GString惰性求值

1. 当对一个GString实例求值时，如果其中包含一个变量，该变量的值会被简单地打印到一个Writer，通常是一个StringWriter。然而，如果GString中包含的是一个闭包，而非变量，该闭包就会被调用。如果闭包接受一个参数，GString会把Writer对象当做一个参数发送给它。如果闭包不接受任何参数，GString会简单地调用该闭包，并打印我们想返回Writer的结果。如果闭包接受的参数不止一个，调用则会失败，并抛出一个异常。
2. 如果希望改变表达式中使用的引用，而且希望它们的当前值被用于惰性求值中，请必须记住，不要再表达式中直接替换他们，而要使用一个无参闭包。

例子对比：

companyClosure = {-> company}
priceClosure = {-> price}
quote = "Today ${companyClosure} stock closed at ${priceClosure}"
stocks = [Apple : 663.01 ,Microsoft : 30.95]
stocks.each{key,value->
    company = key
    price = value
    println quote
}

结果输出：

Today Apple stock closed at 663.01
Today Microsoft stock closed at 30.95

例子对比：

price = 684.71
company = 'Google'
qutoe = "Today $company stock closed at $price"
println qutoe

stocks = [Apple : 663.01 ,Microsoft : 30.95]
stocks.each{key,value->
    company = key
    price = value
    println qutoe
}

结果输出：

Today Google stock closed at 684.71
Today Google stock closed at 684.71
Today Google stock closed at 684.71