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    <p>I have to second (or third) the previously posted answers in that NSNotificationCenter is probably what you're looking for here.</p> <p>Essentially one typically uses notifications when there are potentially many delegates all of which need to do <em>something</em> in response to a single action or event that has occurred. Think of it as a one-to-many sort of delegation or an implementation of the observer pattern. The basic things to know are:</p> <ol> <li><p>NSNotifications have a name that you define which is just an NSString. Notifications can be posted by name and objects register to receive notifications by name.</p></li> <li><p>When a notification is posted a notificationSender object and/or userInfo dictionary can be provided. The notificationSender is the direct way of determining who posted a given notification when it is being handled by the receiver. The userInfo is an NSDictionary that can be used to provide additional context info along with the notification. </p></li> </ol> <p>So, rather than forcing all of the workers to adopt to an informal protocol and messing around with reflection style calling-methods-at runtime you just register instances of the workers with NSNotificationCenter. Typically the registration with the NSNotificationCenter is done in an init method of each worker class. Instances of each type of worker are then typically set up as "freeze dried" objects in a NIB or can be programatically instantiated in the app delegate so that they get registered with the notification center early on in the app's life.</p> <p>When the <em>thing</em> occurs you post a NSNotification to the NSNotificationCenter (which is essentially a singleton) and then everything else that has registered to receive that particular type of notification will have the method called that was specified to handle that type of notification. When done these methods can then call a common method back on the sender (obtained via NSNotification's object method) to tell the sender that they've completed their work.</p> <p>Once each known worker has checked in the the common method on the sender can then go on to whatever post-worker-completion code is to be performed.</p>
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    1. POHow should I deal with the need for multiple callbacks for the same delegate in Objective-C?
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    1. USTerry Longrie
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    1. POHow should I deal with the need for multiple callbacks for the same delegate in Objective-C?
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    1. COSo any context can be placed into the userInfo NSDictionary? It looks like the notifications could be used in more than one class instance as well as long as they both are set up that way. This list of links from Apple should help me get started. http://developer.apple.com/mac/library/documentation/Cocoa/Conceptual/Notifications/Introduction/introNotifications.html Thanks Terry!
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    2. COAs long as you can store it as a value in a dictionary you can pass around things as needed. Absolutely notifications can be sent by and received by more than one class. In general they promote decoupling where you want class A, B, C, and D to carry out some action when some sort of event occurs but you don't want your event processing code to necessarily have to know about classes A, B, C, or D.
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    3. COOne tip - be sure you remove() your observers, the objects you register to receive notifications, from the NSNotificationCenter when they are no longer needed. Typically, if an object is registered as an observer in an init method then a good place to remove them is in their dealloc( ) implementation.
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