iOS Runtime特性之关联对象

前言

现在你准备用一个系统的类或者是你写的类，但是这个类并不能满足你的需求，你需要额外添加一个属性。
一般解决办法要么是extends(继承)，要么使用category(类别)。
而我并不推荐使用extends，主要是耦合性太强，一般我使用category。
我们都知道，分类中是无法设置属性的，如果在分类的声明中写@property 只能为其生成get 和 set 方法的声明，
但是有时候使用类别也需要增加一个额外属性，
那么怎么办呢？
这个时候，runtime的关联属性就能发挥它的作用了。
一般都是key value 的存在。

有关的方法

objc_setAssociatedObject 设置关联对象使用
objc_getAssociatedObject 获取关联对象使用
objc_removeAssociatedObjects 移除关联对象使用

用法

一般我用在category里，合理使用它能让category发挥更大的作用。

• UIView的category

.h文件

#import <UIKit/UIKit.h>
@interface UIView (WT)
typedef void (^GestureActionBlock)(UIGestureRecognizer *ges);
/** 单点击手势 */
- (void)tapGesture:(GestureActionBlock)block;
/** 长按手势 */
- (void)longPressGestrue:(GestureActionBlock)block;
@end

.m文件

#import "UIView+WT.h"
#import <objc/runtime.h>
@implementation UIView (WT)

static char kActionHandlerTapBlockKey;
static char kActionHandlerTapGestureKey;
static char kActionHandlerLongPressBlockKey;
static char kActionHandlerLongPressGestureKey;

//单点击手势
- (void)tapGesture:(GestureActionBlock)block {
    self.userInteractionEnabled = YES;
    UITapGestureRecognizer *gesture = objc_getAssociatedObject(self, &kActionHandlerTapGestureKey);
    if (!gesture) {
        gesture = [[UITapGestureRecognizer alloc] initWithTarget:self action:@selector(handleActionForTapGesture:)];
        [self addGestureRecognizer:gesture];
        objc_setAssociatedObject(self, &kActionHandlerTapGestureKey, gesture, OBJC_ASSOCIATION_RETAIN);
    }
    objc_setAssociatedObject(self, &kActionHandlerTapBlockKey, block, OBJC_ASSOCIATION_COPY);
}

- (void)handleActionForTapGesture:(UITapGestureRecognizer *)gesture {
    if (gesture.state == UIGestureRecognizerStateRecognized) {
        GestureActionBlock block = objc_getAssociatedObject(self, &kActionHandlerTapBlockKey);
        if (block) {
            block(gesture);
        }
    }
}

//长按手势
- (void)longPressGestrue:(GestureActionBlock)block {
    self.userInteractionEnabled = YES;
    UILongPressGestureRecognizer *gesture = objc_getAssociatedObject(self, &kActionHandlerLongPressGestureKey);
    if (!gesture) {
        gesture = [[UILongPressGestureRecognizer alloc] initWithTarget:self action:@selector(handleActionForLongPressGesture:)];
        [self addGestureRecognizer:gesture];
        objc_setAssociatedObject(self, &kActionHandlerLongPressGestureKey, gesture, OBJC_ASSOCIATION_RETAIN);
    }
    objc_setAssociatedObject(self, &kActionHandlerLongPressBlockKey, block, OBJC_ASSOCIATION_COPY);
}

- (void)handleActionForLongPressGesture:(UITapGestureRecognizer *)gesture {
    if (gesture.state == UIGestureRecognizerStateBegan) {
        GestureActionBlock block = objc_getAssociatedObject(self, &kActionHandlerLongPressBlockKey);
        if (block) {
            block(gesture);
        }
    }
}

@end

我解释下里面的一些关键字段，比如OBJC_ASSOCIATION_RETAIN这个字段实际上是个枚举来的

typedef OBJC_ENUM(uintptr_t, objc_AssociationPolicy) {
    OBJC_ASSOCIATION_ASSIGN = 0,           /**< Specifies a weak reference to the associated object. */
    OBJC_ASSOCIATION_RETAIN_NONATOMIC = 1, /**< Specifies a strong reference to the associated object. 
                                            *   The association is not made atomically. */
    OBJC_ASSOCIATION_COPY_NONATOMIC = 3,   /**< Specifies that the associated object is copied. 
                                            *   The association is not made atomically. */
    OBJC_ASSOCIATION_RETAIN = 01401,       /**< Specifies a strong reference to the associated object.
                                            *   The association is made atomically. */
    OBJC_ASSOCIATION_COPY = 01403          /**< Specifies that the associated object is copied.
                                            *   The association is made atomically. */
};

用法跟@property中的strong 、weak、copy 、assign 、retain等声明属性的修饰符一样，我上面用到了block就对应OBJC_ASSOCIATION_COPY,而UITapGestureRecognizer与UILongPressGestureRecognizer则对应OBJC_ASSOCIATION_RETAIN进行修饰。
当然实际上我的UIView的category不止这些，可以参考我开发项目总结的一套库
WTSDK
可能有些地方描述得不是很好，或者描述错误了，希望你们能给我留言，thank！