KVC底层原理和实现 KVC底层原理和实现

关于KVC
一。 valueForKey:key的调用顺序
1.先用相关方法
（1）先调用getter方法:有三种getter方法优先级是：getKey>key>isKey
（2）NSArry方法：- (NSInteger)countOfKey- (id)objectInKeyAtIndex:(NSInteger)index
2.如果没有相关方法：看+(BOOL)accessInstanceVariablesDirectly返回值
如果返回YES,找到成员变量，先后顺序是：_key>_isKey>key>isKey
如果返回NO，抛出异常valueForUndefinedKey(此时如果重写-(id)valueForUndefinedKey:(NSString *)key就不会抛出异常)

当属性为一个对象的时候，取得属性可以通过valueForKeyPath来获取，路径通过.获取值
//NSObject *name = [dogg valueForKey:@"person"];
//NSString *personName = [name valueForKey:@"myName"];
NSLog(@"%@",[dogg valueForKeyPath:@"person.myName"]);

二。setValue：forKey:的调用顺序
1.找set方法：setKey>setIsKey
2.如果没有相关方法：看+(BOOL)accessInstanceVariablesDirectly返回值
如果返回YES,找到成员变量，先后顺序是：_key>_isKey>key>isKey
如果返回NO，抛出异常valueForUndefinedKey(此时如果重写- (void)setValue:(id)value forUndefinedKey:(NSString*)key不会抛出异常),当赋值为nil的时候可以- (void)setNilValueForKey:(NSString*)key来防止抛出异常

当清楚原理，可以自己写setValue：forKey:

- (void)MT_setValue:(id)value forKey:(NSString*)key

{
if(key ==nil|| key.length==0) {
return;
}
//用setKey方法
NSString *setKey = [NSString stringWithFormat:@"set%@:",key.capitalizedString];
if ([self respondsToSelector:NSSelectorFromString(setKey)]) {
[self performSelector:NSSelectorFromString(setKey) withObject:value];
return;
}
//用setIsKey
NSString *setIsKey = [NSString stringWithFormat:@"setIs%@:",key.capitalizedString];
if ([setKey respondsToSelector:NSSelectorFromString(setIsKey)]) {
[self performSelector:NSSelectorFromString(setIsKey) withObject:value];
return;
}
//accessInstanceVariablesDirectly为NO抛出异常
if (![self.class accessInstanceVariablesDirectly]) {
NSException *exception = [NSException exceptionWithName:@"MTKVC exception" reason:@"accessInstanceVariablesDirectly为NO，不可以找到对于属性" userInfo:nil];
@throwexception;
return;
}
unsignedintcount =0;
Ivar*ivars =class_copyIvarList([selfclass], &count);
//找_key
for(inti =0; i< count; i++) {
Ivarivar = ivars[i];
NSString *keyName = [NSString stringWithUTF8String:ivar_getName(ivar)];
if([keyNameisEqualToString:[NSStringstringWithFormat:@"_%@",key]]) {
object_setIvar(self, ivar, value);
free(ivars);
return;
}

}
//找_isKey
for(inti =0; i< count; i++) {
Ivarivar = ivars[i];
NSString *keyName = [NSString stringWithUTF8String:ivar_getName(ivar)];
if([keyNameisEqualToString:[NSStringstringWithFormat:@"_is%@",key.capitalizedString]]) {
object_setIvar(self, ivar, value);
free(ivars);
return;
}

}
//找key
for(inti =0; i< count; i++) {
Ivarivar = ivars[i];
NSString *keyName = [NSString stringWithUTF8String:ivar_getName(ivar)];
if([keyNameisEqualToString:key]) {
object_setIvar(self, ivar, value);
free(ivars);
return;
}

}
//找isKey
for(inti =0; i< count; i++) {
Ivarivar = ivars[i];
NSString *keyName = [NSString stringWithUTF8String:ivar_getName(ivar)];
if([keyNameisEqualToString:[NSStringstringWithFormat:@"is%@",key.capitalizedString]]) {
object_setIvar(self, ivar, value);
free(ivars);
return;
}

}
//异常
[self setValue:value forUndefinedKey:key];
}

valueForKey:key也可以这样写

- (id)MT_valueForKey:(NSString*)key
{
if(key ==nil|| key.length==0) {
returnnil;
}
//用getKey
NSString *getKey = [NSString stringWithFormat:@"get%@",key.capitalizedString];
if ([self respondsToSelector:NSSelectorFromString(getKey)]) {
idvalue =https://www.it610.com/article/[self performSelector:NSSelectorFromString(getKey)];
returnvalue;
}
//用key
if ([self respondsToSelector:NSSelectorFromString(key)]) {
idvalue =https://www.it610.com/article/[self performSelector:NSSelectorFromString(key)];
returnvalue;
}
//用isKey
NSString *isKey = [NSString stringWithFormat:@"is%@",key.capitalizedString];
if ([self respondsToSelector:NSSelectorFromString(isKey)]) {
idvalue =https://www.it610.com/article/[self performSelector:NSSelectorFromString(isKey)];
returnvalue;
}
//accessInstanceVariablesDirectly为NO抛出异常
if (![self.class accessInstanceVariablesDirectly]) {
NSException *exception = [NSException exceptionWithName:@"MT_valueForKey异常" reason:@"accessInstanceVariablesDirectly为NO,不可以查找属性值" userInfo:nil];
@throwexception;
returnnil;
}
unsignedintcount =0;
//找_key
Ivar*ivars =class_copyIvarList([selfclass], &count);
for(inti =0; i< count; i++) {
Ivarivar = ivars[i];
NSString *keyName = [NSString stringWithUTF8String:ivar_getName(ivar)];
if([keyNameisEqualToString:[NSStringstringWithFormat:@"_%@",key]]) {
free(ivars);
returnobject_getIvar(self, ivar);
}
}
//找_isKey
for(inti =0; i< count; i++) {
Ivarivar = ivars[i];
NSString *keyName = [NSString stringWithUTF8String:ivar_getName(ivar)];
if([keyNameisEqualToString:[NSStringstringWithFormat:@"_is%@",key.capitalizedString]]) {
free(ivars);
returnobject_getIvar(self, ivar);
}
}
//找key
for(inti =0; i< count; i++) {
Ivarivar = ivars[i];
NSString *keyName = [NSString stringWithUTF8String:ivar_getName(ivar)];
if([keyNameisEqualToString:key]) {
【KVC底层原理和实现】free(ivars);
returnobject_getIvar(self, ivar);
}
}
//找iskey
for(inti =0; i< count; i++) {
Ivarivar = ivars[i];
NSString *keyName = [NSString stringWithUTF8String:ivar_getName(ivar)];
if([keyNameisEqualToString:[NSStringstringWithFormat:@"is%@",key.capitalizedString]]) {
free(ivars);
returnobject_getIvar(self, ivar);
}
}
return[self valueForUndefinedKey:key];
}