Delegates: Difference between revisions
Content added Content deleted
(Simplify the task to the minimum required to show the pattern) |
(Fix python section) |
||
Line 75: | Line 75: | ||
return 0; |
return 0; |
||
} |
} |
||
==[[Python]]== |
|||
[[Category:Python]] |
|||
<pre> |
|||
class Delegator: |
|||
def __init__(self, delegate=None): |
|||
self.delegate = delegate |
|||
def thing(self): |
|||
if hasattr(self.delegate, 'thing'): |
|||
return self.delegate.thing() |
|||
return 42 |
|||
def other(self): |
|||
if hasattr(self.delegate, 'other'): |
|||
return self.delegate.thing() |
|||
return 5 |
|||
class Delegate: |
|||
def thing(self): |
|||
return 37 |
|||
if __name__ == '__main__': |
|||
# No delegate |
|||
a = Delegator() |
|||
assert a.thing() == 42 |
|||
assert a.other() == 5 |
|||
# With delegate |
|||
a.delegate = Delegate() |
|||
assert a.thing() == 37 |
|||
assert a.other() == 5 |
|||
</pre> |
Revision as of 21:25, 3 October 2007
![Task](http://static.miraheze.org/rosettacodewiki/thumb/b/ba/Rcode-button-task-crushed.png/64px-Rcode-button-task-crushed.png)
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 not 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 implementaion"
Show how objets are created and used. First, without a delegate, then with a delegate.
Objective-C
@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 Delegate *d = [[Delegate alloc] init]; [a setDelegate:d]; assert([isEqualToString:@"delegate implementation"]); return 0; }