Delegates: Difference between revisions
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Delegate: |
Delegate: |
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* Implement "thing" and return the string "delegate |
* Implement "thing" and return the string "delegate implementation" |
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Show how |
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". |
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=={{header|Objective-C}}== |
=={{header|Objective-C}}== |
Revision as of 01:02, 11 January 2008
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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".
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 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; }
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
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'