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groovymetaprogrammingmetaclassexpandometaclass

How to add a new closure to a class in groovy


From Snipplr

Ok here is the script code, in the comments is the question and the exception thrown

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()

//Now I want to add a new closure to Class1

def newClosure = {
    println "new Closure"
    println this.class.name
    println delegate.class.name
    def nestedClos = {
        println owner.class.name
    }
    nestedClos()
}

//getAbc to create a property, not a method
Class1.metaClass.getAbc = newClosure

//What happens here is that the property abc is not used as a closure per se, it's used
//as a property and when I execute it just run the closure and when I want to change
//the delegate, a null pointer is thrown
clos = new Class1().abc //abc executed instead of passing the reference closure
clos.delegate = this  //Exception!!!!
clos()

Solution

  • Ok, It's done it's not a fancy way, but I have the solution....yeah!

    Create the property as object and later assign the closure

    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()
    
    //Now I want to add a new closure to Class1
    
    def newClosure = {
        println "new Closure"
        println this.class.name
        println delegate.class.name
        def nestedClos = {
            println owner.class.name
        }
        nestedClos()
    }
    
    //before edit
    //Class1.metaClass.abc = new Object()
    Class1.metaClass.abc = newClosure    
    
    
    def cl = new Class1()
    //Before edit
    //For the sake of simplicity we are going to use & for the method
    //clos = cl.abc 
    closs = cl.&abc
    clos.delegate = this
    clos()