Programming Grails - Cdn.oreilly.com

}
};
This can be invoked from Groovy just like one created the typical way:
closure(6) // prints "6"
closure(6, 2) // prints "12"
closure(1, 2, 3) // throws a groovy.lang.MissingMethodException
// since there's no 3-param doCall()
Owner, Delegate, and this
this inside a closure is probably not what you expect. Intuitively, it seems like it should
be the closure itself, but it turns out that it’s actually the class instance where the closure
is defined. As such, it’s probably not of much use—the owner and delegate are much
more useful.
The owner of a closure is the surrounding object that contains the closure. It functions
as the target of method invocations inside the closure, and if the method isn’t defined,
then a MissingMethodException will be thrown. In this example, if we create a new
class instance and call the callClosure method (new SomeClass().callClosure()),
it will print Woof!, because the dogAndCat closure calls the existing woof method, but
it will then fail on the meow call because it doesn’t exist:
class SomeClass {
}
void callClosure() {
}
def dogAndCat = {
woof()
meow()
}
dogAndCat()
void woof() {
println "Woof!"
}
You can assign a delegate for a closure to handle method calls. By default, the delegate
is the owner, but you can change it with the setDelegate method. This is frequently
used when parsing DSLs. The DSL can be implemented as a closure, and inner method
calls and property access can be routed to a helper (i.e., the DSL builder), which im‐
plements the logic required when a method or property is called that’s not locally defined
but is valid in the DSL.
One example is the mapping block in Grails domain classes. This is a static closure that,
if defined, will be used to customize how the class and fields map to the database:
12 | Chapter 1: Introduction to Groovy