Delegates: Difference between revisions
Content deleted Content added
→{{header|Racket}}: added |
|||
Line 1,516:
a.delegate = Delegate()
assert a.operation() == 'delegate implementation'</lang>
=={{header|Racket}}==
<code>[[http://docs.racket-lang.org/reference/objectutils.html?q=is-a%3F#%28def._%28%28lib._racket%2Fprivate%2Fclass-internal..rkt%29._object-method-arity-includes~3f%29%29 object-method-arity-includes?]]</code>
tests for the existence of the method in an object.
<code>[[http://docs.racket-lang.org/reference/objectutils.html?q=is-a%3F#%28def._%28%28lib._racket%2Fprivate%2Fclass-internal..rkt%29._is-a~3f%29%29 is-a?]]</code>
can be used to test for a class instance or interface implementor; and is
probably more likely to be used in anger. But
<code>object-method-arity-includes?</code> can be used generally; and actually
follows the requirement of the task better.
<lang racket>#lang racket
;; Delegates. Tim Brown 2014-10-16
(define delegator%
(class object%
(init-field [delegate #f])
(define/public (operation)
(cond [(and (object? delegate) (object-method-arity-includes? delegate 'thing 0))
(send delegate thing)]
[else "default implementation"]))
(super-new)))
(define non-thinging-delegate% (class object% (super-new)))
(define thinging-delegate%
(class object%
(define/public (thing) "delegate implementation")
(super-new)))
(module+ test
(require tests/eli-tester)
(define delegator-1 (new delegator%))
(define delegator-2 (new delegator%))
(define non-thinging-delegate (new non-thinging-delegate%))
(define thinging-delegate (new thinging-delegate%))
(test
(send delegator-1 operation) => "default implementation"
(send delegator-2 operation) => "default implementation"
(set-field! delegate delegator-1 non-thinging-delegate) => (void)
(set-field! delegate delegator-2 thinging-delegate) => (void)
(send delegator-1 operation) => "default implementation"
(send delegator-2 operation) => "delegate implementation"
(send (new delegator% [delegate thinging-delegate]) operation) => "delegate implementation"))
</lang>
All the tests pass. Believe me.
=={{header|Ruby}}==
| |||