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 == 0) {
        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 = 0;
    //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 = 0; 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 = 0; 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 = 0; 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 = 0; 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);
    
}