In a previous post we learned about the Writable interface and how the GString implementation implements this interface. In Groovy we can also use a closure as an implementation of the Writable interface. The Closure class has the method asWritable() that will return a version of the closure with an implementation of the writeTo() method. The Writer object that is used as an argument for the writeTo() method will be passed as argument to the closure. The asWritable() method also adds a toString() implementation for the closure to return the result of a closure as aString.

In the following code we write a sample make() method. The make() method return a Writable closure. The closure is only executed when thewriteTo() or toString() method is invoked.

Writable make(Map binding = [:], Closure template) {
    // Use asWritable() to make the closure
    // implement the Writable interface.
    def writableTemplate = template.asWritable()
    
    // Assing binding map as delegate so we can access
    // the keys of the maps as properties in the 
    // closure context.
    writableTemplate.delegate = binding
    
    // Return closure as Writable.
    writableTemplate
}

// Use toString() of Writable closure.
assert make { Writer out -> out <<  "Hello world!" }.toString() == 'Hello world!'

// Provide data for the binding.
// The closure is not executed when the 
// make method is finished.
final writable = make(user:'mrhaki', { out ->
    out.println "Welcome ${user},"
    out.print "Today on ${new Date(year: 114, month: 3, date: 4).format('dd-MM-yyyy')}, "
    out.println "we have a Groovy party!"
})

// We invoke toString() and now the closure
// is executed.
final result = writable.toString()

assert result == '''Welcome mrhaki,
Today on 04-04-2014, we have a Groovy party!
'''

// Append contents to a file.
// NOTE: The leftShift (<<) operator on File is implemented
// in Groovy to use the File.append() method.
// The append() method creates a new Writer and
// invokes the write() method which 
// is re-implemented in Groovy if the argument
// is a Writable object. Then the writeTo() method
// is invoked:
// Writer.write(Writable) becomes Writable.writeTo(Writer).
// So a lot of Groovy magic allows us to use the following one-liner
// and still the writeTo() method is used on Writable.
new File('welcome.txt') << writable

assert new File('welcome.txt').text == '''Welcome mrhaki,
Today on 04-04-2014, we have a Groovy party!
'''