Groovy for Domain-specific Languages - Second Edition

Chapter 5

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Groovy has special handling for methods whose last parameter is a closure. When
invoking these methods, the closure can be defined anonymously after the method
call parentheses. So, yet another legitimate way to call the preceding line is:

flintstones.each() { println "hello, ${it}" }

The general convention is not to use parentheses unless there are parameters in
addition to the closure:

given:

def flintstones = ["Fred", "Barney", "Wilma"]

when: "we call findIndexOf passing int and a Closure"

def result = flintstones.findIndexOf(0) { it == 'Wilma'}

then:

result == 2

The signature of the GDK findIndexOf method is:

int findIndexOf(int, Closure)

We can define our own methods that accept closures as parameters. The simplest
case is a method that accepts only a single closure as a parameter:

def closureMethod(Closure c) {

c.call()

}

when: "we invoke a method that accepts a closure"

closureMethod {

println "Closure called"

}

then: "the Closure passed in was executed"

"Closure called" == output()

Method parameters as DSL

Method parameters as DSL is an extremely useful construct when we want to wrap a
closure in some other code. Suppose we have some locking and unlocking that needs
to occur around the execution of a closure. Rather than the writer of the code locking
via a locking API call, we can implement the locking within a locker method that
accepts the closure:

def locked(Closure c) {

callToLockingMethod()

c.call()

callToUnLockingMethod()

}