iOS學習系列 - 擴展機制category與associative

category與associative作為objective-c的擴展機制的兩個特性，category即類型，可以通過它來擴展方法；associative，可以通過它來擴展屬性；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>的頭文件，然后就可以自由使用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 參數作為待擴展的對象實例，key作為該對象實例的屬性的鍵，而value就是對象實例的屬性的值，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可以刪除指定對象實例的所有擴展屬性。

現在看一個簡單的例子：（引用網上的例子:http://code4app.com/ios/Block-UI/504fe65d6803faa33f000003 ）

這里，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對其進行方法擴展：

 
          @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進行屬性擴展：

OperationKey是一個字符類型，這里表示一個鍵，如果Opreations為空，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];} 
         

注意到這里會去執行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); 
          
} 
         

最終會從擴展屬性中得到^block，關於^block可以參考：http://blog.csdn.net/pjk1129/article/details/6577097

最后調用：

           [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的兩種擴展機制。