Runtime（三）动态添加一个类，KVO的底层实现原理

动态创建一个类

//创建一个类
Class clazz = objc_allocateClassPair([NSObject class], "GoodPerson", 0);

//添加Ivar
class_addIvar(clazz, "_name", sizeof(NSString *), log2(sizeof(NSString *)), @encode(NSString *));

class_addIvar(clazz, "_age", sizeof(NSUInteger), log2(sizeof(NSUInteger)), @encode(NSUInteger));

//注册该类
objc_registerClassPair(clazz);

//创建实例对象
id object = [[clazz alloc] init];

//给Ivar赋值
[object setValue:@"Tracy" forKey:@"name"];

Ivar ageIvar = class_getInstanceVariable(clazz, "_age");
object_setIvar(object, ageIvar, @18);

//打印对象的属性值
NSLog(@"name = %@, age = %@", [object valueForKey:@"name"], object_getIvar(object, ageIvar));

//销毁类
object = nil;
objc_disposeClassPair(clazz);

KVO的底层探究

Person *p = [Person new];

[p addObserver:self forKeyPath:@"name" options:(NSKeyValueObservingOptionNew) context:nil];

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSKeyValueChangeKey,id> *)change context:(void *)context
{
    NSLog(@"改变了%@",change);
}

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

通过打断点可以发现其实系统动态生成了一个NSKVONotifying_Person类，然后重写了NSKVONotifying_Person的set方法，当有新值的时候，会发送通知，调用- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSKeyValueChangeKey,id> *)change context:(void *)context，将变化的值传递回来，从而实现KVO监听属性。

利用下面方法，将Person作为参数传入，可以货渠道Person类和他的子类的数组，可以发现(Person, "NSKVONotifying_Person")。

//获取指定类的子类
+ (NSArray *)findSubClass:(Class)defaultClass
{
    //注册类的总数
    int count = objc_getClassList(NULL, 0);
    
    //创建一个数组，其中包含给定对象
    NSMutableArray *array = [NSMutableArray arrayWithObject:defaultClass];
    
    //获取所有已注册的类
    Class *classes = (Class *)malloc(sizeof(Class)*count);
    objc_getClassList(classes, count);
    
    //遍历
    for (int i = 0 ; i<count; i++) {
        if (defaultClass == class_getSuperclass(classes[i])) {
            [array addObject:classes[i]];
        }
    }
    
    free(classes);
    
    return array;
}

给NSObject创建一个分类，添加一个自己的方法，模拟实现KVO的监听机制。创建一个新的类，作为self的属性，重写这个类的set方法，关联这个类的属性，当有新值赋值的时候，通过objc_msgSend调用- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSKeyValueChangeKey,id> *)change context:(void *)context进行通知。

#import <Foundation/Foundation.h>

NS_ASSUME_NONNULL_BEGIN

@interface NSObject (KVO)

- (void)lwn_addObserver:(NSObject *)observer forKeyPath:(NSString *)keyPath options:(NSKeyValueObservingOptions)options context:(nullable void *)context;

@end

NS_ASSUME_NONNULL_END

#import "NSObject+KVO.h"
#import <objc/runtime.h>
#import <objc/message.h>

@implementation NSObject (KVO)

- (void)lwn_addObserver:(NSObject *)observer forKeyPath:(NSString *)keyPath options:(NSKeyValueObservingOptions)options context:(nullable void *)context
{
    //1.动态注册一个新的类，这个类是继承当前类的类
    NSString *oldName =  NSStringFromClass([self class]);
    NSString *newName = [NSString stringWithFormat:@"lwnKVO%@",oldName];
    Class customClass = objc_allocateClassPair([self class], newName.UTF8String, 0);
    objc_registerClassPair(customClass);
    
    //2.修改self属性
    object_setClass(self, customClass);
    
    
    //3.重写子类的set方法
    NSString *methodName = [NSString stringWithFormat:@"set%@:",keyPath.capitalizedString];
    SEL sel = NSSelectorFromString(methodName);
    class_addMethod(customClass, sel, (IMP)setName, "v@:@");
    
    
    //关联属性
    objc_setAssociatedObject(self, (__bridge const void *)@"objc", observer,OBJC_ASSOCIATION_ASSIGN);
}

void setName(id self, SEL _cmd, NSString *name) {
    NSLog(@"%@",name);
    
    struct objc_super superClass = {self,class_getSuperclass([self class])};
    
    objc_msgSendSuper(&superClass, _cmd,name);

    id observer = objc_getAssociatedObject(self, (__bridge const void *)@"objc");
    NSString *methodName = NSStringFromSelector(_cmd);
    NSString *key = @"name";
    
    objc_msgSend(observer, @selector(observeValueForKeyPath: ofObject: change: context:),key,self,@{key:name},nil);
}