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    <p>Subclassing <code>NSNotification</code> is an atypical operation. I think I've only seen it done once or twice in the past few years.</p> <p>If you're looking to pass things along with the notification, that's what the <code>userInfo</code> property is for. If you don't like accessing things through the <code>userInfo</code> directly, you could use a category to simplify access:</p> <pre><code>@interface NSNotification (EasyAccess) @property (nonatomic, readonly) NSString *foo; @property (nonatomic, readonly) NSNumber *bar; @end @implementation NSNotification (EasyAccess) - (NSString *)foo { return [[self userInfo] objectForKey:@"foo"]; } - (NSNumber *)bar { return [[self userInfo] objectForKey:@"bar"]; } @end </code></pre> <p>You can also use this approach to simplify <code>NSNotification</code> creation. For example, your category could also include:</p> <pre><code>+ (id)myNotificationWithFoo:(NSString *)foo bar:(NSString *)bar object:(id)object { NSDictionary *d = [NSDictionary dictionaryWithObjectsForKeys:foo, @"foo", bar, @"bar", nil]; return [self notificationWithName:@"MyNotification" object:object userInfo:d]; } </code></pre> <p>If, for some strange reason, you'd need the properties to be mutable, then you'd need to use <a href="http://developer.apple.com/library/ios/#documentation/Cocoa/Conceptual/ObjectiveC/Chapters/ocAssociativeReferences.html" rel="noreferrer">associative references</a> to accomplish that:</p> <pre><code>#import &lt;objc/runtime.h&gt; static const char FooKey; static const char BarKey; ... - (NSString *)foo { return (NSString *)objc_getAssociatedObject(self, &amp;FooKey); } - (void)setFoo:(NSString *)foo { objc_setAssociatedObject(self, &amp;FooKey, foo, OBJC_ASSOCIATION_RETAIN); } - (NSNumber *)bar { return (NSNumber *)objc_getAssociatedObject(self, &amp;BarKey); } - (void)setBar:(NSNumber *)bar { objc_setAssociatedObject(self, &amp;BarKey, bar, OBJC_ASSOCIATION_RETAIN); } ... </code></pre>
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    1. POIs subclassing NSNotification the right route if I want to add typed properties?
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    1. COGreat explaination. Thanks a lot
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    2. COOn further reflection this doesn't really solve the problem I have. Adding categories is fine if I want all instances of NSNotification to gain this extra functionality, but I want the option to have different specialised notifications. Perhaps one will be carrying a payload of type NSArray, whilst another might be carrying a Some Class as its payload. I understand that I can use userInfo as a kind of generic dump for the data a notification carries, but I think its much cleaner to have a notification specialise.
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