CODEWITHSUNDEEP
objective-c
johnbakers
johnbakers

Reputation: 24750

2 quickies about NSNotification Center

In all the tutorials or examples I find, they show a colon after the selector name (getCurrentData:), like so:

[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(getCurrentData:) name:@"mapsReceived" object:nil ];

Since there is nothing after this colon in the examples, I assume that the method specified does not take any arguments. And mine doesn't either, but when I use the above syntax my app crashes and says unrecognized selector sent to instance I remove the colon, and it works. So why do all examples show a colon?

Secondly, if I did use a selector that required an argument, how can I do that? If I do this:

[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(createButtons:@"ten grand";) name:@"mapsReceived" object:nil ];

It won't even compile; I get tons of errors until I remove the text string and the colon. Yet I can call this method, with the string, just fine when using it outside NSNotification. Is there a way to do what I want here?

Upvotes: 2

Views: 187

Answers (5)

Joe
Joe

Reputation: 57169

If there is a colon at the end then it takes a parameter. You do not pass it anything in the @selector call. Your first example is correct and your getCurrentData: method should look like this.

-(void)getCurrentData:(NSNotification*)note
{
     //Implementation
}

For each colon you see in a selector that is how parameters it takes, for example both @selector(aMethod:secondArg:thirdArg:) and @selector(aMethod:::) have different signatures but both would take 3 arguments.

Upvotes: 1

Kheldar
Kheldar

Reputation: 5389

You're trying ti add an observer and send a message in the same code, it doesn't work that way. You declare that THIS method is used to react to an event as in [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(myMethod:) name:@"behaviorName" object:nil];

Then it will be called when the event happens, and you can pass it a special object via the object parameter to work on. However, behind the colon is supposed to be passed a special event encapsulation object (NSNotification obviously), see the documentation, which you are supposed to design your myMethod: method for. If you omitted the object in your method code, then it makes sense trying to call it fails, since you're calling another method, in effect an overload :p

Upvotes: 0

Holger
Holger

Reputation: 1839

The answer to the first question is that in Objective C the colon is part of the function name. If you omit it you refer to a different function, one that doesn't take any parameters. However, notifications methods are always called with (and must therefore accept) one parameter: the Notification object:

Here is an excerpt of the NSNotificationCenter Class Reference:

"The method specified by notificationSelector must have one and only one argument (an instance of NSNotification)"

Upvotes: 2

dtuckernet
dtuckernet

Reputation: 7895

First, most all examples assume that you are passing the notification to the method. For example:

- (void)respondToNotification:(NSNotification *);

This would be represented as:

@selector(respondToNotification:)

If you removed this argument, it would be

@selector(respondToNotification)

As for the second item, you cannot specify arguments in that manner. You need to architect your methods in such a way that it receives the notification as the argument, and then you can inspect the notification and call another method to perform whatever action you need.

Upvotes: 3

marramgrass
marramgrass

Reputation: 1411

When the NSNotificationCenter sends a notification (via the selector you specify - eg getCurrentData:), it will pass an instance of NSNotification, so your method must accept a single argument which is such an instance.

See http://developer.apple.com/library/mac/#documentation/Cocoa/Reference/Foundation/Classes/NSNotificationCenter_Class/Reference/Reference.html

Upvotes: 2

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