KVC是由NSKeyValueCoding非正式协议启用的一种机制，对象采用该协议提供对其属性的间接访问。当对象符合键值编码时，其属性可以通过简洁、统一的消息传递接口通过字符串参数进行寻址。这种间接访问机制补充了实例变量及其相关访问器方法提供的直接访问。键值编码兼容对象提供了一个简单的消息传递接口，该接口在其所有属性中都是一致的。键值编码是许多其他Cocoa技术的基础概念，如键值观察、Cocoa绑定、核心数据和AppleScript功能。在某些情况下，键值编码也有助于简化代码。
1.KVC键值查找流程
1.1.KVC-setValue:(id)value forKey:(NSString *)key的查找流程如下图：
KVC-setValue.png
简单来说，会依次查找-(void)setKey:(id)value,-(void)_setKey:(id)value,-(void)_setIsKey:(id)value这3个实例方法，如果存在则执行相应的方法，如果不存在则判断accessInstanceVariablesDirectly是否等于YES。如果为真则会依次查找成员_key,_isKey,key,isKey,如果存在则使用runtime进行赋值，否则或accessInstanceVariablesDirectly为假，则看是否实现了-(void)setValue:(id)value forUndefinedKey:(NSString *)key方法，未实现则抛出异常。
1.2.KVC-valueForKey:(NSString *)key的查找流程如下图：
KVC-valueForKey.png
简单来说，会依次查找-(id)getKey,-(id)key,-(void)isKey ,-(void)_key这4个实例方法，如果存在则执行相应的方法，如果不存在则判断accessInstanceVariablesDirectly是否等于YES。如果为真则会依次查找成员_key,_isKey,key,isKey,如果存在则使用runtime进行取值，否则或accessInstanceVariablesDirectly为假，则看是否实现了-(id)valueForUndefinedKey:(NSString *)key方法，未实现则抛出异常。
以上的存取规则在
- (void)setValue:(id)value forKeyPath:(NSString *)keyPath和- (id)valueForKeyPath:(NSString *)keyPath中同样适用。
2.KVC对结构体类型的存取
定义一个结构体类型 NXBookstruct，和一个类类型NXBookclass。
typedef struct {
NSString *name;
NSInteger priceValue;
}NXBookstruct;
@interface NXBookclass : NSObject
@property (nonatomic) NXBookstruct bookstruct;
@end
@implementation NXBookclass
@end
那么采用KVO的方式如何设置和读取结构体类型属性的值呢？
NXBookclass *book = [[NXBookclass alloc] init];
{
NXBookstruct bookstruct = {@"Chinese", 28};
NSValue *value = [NSValue valueWithBytes:&bookstruct objCType:@encode(NXBookstruct)];
[bookclass setValue:value forKey:@"bookstruct"];
NSLog(@"通过类属性直接访问bookstruct:name=%@, priceValue=%@", bookclass.bookstruct.name, @(bookclass.bookstruct.priceValue));
//通过类属性直接访问bookstruct:name=Chinese, priceValue=28
}
{
NXBookstruct bookstruct;
NSValue *value = [bookclass valueForKey:@"bookstruct"];
if (@available(iOS 11.0, *)) {
[value getValue:&bookstruct size:sizeof(NXBookstruct)];
} else {
[value getValue:&bookstruct];
}
NSLog(@"通过KVC间接访问bookstruct:name=%@, priceValue=%@", bookstruct.name, @(bookstruct.priceValue));
//通过KVC间接访问bookstruct:name=Chinese, priceValue=28
}
3.KVC操作符方面的运用
集合操作符方面的运用常见列举如下：
-
@count:数组中元素的个数 -
@avg.key:数组中key对应的值的平均值 -
@max.key:数组中key对应的值的最大值 -
@min.key:数组中key对应的值的最小值 -
@sum.key:数组中key对应的值的总和 -
@distinctUnionOfObjects.key:返回数组中key对应值构成的数组，去除重复 -
@unionOfObjects.key:返回数组中key对应值构成的数组，不去除重复 -
@distinctUnionOfArrays.key:返回二维数组中key对应值构成的数组，去除重复 -
@unionOfArrays.key:返回二维数组中key对应值构成的数组，不去除重复
定义一个class NXBookclass：
@interface NXBookclass : NSObject
@property (nonatomic) NXBookstruct bookstruct;
@property (nonatomic, copy) NSString *name;
@property (nonatomic, assign) NSInteger priceValue;
@end
@implementation NXBookclass
@end
执行以下代码，可以进行验证:
NSMutableArray *bookclasses = [NSMutableArray arrayWithCapacity:5];
for (NSInteger i = 1; i <= 9; i++){
NXBookclass *bookclass = [[NXBookclass alloc] init];
bookclass.name = @(i).stringValue;
bookclass.priceValue = 10 * i;
if(bookclass.priceValue > 80){
bookclass.priceValue = 80;
}
if(bookclass.priceValue < 20){
bookclass.priceValue = 20;
}
[bookclasses addObject:bookclass];
}
/*bookclasses:20,20,30,40,50,60,70,80,80*/
NSLog(@"@count=%@", [bookclasses valueForKeyPath:@"@count"]);
//@count=9
NSLog(@"@avg.priceValue=%@", [bookclasses valueForKeyPath:@"@avg.priceValue"]);
//@avg.priceValue=50
NSLog(@"@max.priceValue=%@", [bookclasses valueForKeyPath:@"@max.priceValue"]);
//@max.priceValue=80
NSLog(@"@min.priceValue=%@", [bookclasses valueForKeyPath:@"@min.priceValue"]);
//@min.priceValue=20
NSLog(@"@sum.priceValue=%@", [bookclasses valueForKeyPath:@"@sum.priceValue"]);
//@sum.priceValue=450
NSLog(@"@distinctUnionOfObjects.priceValue=%@", [bookclasses valueForKeyPath:@"@distinctUnionOfObjects.priceValue"]);
//@distinctUnionOfObjects.priceValue=(70,40,80,50,20,60,30)
NSLog(@"@unionOfObjects.priceValue=%@", [bookclasses valueForKeyPath:@"@unionOfObjects.priceValue"]);
//@unionOfObjects.priceValue=(20,20,30,40,50,60,70,80,80)
NSMutableArray *unionOfBookclasses = [NSMutableArray arrayWithCapacity:2];
[unionOfBookclasses addObject:bookclasses];
[unionOfBookclasses addObject:bookclasses];
NSLog(@"@distinctUnionOfArrays.priceValue=%@", [unionOfBookclasses valueForKeyPath:@"@distinctUnionOfArrays.priceValue"]);
//@distinctUnionOfArrays.priceValue=(40,80,30,70,20,60,50)
NSLog(@"@unionOfArrays.priceValue=%@", [unionOfBookclasses valueForKeyPath:@"@unionOfArrays.priceValue"]);
//@unionOfArrays.priceValue=(20,20,30,40,50,60,70,80,80,20,20,30,40,50,60,70,80,80)
4.自定义KVC
我们搞清楚了KVC的超着流程之后，就可以按照KVC的原理来自定义一套KVC。
4.1.准备工作
获取一个类的成员变量列表和方法列表
@interface NXApi : NSObject
/*获取成员变量列表:cls当前类，forward是否向上查询父类*/
+ (NSArray *)varList:(Class)cls forward:(BOOL)forward;
/*获取方法变量列表:cls当前类，forward是否向上查询父类*/
+ (NSArray *)methodList:(Class)cls forward:(BOOL)forward;
@end
@implementation NXApi
+ (NSArray *)varList:(Class)cls forward:(BOOL)forward{
if(!cls || [cls isEqual:[NSObject class]]){
return @[];
}
NSMutableArray *retValue = [NSMutableArray arrayWithCapacity:2];
unsigned int count;
Ivar * rss = class_copyIvarList(cls, &count);
for (unsigned int i = 0; i < count; i++) {
Ivar rs = rss[i];
NSString *name = [NSString stringWithFormat:@"%s", ivar_getName(rs)];
[retValue addObject:name];
}
free(rss);
Class superclass = [cls superclass];
if(superclass && forward){
[retValue addObjectsFromArray:[NXApi varList:superclass forward:forward]];
}
return retValue;
}
+ (NSArray *)methodList:(Class)cls forward:(BOOL)forward{
if(!cls || [cls isEqual:[NSObject class]]){
return @[];
}
NSMutableArray *retValue = [NSMutableArray arrayWithCapacity:2];
unsigned int count;
Method *rss = class_copyMethodList(cls, &count);
for (unsigned int i = 0; i < count; i++) {
Method rs = rss[i];
NSString *name = [NSString stringWithFormat:@"%s", sel_getName(method_getName(rs))];
[retValue addObject:name];
}
free(rss);
Class superclass = [cls superclass];
if(superclass && forward){
[retValue addObjectsFromArray:[NXApi methodList:superclass forward:forward]];
}
return retValue;
}
@end
4.2 自定义KVC方法
@interface NSObject(NX)
- (void)nx_setValue:(id)value forKey:(NSString *)key;
- (id)nx_valueForKey:(NSString *)key;
@end
@implementation NSObject(NX)
- (void)nx_setValue:(id)value forKey:(NSString *)key{
if (key == nil || key.length == 0) {
return ;
}
NSString *Key = key.capitalizedString;
//1、 set: / _set: /setIs:
NSMutableArray *__selStringArray = [NSMutableArray arrayWithCapacity:3];
[__selStringArray addObject:[NSString stringWithFormat:@"set%@:", Key]];
[__selStringArray addObject:[NSString stringWithFormat:@"_set%@:", Key]];
[__selStringArray addObject:[NSString stringWithFormat:@"setIs%@:", Key]];
for(NSString *selString in __selStringArray){
SEL sel = NSSelectorFromString(selString);
if(sel && [self respondsToSelector:sel]){
[self performSelector:sel withObject:value];
NSLog(@"set/sel:%@", selString);
return;
}
}
//2、 self.class.accessInstanceVariablesDirectly == YES => _，_is，, is
if (self.class.accessInstanceVariablesDirectly) {
//获取实例变量列表
NSArray *varStringArray = [NXApi varList:self.class forward:YES];
NSMutableArray *__varStringArray = [NSMutableArray arrayWithCapacity:4];
[__varStringArray addObject:[NSString stringWithFormat:@"_%@", key]];
[__varStringArray addObject:[NSString stringWithFormat:@"_is%@", Key]];
[__varStringArray addObject:key];
[__varStringArray addObject:[NSString stringWithFormat:@"is%@", Key]];
for (NSString *varString in __varStringArray){
if([varStringArray containsObject:varString]){
Ivar ivar = class_getInstanceVariable(self.class, varString.UTF8String);
object_setIvar(self, ivar, value);
NSLog(@"set/var:%@", varString);
return;
}
}
}
//3、 setValue:forUndefinedKey:
if([[NXApi methodList:self.class forward:YES] containsObject:@"setValue:forUndefinedKey:"]){
[self setValue:value forUndefinedKey:key];
}
//4、Exception
}
- (id)nx_valueForKey:(NSString *)key{
if (key == nil || key.length == 0) {
return [NSNull null];
}
NSString *Key = key.capitalizedString;
//1、 搜索实例的方法 get, key, isKey, _key
NSMutableArray *__selStringArray = [NSMutableArray arrayWithCapacity:4];
[__selStringArray addObject:[NSString stringWithFormat:@"get%@", Key]];
[__selStringArray addObject:key];
[__selStringArray addObject:[NSString stringWithFormat:@"is%@", Key]];
[__selStringArray addObject:[NSString stringWithFormat:@"_%@", key]];
for(NSString *selString in __selStringArray){
SEL sel = NSSelectorFromString(selString);
if(sel && [self respondsToSelector:sel]){
NSLog(@"get/sel:%@", selString);
return [self performSelector:sel];
}
}
// 2、 self.class.accessInstanceVariablesDirectly == YES => _，_is，, is
if (self.class.accessInstanceVariablesDirectly) {
//获取实例变量列表
NSArray *varStringArray = [NXApi varList:self.class forward:YES];
NSMutableArray *__varStringArray = [NSMutableArray arrayWithCapacity:4];
[__varStringArray addObject:[NSString stringWithFormat:@"_%@", key]];
[__varStringArray addObject:[NSString stringWithFormat:@"_is%@", Key]];
[__varStringArray addObject:key];
[__varStringArray addObject:[NSString stringWithFormat:@"is%@", Key]];
for (NSString *varString in __varStringArray){
if([varStringArray containsObject:varString]){
Ivar ivar = class_getInstanceVariable(self.class, varString.UTF8String);
NSLog(@"get/var:%@", varString);
return object_getIvar(self, ivar);
}
}
}
// 3、 valueForUndefinedKey:
// 经过测试只要在子类中实现了valueForUndefinedKey:方法就可以正常调用不会报错
if([[NXApi methodList:self.class forward:YES] containsObject:@"valueForUndefinedKey:"]){
return [self valueForUndefinedKey:key];
}
// 4. Exception
return nil;
}
@end
使用称为isa swizzling的技术实现自动键值观察。顾名思义，isa指针指向维护分派表的对象类。这个分派表本质上包含指向类实现的方法的指针以及其他数据。
当一个观察者为一个对象的属性注册时，被观察对象的isa指针被修改，指向一个中间类而不是真正的类。因此，isa指针的值不一定反映实例的实际类。
决不能依赖isa指针来确定类成员身份。相反，您应该使用class方法来确定对象实例的类。
5.KVO准备工作
先定义一个类NXBookclass，再定义好一个工具方法打印前后类的变化，
@interface NXBookclass : NSObject {
@public NSInteger _priceValue;
}
@property (nonatomic, copy) NSString *name;
@property (nonatomic, strong) NSMutableArray *descriptions;
@end
@implementation NXBookclass
//+ (BOOL)automaticallyNotifiesObserversForKey:(NSString *)key {
// return YES;
//}
@end
@interface NXApi : NSObject
+ (NSMutableDictionary *)descriptionClass:(Class)cls;
@end
@implementation NXApi
+ (NSMutableDictionary *)descriptionClass:(Class)cls {
NSMutableDictionary *dicValue = [NSMutableDictionary dictionaryWithCapacity:5];
while (cls && cls != [NSObject class]) {
NSMutableDictionary *dicSubvalue = [NSMutableDictionary dictionaryWithCapacity:5];
dicSubvalue[@"varList"] = [NXApi varList:cls forward:false];
dicSubvalue[@"methodList"] = [NXApi methodList:cls forward:false];
dicSubvalue[@"metaClass"] = [NXApi metaClass:cls forward:false];
[dicValue setObject:dicSubvalue forKey:[NSString stringWithCString:class_getName(cls) encoding:NSUTF8StringEncoding]];
cls = class_getSuperclass(cls);
}
return dicValue;
}
@end
6.KVO属性观察
@interface NXKVO : NSObject
- (void)KVO;
@end
@implementation NXKVO
- (void)KVO{
NXBookclass *bookclass = [[NXBookclass alloc] init];
NSLog(@"观察前:%@", [NXApi descriptionClass:object_getClass(bookclass)]);//打印观察之前的类信息
{
[bookclass addObserver:self forKeyPath:@"name" options:NSKeyValueObservingOptionOld|NSKeyValueObservingOptionNew context:NULL];
NSLog(@"观察中:%@", [NXApi descriptionClass:object_getClass(bookclass)]);//打印观察之后的类信息
bookclass.name = @"Swift";
[bookclass removeObserver:self forKeyPath:@"name"];
}
NSLog(@"取消观察后:%@", [NXApi descriptionClass:object_getClass(bookclass)]);//打印取消观察之后的类信息
}
- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context{
NSLog(@"%s, key=%@, change=%@", __func__, keyPath, change);
}
@end
运行如上代码，打印信息如下：
观察前或取消观察后:
{
NXBookclass = {
metaClass = (
NXBookclass
);
methodList = (
name,
".cxx_destruct",
"setName:",
descriptions,
"setDescriptions:"
);
propertyList = (
name,
descriptions
);
varList = (
"_priceValue",
"_name",
"_descriptions"
);
};
}
观察中:
{
"NSKVONotifying_NXBookclass" = {
metaClass = (
"NSKVONotifying_NXBookclass"
);
methodList = (
"setName:",
class,
dealloc,
"_isKVOA"
);
propertyList = (
);
varList = (
);
};
NXBookclass = {
metaClass = (
NXBookclass
);
methodList = (
name,
".cxx_destruct",
"setName:",
descriptions,
"setDescriptions:"
);
propertyList = (
name,
descriptions
);
varList = (
"_priceValue",
"_name",
"_descriptions"
);
};
}
观察的回调信息：
key=name, change={
kind = 1;
new = Swift;
old = "";
}
通过打印的日志信息，我们可以看到，我们的bookclass的的name属性的观察是成功的，name的值由null变成了Swift。
通过打印的日志我们可以看到如下现象：观察中的类与观察前后的类信息不一样，默认的类是NXBookclass->NSObject;而观察中的类是NSKVONotifying_NXBookclass->NXBookclass->NSObject。并且在NSKVONotifying_NXBookclass这个类中出现了setName:,class,dealloc,_isKVOA4个新增的方法。
1.iOS对KVO底层的实现，其实是通过派生一个NSKVONotifying_XXXX的类。bookclass.is指向NSKVONotifying_NXBookclass, 我们通过访问[bookclass class]会发现返回的仍然是NXBookclass, 这是因为NSKVONotifying_NXBookclass重写了class类，返回了NXBookclass。
2.观察实际上是通过重写观察的属性的setter方法来实现的。重写setName:方法：
- (void)setName:(NSString *)name {
if(_name != name){
[self willChangeValueForKey:@"name"];
_name = name;
[self didChangeValueForKey:@"name"];
}
}
3._isKVOA,用来标记是否是一个KVO的实例，通过调试发现_isKVOA == YES。在最后一个removeObserver:forKeyPath:之后变成NO，并将isa指回去。
4.通过观察前，观察后，取消观察后对NSClassFromString(@"NSKVONotifying_NXBookclass")访问，可以发现首次观察之前结果为null,观察中和取消观察后都存在，也就是取消观察后NSKVONotifying_NXBookclass不会消失。如果存在自定义的类NSKVONotifying_NXBookclass，那么在进行注册观察的时候会失败，也就是观察并不会生效。
5.默认情况下类的automaticallyNotifiesObserversForKey :返回YES，runtime会自动派生子类帮助我们做好监听。我们可以重写该方法 返回NO，并且自己在当前类中重写setName:方法，这样系统不会派生新的类。
7.KVO对可变数组的观察
准备代码，对可变数组的观察需要借用KVC的mutableArrayValueForKey方法才能观察得到：
{
[bookclass addObserver:self forKeyPath:@"descriptions" options:NSKeyValueObservingOptionOld|NSKeyValueObservingOptionNew context:NULL];
NSLog(@"观察之中:%@", [NXApi descriptionClass:object_getClass(bookclass)]);
bookclass.descriptions = [NSMutableArray arrayWithCapacity:2];
[[bookclass mutableArrayValueForKey:@"descriptions"] addObject:@"A"];
[[bookclass mutableArrayValueForKey:@"descriptions"] setObject:@"B" atIndexedSubscript:0];
[[bookclass mutableArrayValueForKey:@"descriptions"] removeObjectAtIndex:0];
[bookclass removeObserver:self forKeyPath:@"descriptions"];
}
观察打印的日志发现有4次KVO的回调：
分别为：
key=descriptions, change={
kind = 1;
new = ();
old = "";
}
key=descriptions, change={
indexes = "<_NSCachedIndexSet: 0x600000d53420>[number of indexes: 1 (in 1 ranges), indexes: (0)]";
kind = 2;
new = (A);
}
key=descriptions, change={
indexes = "<_NSCachedIndexSet: 0x600000d53420>[number of indexes: 1 (in 1 ranges), indexes: (0)]";
kind = 4;
new = (B);
old = (A);
}
key=descriptions, change={
indexes = "<_NSCachedIndexSet: 0x600000d53420>[number of indexes: 1 (in 1 ranges), indexes: (0)]";
kind = 3;
old = (B);
}
仔细看，你会发现对数组初始化的时候也收到了回调，再仔细看每次回调的kind都不一样，经过查阅文档，我们可以发现有一个NSKeyValueChange的枚举，NSKeyValueChangeSetting = 1设置，NSKeyValueChangeInsertion = 2插入，NSKeyValueChangeReplacement = 4替换，NSKeyValueChangeRemoval = 3移除，刚好跟我们对数组的初始化、插入对象、替换对象、删除对象的操作一致。