In the delegate pattern, only one object can directly listen to another object's events. In the observer pattern, any number of objects can listen to a particular object's events. When designing a class that needs to notify other object(s) of events, why would you ever use the delegate pattern over the observer pattern? I see the observer pattern as more flexible. You may only have one observer now, but a future design may require multiple observers.
What are the advantages of the delegate pattern over the observer pattern
delegatesdesign-patternsobserver-pattern
Best Answer
There is no delegate pattern per se. I am going to assume you mean Delegation Pattern.
As I understand it, they're the complete reverse of each other and used for different purposes.
Generally, with an Observer Pattern, any number of observer objects will listen to an event on a second object and act on the event. The second object has no knowledge of its listeners. It just calls out to them.
A delegate object is passed to the second object which calls methods directly on the delegate. And therein lies the advantage you're looking for. Rather than sending a single message to multiple listeners, it has complete control over a single object (at a given time). See also Inversion of Control.