Delegates

A delegate is a helper object used by another object. The delegator may delegate some operations to the delegate, and provide a default implementation when there is not delegate or the delegate does not implement the operation. This pattern is heavily used in Cocoa framework on Mac OS X

Objects responsibilities:

Delegator:

Keep an optional delegate instance.
Allow setting the delegate when creating the instance or later.
Delegate the methods "thing" and "other" to the delegate.
Provide default implementation if there is no delegate or the delegate does not implement one of the method.

Delegate:

Implement "thing".
May implement "other", but don't implement it for this example.

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]]==
[[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