面试题 - KVC
通过KVC修改属性会触发KVO么？
KVC的赋值和取值过程是怎样的？原理是什么？
KVC的基本使用
image-20210417151743134
KVC的全称是Key-Value Coding，俗称“键值编码”，可以通过一个key来访问某个属性
常见的API有：
- (void)setValue:(id)value forKeyPath:(NSString *)keyPath;
- (void)setValue:(id)value forKey:(NSString *)key;
- (id)valueForKeyPath:(NSString *)keyPath;
- (id)valueForKey:(NSString *)key;
KVC的基本使用：
@interface MJCat : NSObject
@property (assign, nonatomic) int weight;
@end
@interface MJPerson : NSObject
@property (assign, nonatomic) int age;
@property (retain, nonatomic) MJCat *cat;
@end
@implementation MJPerson
@end
@implementation MJCat
@end
person.age = 10;
NSLog(@"%@", [person valueForKey:@"age"]);
NSLog(@"%@", [person valueForKeyPath:@"cat.weight"]);
NSLog(@"%d", person.age);
[person setValue:[NSNumber numberWithInt:10] forKey:@"age"];
[person setValue:@30 forKey:@"age"];
person.cat = [[MJCat alloc] init];
[person setValue:@20 forKeyPath:@"cat.weight"];
NSLog(@"%d", person.age);
NSLog(@"%d", person.cat.weight);
RUN>
=================打印结果================= 2021-04-17 15:29:31.379302+0800 Interview01-KVC[2187:88935] 10 2021-04-17 15:29:31.379738+0800 Interview01-KVC[2187:88935] (null) 2021-04-17 15:29:31.379774+0800 Interview01-KVC[2187:88935] 10 2021-04-17 15:29:31.380053+0800 Interview01-KVC[2187:88935] 30 2021-04-17 15:29:31.380093+0800 Interview01-KVC[2187:88935] 20
通过KVC给属性赋值能触发KVO吗？
能不能试一下就知道了：
我们创建一个MJPerson类,添加一个age属性,然后给这个age属性添加一个KVO,使用KVC修改age的值:
#import "MJObserver.h"
@implementation MJObserver
- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context
{
NSLog(@"observeValueForKeyPath - %@", change);
}
@end
MJObserver *observer = [[MJObserver alloc] init];
MJPerson *person = [[MJPerson alloc] init];
// 添加KVO监听
[person addObserver:observer forKeyPath:@"age" options:NSKeyValueObservingOptionNew | NSKeyValueObservingOptionOld context:NULL];
[person setValue:@10 forKey:@"age"];
RUN>
=================打印结果================= 2021-04-17 15:34:55.609944+0800 Interview01-KVC[2235:92111] observeValueForKeyPath - { kind = 1; new = 10; old = 0; }
可以看到KVC访问属性的时候,的确调用了setAge:方法.那么setVlaue:forKey:是如何设值的呢?
通过KVC修改age属性会触发KVO。为什么呢？先往下看
接下来看看setValue:forKey:设值原理和valueForKey:取值原理。
KVC设值原理
KVC赋值流程
执行步骤:
1:setVlaue:forKey:首先查找setKey:方法,如果有就执行;如果没有再去查找_setKey:方法.
2:如果没有找到_setKey:方法,就去查看accessInstanceVariablesDirectly方法的返回值,这个方法的意思是:是否允许直接进入对象的成员变量?,如果返回YES,就按照_key,_isKey,key,isKey的顺序查找成员变量,如果找到直接赋值;如果没找到就抛出异常NSUnknownKeyException.
3:如果accessInstanceVariablesDirectly方法直接返回NO(默认返回YES),就直接抛出异常NSUnknownKeyException.
设值原理解释：
1.寻找setAge方法
- (void)setAge:(int)age
{
NSLog(@"setAge: - %d", age);
}
2.寻找_setAge方法
- (void)_setAge:(int)age
{
NSLog(@"_setAge: - %d", age);
}
3.找不到上面两个方法就调用accessInstanceVariablesDirectly问问能不能直接访问成员变量
默认的返回值就是YES
+ (BOOL)accessInstanceVariablesDirectly
{
return YES;
}
4.1 如果返回NO,就调用setValue:forUndefinedKey:并抛出异常NSUnknownKeyException
4.2如果返回YES,会按顺序_key,_isKey,key,isKey赋值,如果四个都找不到就报上面的错
上面我们知道KVC设值会触发KVO，但是如果没有set方法呢？就算没有set方法只有成员变量，通过KVC进行赋值也会触发KVO，可以理解它们是配套使用的。
我们先证明第一点(执行步骤1),把MJPerson类中属性删掉,然后重写setAge:,_setAge:方法:
@implementation MJPerson
- (void)setAge:(int)age{
NSLog(@"setAge方法被调用");
}
- (void)_setAge:(int)age{
NSLog(@"_setAge被调用");
}
@end
MJPerson *person = [[MJPerson alloc] init];
// 通过KVC修改age属性
[person setValue:@10 forKey:@"age"];
RUN>
=================打印结果================= 2021-04-17 15:42:35.598146+0800 Interview01-KVC[2348:98038] setAge方法被调用我们把
setAge:方法注释掉,再运行:=================打印结果================= 2021-04-17 15:45:15.169572+0800 Interview01-KVC[2404:100780] _setAge被调用通过运行结果可以很清楚的看到,
[person setValue:@10 forKey:@"age"];会先去查找setAge:方法,如果有就调用,如果没有再去查找_setAge:方法.
接下来验证第二点(执行步骤2),我们在MJPerson注释掉刚才的setAge:方法,然后重写accessInstanceVariablesDirectly方法,然后返回NO,运行下看看效果:
@implementation MJPerson
+ (BOOL)accessInstanceVariablesDirectly{
return NO;
}
@end
MJPerson *person = [[MJPerson alloc] init];
// 通过KVC修改age属性
[person setValue:@10 forKey:@"age"];
RUN>
=================打印结果================= 2021-04-17 15:48:20.837291+0800 Interview01-KVC[2437:102949] *** Terminating app due to uncaught exception 'NSUnknownKeyException', reason: '[setValue:forUndefinedKey:]: this class is not key value coding-compliant for the key age.' image-20210417154937044
再修改为return YES;然后添加如下成员变量:
@interface MJPerson : NSObject
{
@public
int _age;
int _isAge;
int age;
int isAge;
}
@end
@implementation MJPerson
+ (BOOL)accessInstanceVariablesDirectly{
return YES;
}
@end
int main(int argc, const char * argv[]) {
@autoreleasepool {
MJPerson *person = [[MJPerson alloc] init];
// 通过KVC修改age属性
[person setValue:@10 forKey:@"age"];
NSLog(@"%@", [person valueForKey:@"age"]);
}
return 0;
}
RUN>
=================打印结果================= 2021-04-17 16:05:21.917933+0800 Interview01-KVC[2574:111457] 10image-20210417160635084image-20210417160710325image-20210417160805503
有人可能会以为是成员变量顺序导致的,我们把成员变量的顺序打乱在执行看看:
image-20210417160931763
我们把4个属性全注释掉,运行一下看看效果:
@interface MJPerson : NSObject
{
}
RUN>
=================打印结果================= 2021-04-17 16:10:24.189710+0800 Interview01-KVC[2648:117035] *** Terminating app due to uncaught exception 'NSUnknownKeyException', reason: '[setValue:forUndefinedKey:]: this class is not key value coding-compliant for the key age.' 如果找到直接赋值;如果没找到就抛出异常
NSUnknownKeyException.
如果用KVC修改一个类的成员变量就会触发KVO.我们来验证一下
@interface MJPerson : NSObject
{
@public
int age;
}
@end
int main(int argc, const char * argv[]) {
@autoreleasepool {
MJObserver *observer = [[MJObserver alloc] init];
MJPerson *person = [[MJPerson alloc] init];
// 添加KVO监听
[person addObserver:observer forKeyPath:@"age" options:NSKeyValueObservingOptionNew | NSKeyValueObservingOptionOld context:NULL];
// 通过KVC修改age属性
[person setValue:@10 forKey:@"age"];
}
return 0;
}
RUN>
=================打印结果================= 2021-04-17 16:15:52.595089+0800 Interview01-KVC[2690:120312] observeValueForKeyPath - { kind = 1; new = 10; old = 0; }
其实KVC内部调用了下面方法才会触发KVO的
[person willChangeValueForKey:@"age"];
person->_age = 10;
[person didChangeValueForKey:@"age"];
因为KVC赋值同样会调用willChangeValueForKey:,didChangeValueForKey:两个方法,我们在MJPerson中重写这两个方法:
- (void)willChangeValueForKey:(NSString *)key
{
[super willChangeValueForKey:key];
NSLog(@"willChangeValueForKey - %@", key);
}
- (void)didChangeValueForKey:(NSString *)key
{
NSLog(@"didChangeValueForKey - begin - %@", key);
[super didChangeValueForKey:key];
NSLog(@"didChangeValueForKey - end - %@", key);
}
RUN>
=================打印结果================= 2021-04-17 16:18:17.974684+0800 Interview01-KVC[2707:121945] willChangeValueForKey - age 2021-04-17 16:18:17.975060+0800 Interview01-KVC[2707:121945] didChangeValueForKey - begin - age 2021-04-17 16:18:17.975254+0800 Interview01-KVC[2707:121945] observeValueForKeyPath - { kind = 1; new = 10; old = 0; } 2021-04-17 16:18:17.975309+0800 Interview01-KVC[2707:121945] didChangeValueForKey - end - age可以看到,这两个方法的确都被调用了,所以KVC给一个类的属性赋值也能触发KVO.
KVC取值原理
取值原理.png
KVC取值原理解释
1.getAge
- (int)getAge
{
return 11;
}
2.age
- (int)age
{
return 12;
}
3.isAge
- (int)isAge
{
return 13;
}
4._age
- (int)_age
{
return 14;
}
5.3.找不到上面四个方法就调用accessInstanceVariablesDirectly问问能不能直接访问成员变量
默认的返回值就是YES
+ (BOOL)accessInstanceVariablesDirectly
{
return YES;
}
5.1 如果返回NO,就调用valueforUndefinedKey:并抛出异常NSUnknownKeyException
5.2如果返回YES,会按顺序_key,_isKey,key,isKey赋值,如果四个都找不到就报上面的错
@implementation MJPerson
- (int)getAge
{
return 11;
}
- (int)age
{
return 12;
}
- (int)isAge
{
return 13;
}
- (int)_age
{
return 14;
}
@end
int main(int argc, const char * argv[]) {
@autoreleasepool {
MJPerson *person = [[MJPerson alloc] init];
[person setValue:@18 forKey:@"age"];
NSLog(@"%@", [person valueForKey:@"age"]);
}
return 0;
}
Run>
2021-04-17 16:21:41.302038+0800 Interview01-KVC[2757:124317] 11image-20210417162323944image-20210417162342102
找不到上面四个方法就调用accessInstanceVariablesDirectly
image-20210417162508473
@implementation MJPerson
- (instancetype)init
{
self = [super init];
if (self) {
_age=11;
_isAge=21;
age=31;
isAge = 41;
}
return self;
}
// 默认的返回值就是YES
+ (BOOL)accessInstanceVariablesDirectly
{
return YES;
}
@end
int main(int argc, const char * argv[]) {
@autoreleasepool {
MJPerson *person = [[MJPerson alloc] init];
NSLog(@"%@", [person valueForKey:@"age"]);
}
return 0;
}
RUN>
=================打印结果================= 2021-04-17 16:32:46.330006+0800 Interview01-KVC[2902:132396] 11
面试题 - KVC
通过KVC修改属性会触发KVO么？
会触发KVO.
KVC的赋值和取值过程是怎样的？原理是什么？
看 KVC设值原理 KVC取值原理 步骤，两张图的流程
特别备注
本系列文章总结自MJ老师在腾讯课堂iOS底层原理班（下）/OC对象/关联对象/多线程/内存管理/性能优化，相关图片素材均取自课程中的课件。如有侵权，请联系我删除，谢谢！