Associated Objects 介绍

Associated Objects

<objc/runtime.h>中的函数号称是iOS中最后一把神兵利刃，具有其他方式做不到的，能为应用和框架提供强大功能的能力。但使用不当也可能废掉代码的,一切代码和逻辑都可能被异常糟糕的副作用影响。

就像和魔鬼做交易一样常常让人怀着巨大的恐惧。

历史

对象关联（或称为关联引用）本来是Objective-C 2.0运行时的一个特性，起始于OS X Snow Leopard和iOS4。

核心函数3个 -- 允许将任何键值在允许时关联到对象上

• objc_setAssociatedObject
• objc_getAssociatedObject
• objc_removeAssociatedObjects

这有什么用呢？这允许开发者对已经存在的类在扩展中添加自定义的属性

实现的3种方式

• static char
• selector
• _cmd_

1.使用 static char 实现

NSObject+AssociatedObject.h

@interface NSObject (AssociatedObject)
@property (nonatomic, strong) id associatedObject;
@end

NSObject+AssociatedObject.m

@implementation NSObject (AssociatedObject)

static char kAssociatedObjectKey;

- (id)associatedObject
{
    return objc_getAssociatedObject(self,&kAssociatedObjectKey);
}

- (void)setAssociatedObject:(id)associatedObject
{
    objc_setAssociatedObject(self, &kAssociatedObjectKey, associatedObject, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}

@end

2.使用 selector 实现

NSObject+AssociatedObject.h

@interface NSObject (AssociatedObject)
@property (nonatomic, strong) id associatedObject;
@end

NSObject+AssociatedObject.m

@implementation NSObject (AssociatedObject)

- (id)associatedObject
{
    return objc_getAssociatedObject(self, @selector(associatedObject));
}

- (void)setAssociatedObject:(id)associatedObject
{
    objc_setAssociatedObject(self, @selector(associatedObject), associatedObject, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}

@end

2.使用 _cmd_ 实现

NSObject+AssociatedObject.h

@interface NSObject (AssociatedObject)
@property (nonatomic, strong) id associatedObject;
@end

NSObject+AssociatedObject.m

@implementation NSObject (AssociatedObject)

- (id)associatedObject
{
    return objc_getAssociatedObject(self, _cmd_);
}

- (void)setAssociatedObject:(id)associatedObject
{
    objc_setAssociatedObject(self, @selector(associatedObject), associatedObject, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}

@end

关联对象的行为

属性根据枚举类型objc_AssociationPolicy来决定被关联在对象上的行为：

OBJC_ASSOCIATION_ASSIGN类型关联对象的弱引用不代表weak的弱引用，行为上更像unsafe_unretained属性。

对象销毁时间

被关联的对象在声明周期内比对象本身释放要晚很多，在对象调用-dealloc中的object_dispose()中释放。

删除属性

你或许想要使用objc_removeAssociatedObjects（）来进行删除操作，但官方文档不建议手动调用这个函数。

这个函数可能会把其他用户对其添加的属性也移除了，正确的方式是调用objc_setAssociatedObject方法，传入nil值来清楚一个关联。

优秀样例

• 添加私有属性用于更好地去实现细节。
• 添加public属性来增强category的功能。
• 创建一个用于KVO的关联观察者。

应用举例

1.UIImagePickerController 图片选择回调

关联一个block实现完成选择图片后的回调。

UIImagePickerController+RFBlocks.h

@interface UIImagePickerController (RFBlocks)<UIImagePickerControllerDelegate, UINavigationControllerDelegate>

typedef void(^RFImagePickerFinishBlock)(NSDictionary *info);

@property (nonatomic, copy) RFImagePickerFinishBlock rf_finishBlock;

+ (UIImagePickerController *)rf_imagePickerWithFinishBlock:(RFImagePickerFinishBlock)finishBlock;

@end

UIImagePickerController+RFBlocks.m

static const char kFinishBlockKey;

@implementation UIImagePickerController (RFBlocks)

+ (UIImagePickerController *)rf_imagePickerWithFinishBlock:(RFImagePickerFinishBlock)finishBlock
{
    UIImagePickerController *picker = [[UIImagePickerController alloc] init];
    picker.rf_finishBlock = finishBlock;
    
    return picker;
}

- (RFImagePickerFinishBlock)rf_finishBlock {
    return objc_getAssociatedObject(self, &kFinishBlockKey);
}

- (void)setRf_finishBlock:(RFImagePickerFinishBlock)rf_finishBlock {
    self.delegate = self;
    
    objc_setAssociatedObject(self, &kFinishBlockKey, rf_finishBlock, OBJC_ASSOCIATION_COPY_NONATOMIC);
}

#pragma mark - UIImagePickerControllerDelegate

- (void)imagePickerController:(UIImagePickerController *)picker didFinishPickingMediaWithInfo:(NSDictionary<NSString *,id> *)info
{
    RFImagePickerFinishBlock block = self.rf_finishBlock;
    
    if (block)  block(info);
    
    [self dismissViewControllerAnimated:YES completion:nil];
    
    self.rf_finishBlock = nil;
}

@end

使用：

__weak typeof(self)weakSelf = self;

UIImagePickerController *picker =  [UIImagePickerController rf_imagePickerWithFinishBlock:^(NSDictionary *info) {
        NSLog(@"finish picke image\n info:%@",info);
        
        UIImage *img = info[@"UIImagePickerControllerOriginalImage"];
        weakSelf.view.layer.contents = (id)img.CGImage;
    }];
[self presentViewController:picker animated:YES completion:nil];

或者

UIImagePickerController *picker = [[UIImagePickerController alloc] init];
    
__weak typeof(self)weakSelf = self;

picker.rf_finishBlock = ^(NSDictionary *info) {
    NSLog(@"finish picke image\n info:%@",info);
    
    UIImage *img = info[@"UIImagePickerControllerOriginalImage"];
    weakSelf.view.layer.contentsGravity = kCAGravityResizeAspect;
    weakSelf.view.layer.contents = (id)img.CGImage;
};

[self presentViewController:picker animated:YES completion:nil];

1.UIButton 按钮事件回调

关联一个按钮事件block，当触发按钮UIControlEventTouchUpInside事件时回调。

UIButton+RFBlcoks.h

@interface UIButton (RFBlcoks)

typedef void(^RFButtonClickBlock)(UIButton *button);

@property (nonatomic, copy) RFButtonClickBlock rf_buttonClickBlock;

@end

UIButton+RFBlcoks.m

static char kButttonClickBlockKey;

@implementation UIButton (RFBlcoks)

- (RFButtonClickBlock)rf_buttonClickBlock
{
    return objc_getAssociatedObject(self, &kButttonClickBlockKey);
}

- (void)setRf_buttonClickBlock:(RFButtonClickBlock)rf_buttonClickBlock
{
    objc_setAssociatedObject(self, &kButttonClickBlockKey, rf_buttonClickBlock, OBJC_ASSOCIATION_COPY_NONATOMIC);
    
    [self addTarget:self action:@selector(buttonClicked) forControlEvents:UIControlEventTouchUpInside];
}

- (void)buttonClicked
{
    if (self.rf_buttonClickBlock) {
        self.rf_buttonClickBlock(self);
    }
}

@end

使用：

self.button.rf_buttonClickBlock = ^(UIButton *button){
        NSLog(@"%@ clicked",button);
};

Demo地址：

• AssociatedObjectDemo

原文发表于王若风的技术博客

参考资料：

• Associated Objects -- NSHipster
• Objective-C Associated Objects -- King's Cocoa
• Objective-C Associated Objects 的实现原理-- 雷纯峰的技术博客
• Associated Objects by Example -- Sebastian Rehnby