Monkey-Patching

Objective-C

Extending iOS SDK

Defining Classes

@interface MyObject : NSObject

@end

Instance Variables == ivars

@interface MyObject : NSObject {
  @private
  NSUInteger count
  MySubObject* subObject;
}

@end

Methods

    @interface MyObject : NSObject {
      @private
      NSUInteger _count;
      MySubObject* _subObject;
    }

    + (Counter*) counter;
    - (void) increment;
    - (MySubObject*) subObject;
    - (void) setSubObject:(MySubObject*)subObject;

    @end

Implementing Classes

    @implementation MyObject

    ...

    - (MySubObject*) subObject {
      return _subObject;
    }

    - (void) setSubObject:(MySubObject*)subObject {
      _subObject = [subObject retain];
    }

    ...

Properties

    @interface MyObject : NSObject {
      @private
      NSUInteger count;
      MySubObject* subObject;
    }

    + (Counter*) myObject;
    - (void) increment;

    @property (nonatomic,retain) subObject;

    @end

Properties

myObject.subObject;

myObject.subObject = aSubObject;

Implementing Properties

    @implementation MyObject

    ...

    - (MySubObject*) subObject {
      return _subObject;
    }

    - (void) setSubObject:(MySubObject*)subObject {
      _subObject = [subObject retain];
    }

    ...

@synthesize

@implementation MyObject

@synthesize subObject = _subObject;

...

@end

In fact ...

    @interface MyObject : NSObject {
      NSUInteger count;
    }

    ...

    @property (nonatomic,retain) subObject;

    @end

Reopening Classes (Category)

    @interface NSDate (Helpers)

    - (NSString*) timeAgoInWords;

    @end

Reopening Classes (Category)

    @implementation NSDate (Helpers)

    - (NSString *)timeAgoInWords {
      double ti = [self timeIntervalSinceNow];
      ti = ti * -1;
      if(ti < 1) {
        return @"never";
      } else if (ti < 60) {
        return @"less than a minute ago";
      } else if (ti < 3600) {
        int diff = round(ti / 60);
        return [NSString stringWithFormat:@"%d minutes ago", diff];
      } else if (ti < 86400) {
        int diff = round(ti / 60 / 60);
        return[NSString stringWithFormat:@"%d hours ago", diff];
      } else if (ti < 2629743) {
        int diff = round(ti / 60 / 60 / 24);
        return[NSString stringWithFormat:@"%d days ago", diff];
      } else {
        return @"never";
      }   
    }

    @end

SO ...

    @interface UIViewController (Pager)

    @property (nonatomic,retain) 
        PagerController* pagerController;
    @property (nonatomic,retain) 
        ScrollTabBarItem* scrollTabBarItem;

    @end

    @implementation UIViewController (Pager)

    @synthesize pagerController;
    @synthesize scrollTabBarItem;

    @end

BUT ...

Categories Can't Create

New ivars!

What to do?

Associative References!

    @implementation UIViewController (Pager)

    static char pagerControllerKey;
    static char scrollTabBarItemKey;

    - (PagerController *)pagerController
    {
      return (PagerController*)
          objc_getAssociatedObject(self,
              &pagerControllerKey);
    }

    - (void)setPagerController:
        (PagerController *)pagerController
    {
      objc_setAssociatedObject(self, 
        &pagerControllerKey, 
        pagerController,
        OBJC_ASSOCIATION_RETAIN_NONATOMIC);
    }

    - (ScrollTabBarItem *)scrollTabBarItem
    {
      return (ScrollTabBarItem*)
        objc_getAssociatedObject(self,
          &scrollTabBarItemKey);
    }

    - (void)setScrollTabBarItem:(ScrollTabBarItem *)scrollTabBarItem
    {
      objc_setAssociatedObject(self, 
        &scrollTabBarItemKey, scrollTabBarItem,
        OBJC_ASSOCIATION_RETAIN_NONATOMIC);
    }
    @end

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