Delegates: Difference between revisions
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<pre> |
<pre> |
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class Delegator: |
class Delegator: |
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def __init__(self |
def __init__(self): |
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self.delegate = |
self.delegate = None |
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def |
def operation(self): |
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if hasattr(self.delegate, 'thing'): |
if hasattr(self.delegate, 'thing'): |
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return self.delegate.thing() |
return self.delegate.thing() |
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return |
return 'default implementation' |
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def other(self): |
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if hasattr(self.delegate, 'other'): |
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return self.delegate.thing() |
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return 5 |
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class Delegate: |
class Delegate: |
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def thing(self): |
def thing(self): |
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return |
return 'delegate implementation' |
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if __name__ == '__main__': |
if __name__ == '__main__': |
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# No delegate |
# No delegate |
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a = Delegator() |
a = Delegator() |
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assert a. |
assert a.operation() == 'default implementation' |
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assert a.other() == 5 |
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# With delegate |
# With delegate |
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a.delegate = Delegate() |
a.delegate = Delegate() |
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assert a. |
assert a.operation() == 'delegate implementation' |
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assert a.other() == 5 |
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</pre> |
</pre> |
Revision as of 21:29, 3 October 2007
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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 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; }
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 delegate a.delegate = Delegate() assert a.operation() == 'delegate implementation'