Beginner Objective-C: Extending a class?

Question

I'm very new with objective-C, and I am trying to implement a 'factory' style class extending a cocos2D class called CCSprite.

I want there to be a class Monster which extends CCSprite that will create and issue sprites with unique IDs so that I may call and manipulate the generated sprites later on; I am extending CCSprite to add an instance_id property and a static global_id variable.

I want to be able to use all the CCSprite constructors such as spriteWithImageNamed along with all the other functions CCSprite has, the problem is that my instance_id isn't being assigned/incremented properly(It's always 0), and I don't really know where to start to fix this.

Here is what I have:

Monster.h:

#import "CCSprite.h"

@interface Monster : CCSprite

@property (nonatomic, readonly) int instance_id;

@end

Monster.m:

#import "Monster.h"

static int global_id = 0;

@implementation Monster:CCSprite

@synthesize instance_id;


-(id) init{
    self = [super init];

    if(self){
        instance_id = global_id;
        global_id++;
    }
    return self;
}

@end

Is the init function called every time any constructor is called? (eg spriteWithImageNamed)?

How do I ensure my extended properties and variables are applied when I call the parent class's functions?

note that @synthesize is superfluous, Xcode will automatically create the ivar for you with the same name as the property but prefixed with underscore, in this case: _instance_id — LearnCocos2D

Chuck Chuck · Accepted Answer · 2014-07-17T00:26:40

In Objective-C, a class's designated initializer is the one that other initializers are supposed to call into. For many classes, this is indeed init. According to the Cocos2D documentation, however, init is not the designated initializer for CCSprite. Instead, the designated initializer is:

- (id)initWithTexture:(CCTexture *)texture rect:(CGRect)rect rotated:(BOOL)rotated

So that is the method you need to override if you want all the other initializers to call your version.

(If you want to confirm for yourself that this works, you can put a log statement or breakpoint in your implementation of the designated initializer, then call other initializers and make sure the designated initializer is called.)

Beginner Objective-C: Extending a class?

3 Answers