KVC 原理及自定义实现

一.  setValue: forKey: 赋值过程

1.首先寻找setter方法(两个)

- setName:

-setIsName:

2.然后再寻找成员变量

默认 + (BOOL)accessInstanceVariablesDirectly 返回YES;

_key > _isKey > key > isKey

3. 如果以上步骤没有找到key,则会报异常; 也可以重写方法,防止崩溃

-setValue: forUndefinedKey:

二. valueForKey: 取值过程

1.getter方法(三个)

-getKey

-key

-isKey

2.成员变量

_key > _isKey > key > isKey

3.上述没有,可重写,防止崩溃

-valueForUndefinedKey

三. runtime  简单实现 KVC

新建一个NSObject 一个分类

 #import "NSObject+kvc.h"
 #import <objc/runtime.h>
 @implementation NSObject (kvc)
 - (void)xs_setValue:(id)value forKey:(NSString *)key{
     //key 需要合法
     if (key == nil || key.length == ) {
         return;
     }
     //一.setter 方法
     //调用相关setter方法(capitalizedString 首字母大写)
     NSString *setKey = [NSString stringWithFormat:@"set%@:",[key capitalizedString]];
     //调用
     if ([self respondsToSelector:NSSelectorFromString(setKey)]) {
         [self performSelector:NSSelectorFromString(setKey) withObject:value];
         return;
     }
     NSString *setIsKey = [NSString stringWithFormat:@"setIs%@:",[key capitalizedString]];
     if ([self respondsToSelector:NSSelectorFromString(setIsKey)]) {
         [self performSelector:NSSelectorFromString(setIsKey) withObject:value];
         return;
     }

     //异常处理方法
     if (![self.class accessInstanceVariablesDirectly]) {
         NSException *exception = [NSException exceptionWithName:@"XSKVC Exception" reason:@"你返回了NO啊!" userInfo:nil];
         @throw exception;
         return;
     }
     //二.成员变量
     unsigned int count = ;
     //获取所有成员变量
     Ivar *ivars = class_copyIvarList([self class], &count);//解释:在C中,copy new create 在堆区开辟内存空间
 //    NSMutableArray *array = [NSMutableArray array];
 //    for (int i = 0; i<count; i++) {
 //        Ivar ivar = ivars[i];
 //        const char *name = ivar_getName(ivar);
 //        NSString *keyName = [NSString stringWithUTF8String:name];
 //        [array addObject:keyName];
 //    }
     //_key (按照优先级:_key > _isKey > key > isKey)
     for (int i = ; i<count; i++) {
         Ivar ivar = ivars[i];
         NSString *keyName = [NSString stringWithUTF8String:ivar_getName(ivar)];
         if ([keyName isEqualToString:[NSString stringWithFormat:@"_%@",key]]) {
             object_setIvar(self, ivar, value);
             free(ivars);
             return;
         }
     }
     //_isKey
     for (int i = ; i<count; i++) {
         Ivar ivar = ivars[i];
         NSString *keyName = [NSString stringWithUTF8String:ivar_getName(ivar)];
         if ([keyName isEqualToString:[NSString stringWithFormat:@"_is%@",key.capitalizedString]]) {
             object_setIvar(self, ivar, value);
             free(ivars);
             return;
         }
     }
     //key
     for (int i = ; i<count; i++) {
         Ivar ivar = ivars[i];
         NSString *keyName = [NSString stringWithUTF8String:ivar_getName(ivar)];
         if ([keyName isEqualToString:[NSString stringWithFormat:@"%@",key]]) {
             object_setIvar(self, ivar, value);
             free(ivars);
             return;
         }
     }
     //isKey
     for (int i = ; i<count; i++) {
         Ivar ivar = ivars[i];
         NSString *keyName = [NSString stringWithUTF8String:ivar_getName(ivar)];
         if ([keyName isEqualToString:[NSString stringWithFormat:@"is%@",key.capitalizedString]]) {
             object_setIvar(self, ivar, value);
             free(ivars);
             return;
         }
     }

     //处理异常
     [self setValue:value forUndefinedKey:key];
     free(ivars);

 }