iOS中目标操作和键值观察器之间的区别

Question

What is the difference between the two? To me both look like doing the same thing ie notify an event observer about some event.

I read this post http://www.raywenderlich.com/46988/ios-design-patterns regarding the actual pattern in use for the two above mentioned event handling techniques but failed to understand how command pattern is different from observer pattern or am I missing something.

Answer 1

My understanding of them is the next: In the case of target action, you have the Notification Center through which you pass your info between two classes. You have one class which sends out the notification to the Notification Center which will broadcast the info and the second class which has the observers will listen for any broadcast that matches its criteria.

While in the case of KVO you have an object which is capable of observing the change of one of it's properties. So no one is posting any notification in the case of KVO. Hope this cleared things up at least a bit.

Answer 2

In target-action, the object controlling the event is told what to trigger from that event. The most common use of this for iOS developers is not NSInvocation, but Interface Builder. The sender is usually a UIControl.

In NSNotification, a notification is broadcast to whoever is listening for a particular event. These notifications are pretty generic, and can contain any information. The listeners may or may not care who the sender is.

In KVO, the listeners pay attention to specific properties of a particular object.