iOS学习系列 - 扩展机制category与associative

category与associative作为objective-c的扩展机制的两个特性，category即类型，可以通过它来扩展方法；associative，可以通过它来扩展属性；在iOS开发中，可能category比较常见，相对的associative，就用的比较少，要用它必须使用<objc/runtime.h>的头文件，然后就可以自由使用objc_getAssociatedObject以及objc_setAssociatedObject，我们来看下这两个方法：

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);

另外还有一个方法：

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中的外联方法，object 参数作为待扩展的对象实例，key作为该对象实例的属性的键，而value就是对象实例的属性的值，policy作为关联的策略，它的枚举包括：

enum {
OBJC_ASSOCIATION_ASSIGN = 0,
OBJC_ASSOCIATION_RETAIN_NONATOMIC = 1,
OBJC_ASSOCIATION_COPY_NONATOMIC = 3,
OBJC_ASSOCIATION_RETAIN = 01401,
OBJC_ASSOCIATION_COPY = 01403
};

具体我就多说了，可以看得懂的。

另外，objc_removeAssociatedObjects可以删除指定对象实例的所有扩展属性。

现在看一个简单的例子：（引用网上的例子:http://code4app.com/ios/Block-UI/504fe65d6803faa33f000003 ）

这里，定义了一个"alert view"的按钮：

UIButton *button = [UIButton buttonWithType:UIButtonTypeRoundedRect];
[viewController.view addSubview:button];
button.frame = CGRectMake(50, 50, 100, 44);
[button setTitle:@"alert view" forState:UIControlStateNormal];

现在对UIButton通过使用category对其进行方法扩展：

@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进行属性扩展：

OperationKey是一个字符类型，这里表示一个键，如果Opreations为空，会setAssociatedObject一个新的opreations对象到对应键的值中间。

addTarget:action:forControlEvents对于触摸事件进行目标绑定。

NSSelectorFromString(methodName) 将触发方法：

-(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];}

注意到这里会去执行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);
}

最终会从扩展属性中得到^block，关于^block可以参考：http://blog.csdn.net/pjk1129/article/details/6577097

最后调用：

[button handleControlEvent:UIControlEventTouchUpInside withBlock:^(id sender) {
UIAlertView *alert = [[UIAlertView alloc] initWithTitle:@"alert"
message:nil
delegate:nil
cancelButtonTitle:@"ok"
otherButtonTitles:@"other",nil];
}];

点击按钮就会触发一个block函数。

这样就实现了objective-c的两种扩展机制。