category與associative作為objective-c的擴充機制的兩個特性，category即類型，可以通過它來擴充方法；associative，可以通過它來擴充屬性；在iOS開發中，可能category比較常見，相對的associative，就用的比較少，要用它必須使用<objc/runtime.h>的標頭檔，然後就可以自由使用objc_getAssociatedObject以及objc_setAssociatedObject，我們來看下這兩個方法：
OBJC_EXPORT void objc_setAssociatedObject(id object, const void *key, id value, objc_AssociationPolicy policy)
__OSX_AVAILABLE_STARTING(__MAC_10_6, __IPHONE_3_1);
OBJC_EXPORT id objc_getAssociatedObject(id object, const void *key)
__OSX_AVAILABLE_STARTING(__MAC_10_6, __IPHONE_3_1);
另外還有一個方法：
OBJC_EXPORT void objc_removeAssociatedObjects(id object)
__OSX_AVAILABLE_STARTING(__MAC_10_6, __IPHONE_3_1);
objc_getAssociatedObject、objc_setAssociatedObject、objc_removeAssociatedObjects都是Obj-c中的外聯方法，object 參數作為待擴充的對象執行個體，key作為該對象執行個體的屬性的鍵，而value就是對象執行個體的屬性的值，policy作為關聯的策略，它的枚舉包括：
enum {
OBJC_ASSOCIATION_ASSIGN = 0,
OBJC_ASSOCIATION_RETAIN_NONATOMIC = 1,
OBJC_ASSOCIATION_COPY_NONATOMIC = 3,
OBJC_ASSOCIATION_RETAIN = 01401,
OBJC_ASSOCIATION_COPY = 01403
};
具體我就多說了，可以看得懂的。
另外，objc_removeAssociatedObjects可以刪除指定對象執行個體的所有擴充屬性。
現在看一個簡單的例子：（引用網上的例子:http://code4app.com/ios/Block-UI/504fe65d6803faa33f000003 ）
這裡，定義了一個"alert view"的按鈕：
UIButton *button = [UIButton buttonWithType:UIButtonTypeRoundedRect];
[viewController.view addSubview:button];
button.frame = CGRectMake(50, 50, 100, 44);
[button setTitle:@"alert view" forState:UIControlStateNormal];
現在對UIButton通過使用category對其進行方法擴充：
@implementation UIControl (BUIControl)
…...
- (void)handleControlEvent:(UIControlEvents)event withBlock:(void(^)(id sender))block {
NSString *methodName = [UIControl eventName:event];
NSMutableDictionary *opreations = (NSMutableDictionary*)objc_getAssociatedObject(self, &OperationKey);
if(opreations == nil)
{
opreations = [[NSMutableDictionary alloc] init];
objc_setAssociatedObject(self, &OperationKey, opreations, OBJC_ASSOCIATION_RETAIN);
}
[opreations setObject:block forKey:methodName];
[self addTarget:self action:NSSelectorFromString(methodName) forControlEvents:event];
}
objc_getAssociatedObject，objc_setAssociatedObject進行屬性擴充：
OperationKey是一個字元類型，這裡表示一個鍵，如果Opreations為空白，會setAssociatedObject一個新的opreations對象到對應鍵的值中間。
addTarget:action:forControlEvents對於觸摸事件進行目標綁定。
NSSelectorFromString(methodName) 將觸發方法：
-(void)UIControlEventTouchDown{[self callActionBlock:UIControlEventTouchDown];}
-(void)UIControlEventTouchDownRepeat{[self callActionBlock:UIControlEventTouchDownRepeat];}
-(void)UIControlEventTouchDragInside{[self callActionBlock:UIControlEventTouchDragInside];}
-(void)UIControlEventTouchDragOutside{[self callActionBlock:UIControlEventTouchDragOutside];}
-(void)UIControlEventTouchDragEnter{[self callActionBlock:UIControlEventTouchDragEnter];}
-(void)UIControlEventTouchDragExit{[self callActionBlock:UIControlEventTouchDragExit];}
-(void)UIControlEventTouchUpInside{[self callActionBlock:UIControlEventTouchUpInside];}
-(void)UIControlEventTouchUpOutside{[self callActionBlock:UIControlEventTouchUpOutside];}
-(void)UIControlEventTouchCancel{[self callActionBlock:UIControlEventTouchCancel];}
-(void)UIControlEventValueChanged{[self callActionBlock:UIControlEventValueChanged];}
-(void)UIControlEventEditingDidBegin{[self callActionBlock:UIControlEventEditingDidBegin];}
-(void)UIControlEventEditingChanged{[self callActionBlock:UIControlEventEditingChanged];}
-(void)UIControlEventEditingDidEnd{[self callActionBlock:UIControlEventEditingDidEnd];}
-(void)UIControlEventEditingDidEndOnExit{[self callActionBlock:UIControlEventEditingDidEndOnExit];}
-(void)UIControlEventAllTouchEvents{[self callActionBlock:UIControlEventAllTouchEvents];}
-(void)UIControlEventAllEditingEvents{[self callActionBlock:UIControlEventAllEditingEvents];}
-(void)UIControlEventApplicationReserved{[self callActionBlock:UIControlEventApplicationReserved];}
-(void)UIControlEventSystemReserved{[self callActionBlock:UIControlEventSystemReserved];}
-(void)UIControlEventAllEvents{[self callActionBlock:UIControlEventAllEvents];}
注意到這裡會去執行callActionBlock:方法：
- (void)callActionBlock:(UIControlEvents)event {
NSMutableDictionary *opreations = (NSMutableDictionary*)objc_getAssociatedObject(self, &OperationKey);
if(opreations == nil) return;
void(^block)(id sender) = [opreations objectForKey:[UIControl eventName:event]];
if (block) block(self);
}
最終會從擴充屬性中得到^block，關於^block可以參考：http://blog.csdn.net/pjk1129/article/details/6577097
最後調用：
[button handleControlEvent:UIControlEventTouchUpInside withBlock:^(id sender) {
UIAlertView *alert = [[UIAlertView alloc] initWithTitle:@"alert"
message:nil
delegate:nil
cancelButtonTitle:@"ok"
otherButtonTitles:@"other",nil];
}];
點擊按鈕就會觸發一個block函數。
這樣就實現了objective-c的兩種擴充機制。
