第四十四篇：iOS使用 runtime 与 响应式编程 KVO 原理

响应式编程 KVO 的原理

1.简单实现 KVO功能

• 一开始 self.person 的 isa 指针类型为 Person,当控制器被触摸时 self.person.age += 1 (调用了 Person 类的 setAge: 方法), 然后会调用监听方法，如下:

-(void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSKeyValueChangeKey,id> *)change context:(void *)context;

• 此时通过段点可查看到 self.person 的 isa 指针为 NSKVONotifying_Person 类型，如图：
  

//
//  Person.h
//
//  Created by 瞿杰 on 2017/8/16.
//  Copyright © 2017年 iThinkerYZ. All rights reserved.
//

#import <Foundation/Foundation.h>

@interface Person : NSObject

@property (nonatomic , assign) NSInteger age ;

@property (nonatomic , copy) NSString * name ;

@end

//
//  Person.m
//
//  Created by 瞿杰 on 2017/8/16.
//  Copyright © 2017年 iThinkerYZ. All rights reserved.
//

#import "Person.h"

@implementation Person

@end

#import <UIKit/UIKit.h>

@interface ViewController : UIViewController
@end

#import "ViewController.h"
#import "Person.h"

@interface ViewController ()

@property (nonatomic , strong) Person * person ;

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];

    self.person = [[Person alloc] init];
    self.person.age = 1 ;

    [self.person addObserver:self forKeyPath:@"age" options:NSKeyValueObservingOptionNew context:nil];
}

-(void)dealloc
{
    [self.person removeObserver:self forKeyPath:@"age"];
}

-(void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event
{
    // 实际调用了 setAge: 方法
    self.person.age++ ;
}

-(void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSKeyValueChangeKey,id> *)change context:(void *)context
{
    NSLog(@"最新年龄：%ld , 类名：%@",self.person.age,NSStringFromClass([self.person class]));
}

@end

---

2.使用 self.person->_age++ 时 KVO 不会调用 observe 对象的 observeValueForKeyPath:…… 方法。

• 把 Person 对象的 age 属性(默认是 @protected 妨问权限)转变成 @public 的权限

  //
  //  Person.h
  //
  //  Created by 瞿杰 on 2017/8/16.
  //  Copyright © 2017年 qujie. All rights reserved.
  //
  
  
  #import <Foundation/Foundation.h>
  
  
  @interface Person : NSObject
  {
      // 默认是私有变量，把它变成公共变量，外面就可以用 ->_age 这样的格式调用该对象属性,不然会报错
      @public
      NSInteger _age ;
  }
  @property (nonatomic , assign) NSInteger age ;
  
  @property (nonatomic , copy) NSString * name ;
  
  @end

  //
  //  Person.m
  //
  //  Created by 瞿杰 on 2017/8/16.
  //  Copyright © 2017年 qujie. All rights reserved.
  //
  
  
  #import "Person.h"
  
  
  @implementation Person
  
  @end
  

• 在 ViewController 中注意第 46 行代码的使用如下

  //
  //  ViewController.m
  //
  //  Created by 瞿杰 on 2017/8/16.
  //  Copyright © 2017年 qujie. All rights reserved.
  //
  
  
  #import "ViewController.h"
  
  
  #import "Person.h"
  
  
  #import "NSObject+KVO.h"
  
  
  @interface ViewController ()
  
  @property (nonatomic , strong) Person * person ;
  
  @end
  
  @implementation ViewController
  
  - (void)viewDidLoad {
      [super viewDidLoad];
  
      self.person = [[Person alloc] init];
      self.person.age = 1 ;
  
      [self.person addObserver:self forKeyPath:@"age" options:NSKeyValueObservingOptionNew context:nil];
  }
  
  -(void)dealloc
  {
      [self.person removeObserver:self forKeyPath:@"age"];
  }
  
  -(void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event
  {
  //    // 实际调用了 setAge: 方法
  //    self.person.age++ ;
  
      // 实际就只是单纯的赋值
      self.person->_age++ ;
  }
  
  -(void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSKeyValueChangeKey,id> *)change context:(void *)context
  {
      NSLog(@"最新年龄：%ld , 类名：%@",self.person.age,NSStringFromClass([self.person class]));
  }
  
  @end

• 结果却是不会调下面的方法

  -(void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSKeyValueChangeKey,id> *)change context:(void *)context
  {
      NSLog(@"最新年龄：%ld , 类名：%@",self.person.age,NSStringFromClass([self.person class]));
  }

---

3.自定义 KVO

• 以上面的代码为例，系统KVO的实现过程

  • 首先在添加观察者时，用runtime动态创建了一个子类 NSKVONotifying_Person，并修改 self.person 的 isa 指针指向该子类
  • 再用 runtime 动态添加self.person 对象的一些属性如 observe 观察者(strong 强引用，这也解释了为什么每个 dealloc 方法内需要移除监听 )
  • 当设置 self.person.age 时调用 setter 方法触发监听，然后就有了调用 oberve 的方法-(void)observeValueForKeyPath: ……

• 实现自定义KVO

  • Person 类

    //
    //  Person.h
    //
    //  Created by 瞿杰 on 2017/8/16.
    //  Copyright © 2017年 qujie. All rights reserved.
    //
    
    
    #import <Foundation/Foundation.h>
    
    
    @interface Person : NSObject
    
    @property (nonatomic , assign) NSInteger age ;
    
    @property (nonatomic , copy) NSString * name ;
    
    @end
    

    //
    //  Person.m
    //
    //  Created by 瞿杰 on 2017/8/16.
    //  Copyright © 2017年 qujie. All rights reserved.
    //
    
    
    #import "Person.h"
    
    
    @implementation Person
    
    @end

  • 创建 NSObject 分类 NSObject+KVO 实现添加观察者方法：

    //
    //  NSObject+KVO.h
    //
    //  Created by 瞿杰 on 2017/8/16.
    //  Copyright © 2017年 qujie. All rights reserved.
    //
    
    
    #import <Foundation/Foundation.h>
    
    
    @interface NSObject (KVO)
    
    // 添加一个前缀为区分与系统不一样方法
    -(void)qj_addObserver:(NSObject *)observer forKeyPath:(NSString *)keyPath options:(NSKeyValueObservingOptions)options context:(void *)context ;
    
    @end
    

  • 实现 NSObject+KVO 对象方法

    //
    //  NSObject+KVO.m
    //
    //  Created by 瞿杰 on 2017/8/16.
    //  Copyright © 2017年 qujie. All rights reserved.
    //
    
    
    #import "NSObject+KVO.h"
    
    
    // runtime 需要的头文件
    
    #import <objc/message.h>
    
    
    @implementation NSObject (KVO)
    
    -(void)qj_addObserver:(NSObject *)observer forKeyPath:(NSString *)keyPath options:(NSKeyValueObservingOptions)options context:(void *)context
    {
        // 1. 设置该方法的 当前对象的 isa 指针指向动态创建的 QJKVONotifying_Person 类
        Class subClass = [self createClassWithClassName:"QJKVONotifying_Person" superClassName:[self class]];
        object_setClass(self, subClass);
    
        // 2. 动态添加属性 observer 、keyPath 等
        //    OBJC_ASSOCIATION_RETAIN_NONATOMIC 即在 RAC 环境下等同于 @property (nonatomic , strong) [observer class] * observer ;
        objc_setAssociatedObject(self, "observer", observer, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
        objc_setAssociatedObject(self, "keyPath", keyPath, OBJC_ASSOCIATION_COPY_NONATOMIC);
        objc_setAssociatedObject(self, "options", @(options), OBJC_ASSOCIATION_RETAIN_NONATOMIC);
        objc_setAssociatedObject(self, "context", (__bridge id)(context), OBJC_ASSOCIATION_RETAIN_NONATOMIC);
    
        // 3. 方法交换
        // 3.1 获取属性的 setter 方法名
        NSString * methodName = [NSString stringWithFormat:@"set%@%@:",[[keyPath substringToIndex:1] uppercaseString],[keyPath substringFromIndex:1]];
        // 3.2 跟据方法名 获取 方法
        Method InstanceMethod1 = class_getInstanceMethod([self class], NSSelectorFromString(methodName));
        // 3.3 把 setAgeMethod: 方法包装成 Method 类型
        Method InstanceMethod2 = class_getInstanceMethod([self class], @selector(setAgeMethod:));
    
        // 3.4 两个方法交换
        method_exchangeImplementations(InstanceMethod1, InstanceMethod2);
    }
    
    -(void)setAgeMethod:(NSInteger)age
    {
        // 1.取出之前动态添加属性对应的值
        NSObject * observer = objc_getAssociatedObject(self, "observer");
        NSString * keyPath = objc_getAssociatedObject(self, "keyPath");
        void * context = (__bridge void *)(objc_getAssociatedObject(self, "context"));
        NSKeyValueObservingOptions options = [objc_getAssociatedObject(self, @"options") integerValue];
    
        // 2.取出 keyPath 先前的值
        NSInteger oldAge = [[self valueForKeyPath:keyPath] integerValue];
    
        // 3.因为 setAgeMethod: 方法 与 方法名为 methodName 的方法交换了，所以不会告成循环
        [self setAgeMethod:age];
    
        // 4. 调用 KVO 监听的方法
        if ([observer respondsToSelector:@selector(observeValueForKeyPath:ofObject:change:context:)]) {
            NSMutableDictionary * changes = [NSMutableDictionary dictionary];
            [changes setValue:keyPath forKey:@"keyPath"];
    
            // 4.1通过操类型来设置字典的值
            if (options == NSKeyValueObservingOptionOld) {
                [changes setValue:@(oldAge) forKey:NSKeyValueChangeOldKey];
            }
            else if (options == NSKeyValueObservingOptionNew){
                [changes setValue:@([[self valueForKeyPath:keyPath] integerValue]) forKey:NSKeyValueChangeNewKey];
            }// 接下来的枚举就不再写了
    
            // 4.2 调用 observer 观察者实现的 KVO 方法
            [observer observeValueForKeyPath:keyPath ofObject:self change:changes context:context];
        }
    }
    
    /**
        跟据类名 className 和 父类名 superClassName 创建名为 className 的类
     */
    -(Class)createClassWithClassName:(const char * )className superClass:(Class)superClass
    {
        // 1.先查找项目中是否存在这个类
        Class subClass = objc_getClass(className);
    
        // 2.如果不存在则动态创建
        if (!subClass)
        {
            // 2.1 根据父类来创建当前的子类
            subClass = objc_allocateClassPair(superClass, className, 0);
        }
    
        return subClass ;
    }
    
    @end
    

  • 在 ViewController 中的用法如下

    //
    //  ViewController.m
    //
    //  Created by 瞿杰 on 2017/8/16.
    //  Copyright © 2017年 qujie. All rights reserved.
    //
    
    
    #import "ViewController.h"
    
    
    #import "Person.h"
    
    
    #import "NSObject+KVO.h"
    
    
    @interface ViewController ()
    
    @property (nonatomic , strong) Person * person ;
    
    @end
    
    @implementation ViewController
    
    - (void)viewDidLoad {
        [super viewDidLoad];
    
        self.person = [[Person alloc] init];
        self.person.age = 1 ;
    
    //    [self.person addObserver:self forKeyPath:@"age" options:NSKeyValueObservingOptionNew context:nil];
        [self.person qj_addObserver:self forKeyPath:@"age" options:NSKeyValueObservingOptionNew context:nil];
    }
    
    -(void)dealloc
    {
        [self.person removeObserver:self forKeyPath:@"age"];
    }
    
    -(void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event
    {
        // 实际调用了 setAge: 方法
        self.person.age++ ;
    }
    
    -(void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSKeyValueChangeKey,id> *)change context:(void *)context
    {
        NSLog(@"最新年龄：%ld , 类名：%@",self.person.age,NSStringFromClass([self.person class]));
    }
    
    @end

  • 运行结果如下