iOS开发-关于Associated Objects

一、前言

Associated Objects(关联对象)是什么？什么时候用？为什么要用？怎么用？
最开始用到关联对象是源于一个需求（废话，肯定是源于需求）。
大家都知道，Button的点击事件，一定是将本身传入参数：

- (void)setupFoundationUI {
    UIButton *btn = [UIButton buttonWithType:UIButtonTypeSystem];
    btn.frame = (CGRect){0, 0, 30, 30};
    [self.view addSubview:btn];
    [btn addTarget:self action:@selector(btnDidClick:) forControlEvents:UIControlEventTouchUpInside];
}

- (void)btnDidClick:(UIButton *)sender {
    NSLog(@"Btn did click ...");
}

如果想要传入一个特定的参数呢？

• 当时我想传的参数是整型，于是我想到了tag（那时的我还不知道关联对象）
• tag其实是用来标记不同的Button对象，但此时，我也很无奈。。。就先借用一下吧，哈哈
  也就是这样👇👇👇

- (void)setupFoundationUI {
    UIButton *btn = [UIButton buttonWithType:UIButtonTypeSystem];
    btn.frame = (CGRect){0, 0, 30, 30};
    btn.tag = 110;
    [self.view addSubview:btn];
    [btn addTarget:self action:@selector(btnDidClick:) forControlEvents:UIControlEventTouchUpInside];
}

- (void)btnDidClick:(UIButton *)sender {
    NSLog(@"Btn did click ...tag = %zd", sender.tag);
}

后来又有需求，要传的参数是字符串，甚至是对象。。。
就在此时我注意到了关联对象（Associated Objects）

二、关联对象的介绍

1.关联对象解决的问题

我们知道，在 Objective-C 中可以通过 Category (类别、分类，反正你们懂得)给一个现有的类添加属性，但是却不能添加实例变量，这似乎成为了 Objective-C 的一个明显短板，关联对象就可以解决这个问题。

2.如何用关联对象

• 首先要引入 runtime

#import <objc/runtime.h>

• API主要就是(来自系统文件runtime.h的介绍)👇👇👇

/** 
 * Sets an associated value for a given object using a given key and association policy.
 * 
 * @param object The source object for the association.
 * @param key The key for the association.
 * @param value The value to associate with the key key for object. Pass nil to clear an existing association.
 * @param policy The policy for the association. For possible values, see “Associative Object Behaviors.”
 * 
 * @see objc_setAssociatedObject
 * @see objc_removeAssociatedObjects
 */
OBJC_EXPORT void
objc_setAssociatedObject(id _Nonnull object, const void * _Nonnull key,
                         id _Nullable value, objc_AssociationPolicy policy)
    OBJC_AVAILABLE(10.6, 3.1, 9.0, 1.0, 2.0);

/** 
 * Returns the value associated with a given object for a given key.
 * 
 * @param object The source object for the association.
 * @param key The key for the association.
 * 
 * @return The value associated with the key \e key for \e object.
 * 
 * @see objc_setAssociatedObject
 */
OBJC_EXPORT id _Nullable
objc_getAssociatedObject(id _Nonnull object, const void * _Nonnull key)
    OBJC_AVAILABLE(10.6, 3.1, 9.0, 1.0, 2.0);

/** 
 * Removes all associations for a given object.
 * 
 * @param object An object that maintains associated objects.
 * 
 * @note The main purpose of this function is to make it easy to return an object 
 *  to a "pristine state”. You should not use this function for general removal of
 *  associations from objects, since it also removes associations that other clients
 *  may have added to the object. Typically you should use \c objc_setAssociatedObject 
 *  with a nil value to clear an association.
 * 
 * @see objc_setAssociatedObject
 * @see objc_getAssociatedObject
 */
OBJC_EXPORT void
objc_removeAssociatedObjects(id _Nonnull object)
    OBJC_AVAILABLE(10.6, 3.1, 9.0, 1.0, 2.0);

官方的解释已经很清晰了，就不过多解读了(绑定、获取、移除)，值得注意的一点是：objc_removeAssociatedObjects是移除一个对象的所有关联对象，将该对象恢复成“原始”状态，这样的操作风险太大，所以一般的做法是通过给 objc_setAssociatedObject 函数传入 nil 来移除某个已有的关联对象。如下这样👇👇👇

objc_setAssociatedObject(self, &key, nil, OBJC_ASSOCIATION_RETAIN_NONATOMIC);

• 关于key的问题

• 声明 static char kAssociatedObjectKey; 使用 &kAssociatedObjectKey 作为 key 值;
• 声明 static void *kAssociatedObjectKey = &kAssociatedObjectKey; 使用 kAssociatedObjectKey 作为 key 值；
• 用 selector ，使用 getter 方法的名称作为 key 值。

• 关于policy(关联策略)的问题

OBJC_ASSOCIATION_ASSIGN
等价属性@property (assign) or @property (unsafe_unretained)
弱引用关联对象

OBJC_ASSOCIATION_RETAIN_NONATOMIC
等价属性@property (strong, nonatomic)
强引用关联对象，且为非原子操作

OBJC_ASSOCIATION_COPY_NONATOMIC
等价属性@property (copy, nonatomic)
复制关联对象，且为非原子操作

OBJC_ASSOCIATION_RETAIN
等价属性@property (strong, atomic)
强引用关联对象，且为原子操作

OBJC_ASSOCIATION_COPY
等价属性@property (copy, atomic)
复制关联对象，且为原子操作

具体内容可以参考官方文档，这里就不copy了

三、用关联对象解决上述问题

• 传整型数据

NSString *const kButtonKey = @"kButtonKey";

- (void)setupFoundationUI {
    UIButton *btn = [UIButton buttonWithType:UIButtonTypeSystem];
    objc_setAssociatedObject(btn, &kButtonKey, @110, OBJC_ASSOCIATION_ASSIGN);
    [self.view addSubview:btn];
    [btn addTarget:self action:@selector(btnDidClick:) forControlEvents:UIControlEventTouchUpInside];
}

- (void)btnDidClick:(UIButton *)sender {
    NSInteger value = [objc_getAssociatedObject(sender, &kButtonKey) integerValue];
    NSLog(@"btn did click ...value = %zd", value);
}

• 传对象数据

NSString *const kButtonKey = @"kButtonKey";

- (void)setupFoundationUI {
    UIButton *btn = [UIButton buttonWithType:UIButtonTypeSystem];
    btn.frame = (CGRect){0, 0, 30, 30};
    Person *person = [[Person alloc] init];
    person.name = @"LiMing";
    objc_setAssociatedObject(btn, &kButtonKey, person, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
    [self.view addSubview:btn];
    [btn addTarget:self action:@selector(btnDidClick:) forControlEvents:UIControlEventTouchUpInside];
}

- (void)btnDidClick:(UIButton *)sender {
    Person *person = objc_getAssociatedObject(sender, &kButtonKey);
    NSLog(@"person`s name = %@", person.name);
}

四、关联对象用于Category

以实现UIBarButtonItem的扩展为例子，为其增加红点的功能，其中大量的使用了关联对象
需求：

1.显示小红点
2.显示数字红点
3.即有小红点又有数字红点时，优先显示数字红点
4.可自定义红点颜色（默认是红色[UIColor redColor]）
5.数字红点数目大于99时，显示99+

具体代码如下：👇👇👇

#import <UIKit/UIKit.h>

@interface UIBarButtonItem (Badge)

@property (assign, nonatomic) UIColor *badgeColor;

- (void)configBadgeWithBigNum:(NSInteger)bigNum small:(BOOL)isOn;

@end

#import "UIBarButtonItem+Badge.h"
#import <objc/runtime.h>

NSString *const ZYBarButtonItem_hasBadgeKey = @"ZYBarButtonItem_hasBadgeKey";
NSString *const ZYBarButtonItem_badgeKey = @"ZYBarButtonItem_badgeKey";
NSString *const ZYBarButtonItem_badgeSizeKey = @"ZYBarButtonItem_badgeSizeKey";
NSString *const ZYBarButtonItem_badgeOriginXKey = @"ZYBarButtonItem_badgeOriginXKey";
NSString *const ZYBarButtonItem_badgeOriginYKey = @"ZYBarButtonItem_badgeOriginYKey";
NSString *const ZYBarButtonItem_badgeColorKey = @"ZYBarButtonItem_badgeColorKey";
NSString *const ZYBarButtonItem_badgeSizeWKey = @"ZYBarButtonItem_badgeSizeWKey";

@interface UIBarButtonItem ()

@property (nonatomic, assign) CGFloat badgeSizeW;
@property (strong, nonatomic) UILabel *badge;
@property (assign, nonatomic) CGFloat badgeOriginX;
@property (assign, nonatomic) CGFloat badgeOriginY;
@property (assign, nonatomic) CGFloat badgeSize;
@property BOOL hasBadge;

@end

@implementation UIBarButtonItem (Badge)


- (void)initBadge {
    UIView *superview = nil;
    
    if (self.customView) {
        superview = self.customView;
        superview.clipsToBounds = NO;
    } else if ([self respondsToSelector:@selector(view)] && [(id)self view]) {
        superview = [(id)self view];
    }
    [superview addSubview:self.badge];
    
    // 默认设置 default configure
    self.badgeColor = [UIColor redColor];
    self.badgeSize = 10;
    self.badgeSizeW = 10;
    self.badgeOriginX = 28;
    self.badgeOriginY = 8;
    self.badge.hidden = YES;
    self.badge.layer.masksToBounds = YES;
    self.badge.font = [UIFont boldSystemFontOfSize:12];
    self.badge.textAlignment = NSTextAlignmentCenter;
    self.badge.textColor = [UIColor whiteColor];
}

- (void)showBadge {
    self.badge.hidden = NO;
}

- (void)hideBadge {
    self.badge.hidden = YES;
}

- (void)refreshBadge {
    self.badge.frame = (CGRect){self.badgeOriginX,self.badgeOriginY,self.badgeSizeW,self.badgeSize};
    self.badge.backgroundColor = self.badgeColor;
    self.badge.layer.cornerRadius = self.badgeSize/2;
}


#pragma mark ---------- badge getter & setter function -----------

- (UILabel *)badge {
    UILabel *badge = (UILabel *)objc_getAssociatedObject(self, &ZYBarButtonItem_badgeKey);
    if (!badge) {
        badge = [[UILabel alloc] init];
        [self setBadge:badge];
        [self initBadge];
    }
    return badge;
}

- (void)setBadge:(UILabel *)badge {
    objc_setAssociatedObject(self, &ZYBarButtonItem_badgeKey, badge, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}

- (UIColor *)badgeColor {
    return objc_getAssociatedObject(self, &ZYBarButtonItem_badgeColorKey);
}

- (void)setBadgeColor:(UIColor *)badgeColor {
    objc_setAssociatedObject(self, &ZYBarButtonItem_badgeColorKey, badgeColor, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
    if (self.badge) {
        [self refreshBadge];
    }
}

-(CGFloat)badgeSize {
    NSNumber *number = objc_getAssociatedObject(self, &ZYBarButtonItem_badgeSizeKey);
    return number.floatValue;
}

-(void)setBadgeSize:(CGFloat)badgeSize {
    NSNumber *number = [NSNumber numberWithDouble:badgeSize];
    objc_setAssociatedObject(self, &ZYBarButtonItem_badgeSizeKey, number, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
    if (self.badge) {
        [self refreshBadge];
    }
}

- (CGFloat)badgeSizeW {
    NSNumber *number = objc_getAssociatedObject(self, &ZYBarButtonItem_badgeSizeWKey);
    return number.floatValue;
}

- (void)setBadgeSizeW:(CGFloat)badgeSizeW {
    NSNumber *number = [NSNumber numberWithDouble:badgeSizeW];
    objc_setAssociatedObject(self, &ZYBarButtonItem_badgeSizeWKey, number, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
    if (self.badge) {
        [self refreshBadge];
    }
}

-(CGFloat)badgeOriginX {
    NSNumber *number = objc_getAssociatedObject(self, &ZYBarButtonItem_badgeOriginXKey);
    return number.floatValue;
}

-(void)setBadgeOriginX:(CGFloat)badgeOriginX {
    NSNumber *number = [NSNumber numberWithDouble:badgeOriginX];
    objc_setAssociatedObject(self, &ZYBarButtonItem_badgeOriginXKey, number, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
    if (self.badge) {
        [self refreshBadge];
    }
}

-(CGFloat)badgeOriginY {
    NSNumber *number = objc_getAssociatedObject(self, &ZYBarButtonItem_badgeOriginYKey);
    return number.floatValue;
}

-(void)setBadgeOriginY:(CGFloat)badgeOriginY {
    NSNumber *number = [NSNumber numberWithDouble:badgeOriginY];
    objc_setAssociatedObject(self, &ZYBarButtonItem_badgeOriginYKey, number, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
    if (self.badge) {
        [self refreshBadge];
    }
}

- (void)setHasBadge:(BOOL)hasBadge {
    if (hasBadge) {
        [self showBadge];
    }else{
        [self hideBadge];
    }
    
    NSNumber *number = [NSNumber numberWithBool:hasBadge];
    objc_setAssociatedObject(self, &ZYBarButtonItem_hasBadgeKey, number, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}

- (BOOL)hasBadge {
    NSNumber *number = objc_getAssociatedObject(self, &ZYBarButtonItem_hasBadgeKey);
    return number.boolValue;
}

#pragma mark - Public

- (void)configBadgeWithBigNum:(NSInteger)bigNum small:(BOOL)isOn {
    
    if (bigNum > 0) {
        self.hasBadge = YES;
        self.badgeSize = 18;
        self.badgeOriginY = 6;
        NSString *numStr = [NSString stringWithFormat:@"%zd", bigNum];
        if (bigNum < 10) {
            self.badgeSizeW = 18;
        } else if (bigNum < 100) {
            self.badgeSizeW = 25;
        } else {
            self.badgeSizeW = 30;
            numStr = @"99+";
        }
        self.badge.text = numStr;
    } else if (isOn) {
        self.hasBadge = YES;
        self.badgeSizeW = 10;
        self.badgeSize = 10;
        self.badgeOriginY = 8;
        self.badge.text = nil;
    } else {
        self.hasBadge = NO;
    }
}
@end

五、写在最后

• 关联对象与被关联对象本身的存储并没有直接的关系，它是存储在单独的哈希表中的；
• 关联对象的五种关联策略与属性的限定符非常类似，在绝大多数情况下，我们都会使用 OBJC_ASSOCIATION_RETAIN_NONATOMIC 的关联策略，这可以保证我们持有关联对象；
• 关联对象的释放时机与移除时机并不总是一致，比如用关联策略 OBJC_ASSOCIATION_ASSIGN 进行关联的对象，很早就已经被释放了，但是并没有被移除，而再使用这个关联对象时就会造成 Crash 。
  Associated.jpg