Delegates
You are encouraged to solve this task according to the task description, using any language you may know.
A delegate is a helper object used by another object. The delegator may send the delegate certain messages, and provide a default implementation when there is no delegate or the delegate does not respond to a message. This pattern is heavily used in Cocoa framework on Mac OS X
Objects responsibilities:
Delegator:
- Keep an optional delegate instance.
- Implement "operation" method, returning the delegate "thing" if the delegate respond to "thing", or the string "default implementation".
Delegate:
- Implement "thing" and return the string "delegate implementation"
Show how objects are created and used. First, without a delegate, then with a delegate that does not implement "thing", and last with a delegate that implements "thing".
Ada
All that is needed in order to implement this is a common base type. The delegator holds a pointer to an "untyped" object from the base class. Querying if the target implements the delegate interface is done using run-time type identification. <Ada> with Ada.Text_IO; use Ada.Text_IO;
procedure Delegation is
package Things is
-- We need a common root for our stuff
type Object is tagged null record;
type Object_Ptr is access all Object'Class;
-- Objects that have operation thing
type Substantial is new Object with null record;
function Thing (X : Substantial) return String;
-- Delegator objects
type Delegator is new Object with record
Delegate : Object_Ptr;
end record;
function Operation (X : Delegator) return String;
No_Thing : aliased Object; -- Does not have thing
Has_Thing : aliased Substantial; -- Has one
end Things;
package body Things is
function Thing (X : Substantial) return String is
begin
return "delegate implementation";
end Thing;
function Operation (X : Delegator) return String is
begin
if X.Delegate /= null and then X.Delegate.all in Substantial'Class then
return Thing (Substantial'Class (X.Delegate.all));
else
return "default implementation";
end if;
end Operation;
end Things;
use Things;
A : Delegator; -- Without a delegate
begin
Put_Line (A.Operation); A.Delegate := No_Thing'Access; -- Set no thing Put_Line (A.Operation); A.Delegate := Has_Thing'Access; -- Set a thing Put_Line (A.Operation);
end Delegation; </Ada> Sample output:
default implementation default implementation delegate implementation
E
def makeDelegator {
/** construct without an explicit delegate */
to run() {
return makeDelegator(null)
}
/** construct with a delegate */
to run(delegateO) { # suffix because "delegate" is a reserved keyword
def delegator {
to operation() {
return if (delegateO.__respondsTo("thing", 0)) {
delegateO.thing()
} else {
"default implementation"
}
}
}
return delegator
}
}
? def delegator := makeDelegator()
> delegator.operation()
# value: "default implementation"
? def delegator := makeDelegator(def doesNotImplement {})
> delegator.operation()
# value: "default implementation"
? def delegator := makeDelegator(def doesImplement {
> to thing() { return "delegate implementation" }
> })
> delegator.operation()
# value: "default implementation"
JavaScript
<javascript>function Delegator() {
this.delegate = null ;
this.operation = function(){
if(this.delegate && typeof(this.delegate.thing) == 'function')
return this.delegate.thing() ;
return 'default implementation' ;
}
}
function Delegate() {
this.thing = function(){
return 'Delegate Implementation' ;
}
}
function testDelegator(){
var a = new Delegator() ; document.write(a.operation() + "\n") ; a.delegate = 'A delegate may be any object' ; document.write(a.operation() + "\n") ; a.delegate = new Delegate() ; document.write(a.operation() + "\n") ;
}</javascript>
Objective-C
(This code, as is, won't get compiled under GNUstep)
<objc>#import <Cocoa/Cocoa.h>
@interface Delegator : NSObject {
id delegate;
} - (id)delegate; - (void)setDelegate:(id)obj; - (NSString *)operation; @end
@implementation Delegator - (id)delegate; {
return delegate;
} - (void)setDelegate:(id)obj; {
delegate = obj; // Weak reference
} - (NSString *)operation; {
if ([delegate respondsToSelector:@selector(thing)])
return [delegate thing];
return @"default implementation";
} @end
// Any object may implement these @interface NSObject (DelegatorDelegating) - (NSString *)thing; @end
@interface Delegate : NSObject // Don't need to declare -thing because any NSObject has this method @end
@implementation Delegate - (NSString *)thing; {
return @"delegate implementation";
} @end
// Example usage // Memory management ignored for simplification int main() {
// Without a delegate: Delegator *a = [[Delegator alloc] init]; assert([[a operation] isEqualToString:@"default implementation"]);
// With a delegate that does not implement thing: [a setDelegate:@"A delegate may be any object"]; assert([isEqualToString:@"delegate implementation"]);
// With a delegate that implements "thing": Delegate *d = [[Delegate alloc] init]; [a setDelegate:d]; assert([isEqualToString:@"delegate implementation"]);
return 0;
} </objc>
Oz
<ocaml>functor import
Open Application Module
define
Stdout = {New Open.file init(name:stdout)}
%% modify from http://osdir.com/ml/lang.mozart.user/2007-04/msg00042.html %% not known if this is a right way ... fun{IsMethod Klax Meth NArg} B = {NewCell false} in if {IsObject Klax} then [BootObj BootName] = {Module.link ['x-oz://boot/Object' 'x-oz://boot/Name']} MethList = {BootObj.getClass Klax}.{BootName.newUnique 'ooMeth'} MethName = {Dictionary.keys MethList} in {For 1 {Length MethName} 1 proc{$ Idx} if {Value.type MethName.Idx} == 'tuple' andthen MethName.Idx.1 == Meth andthen {Value.type {Dictionary.get MethList MethName.Idx.1}} == 'procedure' andthen {ProcedureArity {Dictionary.get MethList MethName.Idx.1}} == NArg then B := true end end } end @B end
%% translate from Python version
class Delegator
attr delegate
meth operation(?R)
if {IsMethod @delegate 'thing' 1} then
{@delegate thing(R)}
else
R = 'default implementation'
end
end
meth init
delegate := {New BaseObject noop}
end
meth set(DG)
delegate := DG
end
end
class Delegate from BaseObject
meth thing(?R)
R = 'Delegate Implementation'
end
end
A = {New Delegator init}
{Stdout write(vs:{A operation($)}#'\n')}
{A set('A delegate may be any object')}
{Stdout write(vs:{A operation($)}#'\n')}
{A set({New Delegate noop})}
{Stdout write(vs:{A operation($)}#'\n')}
{Application.exit 0}
end</ocaml>
PHP
<php>class Delegator {
function __construct() {
$this->delegate = NULL ;
}
function operation() {
if(method_exists($this->delegate, "thing"))
return $this->delegate->thing() ;
return 'default implementation' ;
}
}
class Delegate {
function thing() {
return 'Delegate Implementation' ;
}
}
$a = new Delegator() ; print "{$a->operation()}\n" ;
$a->delegate = 'A delegate may be any object' ; print "{$a->operation()}\n" ;
$a->delegate = new Delegate() ; print "{$a->operation()}\n" ;</php>
Pop11
uses objectclass;
define :class Delegator;
slot delegate = false;
enddefine;
define :class Delegate;
enddefine;
define :method thing(x : Delegate);
'delegate implementation'
enddefine;
define :method operation(x : Delegator);
if delegate(x) and fail_safe(delegate(x), thing) then
;;; Return value is on the stack
else
'default implementation'
endif;
enddefine;
;;; Default, without a delegate
lvars a = newDelegator();
operation(a) =>
;;; a delegating to itself (works because Delegator does not
;;; implement thing)
a -> delegate(a);
operation(a) =>
;;; delegating to a freshly created Delegate
newDelegate() -> delegate(a);
operation(a) =>
Python
<python>class Delegator:
def __init__(self):
self.delegate = None
def operation(self):
if hasattr(self.delegate, 'thing'):
return self.delegate.thing()
return 'default implementation'
class Delegate:
def thing(self):
return 'delegate implementation'
if __name__ == '__main__':
# No delegate a = Delegator() assert a.operation() == 'default implementation'
# With a delegate that does not implement "thing" a.delegate = 'A delegate may be any object' assert a.operation() == 'default implementation'
# With delegate that implements "thing" a.delegate = Delegate() assert a.operation() == 'delegate implementation'</python>