Apple SeismicXML Example App

In the SeismicXMLAppDelegate implementation file of this class they have the following code:

// forward declarations

@interface SeismicXMLAppDelegate ()



@property (nonatomic, retain) NSURLConnection *earthquakeFeedConnection;

@property (nonatomic, retain) NSMutableData *earthquakeData;    // the data returned from the NSURLConnection

@property (nonatomic, retain) NSOperationQueue *parseQueue;     // the queue that manages our NSOperation for parsing earthquake data



- (void)addEarthquakesToList:(NSArray *)earthquakes;

- (void)handleError:(NSError *)error;

@end

Why do they have a second interface in the implementation file?

http://开发者_JS百科developer.apple.com/library/ios/#samplecode/SeismicXML/Listings/Classes_SeismicXMLAppDelegate_m.html#//apple_ref/doc/uid/DTS40007323-Classes_SeismicXMLAppDelegate_m-DontLinkElementID_10

This is a called an Extension (or an anonymous Category) in Objective-C

You can add properties, change its attributes and declare new methods like in that example. Why not doing it in the interface file? Well there could be a lot of reasons, for design purposes, for not to exposing some properties., etc.

For example, you cannot call myAppDelegate.earthquakeData from RootViewController.m even if you #import "SeismicXMLAppDelegate.h". You can only access to earthquakeDataproperty from inside of SeismicXMLAppDelegate class.

You can read more about Categories and Extensions here: The Objective-C Programming Language