下文會用到const,static關(guān)鍵詞,下一篇文章會針對const,static,extern等關(guān)鍵字的用法詳細介紹,9月18號之前會找空閑編寫上傳,敬請期待,同時歡迎大神指點不足之處-----我的愿望是世界充滿愛

關(guān)于關(guān)聯(lián)引用,簡單的應用場景,舉個栗子:

(1)給一個類動態(tài)添加一個屬性;

(2)給一個類添加一個方法;

官方的API是這樣子的:

//掛連策略,這是類型枚舉,按照字段可以判斷用處
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. */
};

關(guān)聯(lián)對象的三種用法

/*
object 源對象
key 關(guān)鍵字
value 關(guān)聯(lián)的對象
關(guān)鍵策略 上述的枚舉
*/
//設置關(guān)聯(lián)對象
OBJC_EXPORT void objc_setAssociatedObject(id object, const void *key, id value, objc_AssociationPolicy policy)
    __OSX_AVAILABLE_STARTING(__MAC_10_6, __IPHONE_3_1);

//獲取關(guān)聯(lián)對象
OBJC_EXPORT id objc_getAssociatedObject(id object, const void *key)
    __OSX_AVAILABLE_STARTING(__MAC_10_6, __IPHONE_3_1);

//刪除關(guān)聯(lián)對象
OBJC_EXPORT void objc_removeAssociatedObjects(id object)
    __OSX_AVAILABLE_STARTING(__MAC_10_6, __IPHONE_3_1);

情景一:動態(tài)添加屬性

有一個User的類,類里面有關(guān)于User的屬性,但是后來你要新添加屬性,User這個類會經(jīng)常用到,但是你新添加的屬性卻不怎么用到,為了節(jié)省內(nèi)存,造成不必要的系統(tǒng)開銷,所以做一個動態(tài)添加屬性

1.創(chuàng)建一個User類,基于NSObject

#import <Foundation/Foundation.h>

@interface User : NSObject

@property(nonatomic,copy)NSString *userName;

@end

2.需要添加一個userAge的屬性

創(chuàng)建一個基于User的類別

#import "User.h"

@interface User (Extensions)

@property(nonatomic,copy)NSString *userAge;

@end

#import "User+Extensions.h"
#import <objc/runtime.h>

@implementation User (Extensions)
static char userAgeKey;

-(NSString *)userAge{
    return objc_getAssociatedObject(self,&userAgeKey);
}

-(void)setUserAge:(NSString *)userAge{
    objc_setAssociatedObject(self, &userAgeKey, userAge, OBJC_ASSOCIATION_COPY);
}

@end

這就是最簡單的一個使用場景

場景二:給UIViewController添加一個方法

給UIViewController添加一個HUD的方法

1.基于UIViewController創(chuàng)建一個類別,然后聲明一個方法

#import <UIKit/UIKit.h>
#import "MBProgressHUD.h"

@interface UIViewController (HUD)

-(void)showHudInView:(UIView *)view
                hint:(NSString *)hint;

@end

2.實現(xiàn)方法

#import "UIViewController+HUD.h"
#import <objc/runtime.h>

static const void * httpReaHud = &httpReaHud;

@implementation UIViewController (HUD)

-(MBProgressHUD *)hud{
    return objc_getAssociatedObject(self, httpReaHud);
}

-(void)setHud:(MBProgressHUD *)hud{
    objc_setAssociatedObject(self, httpReaHud, hud, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}

-(void)showHudInView:(UIView *)view hint:(NSString *)hint{
    MBProgressHUD *HUD = [[MBProgressHUD alloc] initWithView:view];
    HUD.label.text = hint;
    [view addSubview:HUD];
    [HUD showAnimated:YES];
    [self setHud:HUD];
}

@end

這樣從項目的整潔度,簡潔度還有節(jié)省內(nèi)存都有好處

這個是demo,有需要的可以簡單的看一下https://github.com/summer7659/runtime-