特别的含义

it(注意是小写):

如果你的闭包定义仅含有一个参数，可以将其省略，Groovy自动用it来代替

比如：

Java代码
def clos = { print it }
clos( "hi there" ) //prints "hi there"
def clos = { print it }
clos( "hi there" )              //prints "hi there"
this, owner, 和 delegate

this : 如同在Java中一样，this 代表闭包被定义的类

owner : 包含闭包的对象，有可能是定义闭包的类或者是另外一个闭包

delegate : 默认与owner相同，but changeable for example in a builder or ExpandoMetaClass

Java代码
Example:

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  */
Example:

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  */
作为方法参数的闭包

当一个方法的最后一个参数是闭包的时候，我们可以将闭包紧随其后定义，如

Java代码
def list = ['a','b','c','d']
def newList = []

list.collect( newList ) {
  it.toUpperCase()
}
println newList           //  ["A", "B", "C", "D"]
def list = ['a','b','c','d']
def newList = []

list.collect( newList ) {
  it.toUpperCase()
}
println newList           //  ["A", "B", "C", "D"]
上一个例子也可以用下面的方法实现，就是稍显冗长。

Java代码
def list = ['a','b','c','d']
def newList = []

def clos = { it.toUpperCase() }
list.collect( newList, clos )

assert newList == ["A", "B", "C", "D"]
def list = ['a','b','c','d']
def newList = []

def clos = { it.toUpperCase() }
list.collect( newList, clos ) 

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

Groovy 用很多可以接受闭包作为参数的方法扩展了java.lang.Object

上文提到的 collect 就是这样一个例子

collect(Collection collection) {closure} 返回一个集合，并将该集合的每一项添加到给定的集合中