KVC和KVO

//为对象p添加一个监听者
//Observer: 监听者
//KeyPath: 属性名(需要监听哪个属性)
//options: 监听的哪些选项
[p addObserver:self forKeyPath:@"name" options:kNilOptions context:@"test"];

keyPath: 哪个属性被改了
object: 哪个对象的属性被改了
change: 改成什么样子
context: 当初addObserver时的context参数值

Person *p = [[Person alloc] init];

Dog *dog = [[Dog alloc] init];

[p setValue:@"wangcai" forKeyPath:@"dog.name"];//正确

[p setValue:@"wangcai" forKey:@"dog.name"];//错误
+ forKeyPath可以用来修改模型对象的私有成员变量

[p setValue:@"test" forKeyPath:@"_name"];//正确,直接修改成员变量

HMJBook *book1 = [[HMJBook alloc] init];
book1.name = @"1";
book1.price = 12;
HMJBook *book2 = [[HMJBook alloc] init];
book2.name = @"2";
book2.price = 15;
HMJBook *book3 = [[HMJBook alloc] init];
book3.name = @"3";
book3.price = 22;
NSArray *books = @{book1, book2, book3};
//取得books数组中所有HMJBook对象的name属性值,放在一份新的数组中返回
NSArray *names = [books valueForKeyPath:@"name"];

//等同于
NSArray *names = [p.books valueForKeyPath:@"name"];
//等同于
NSArray *names = [p valueForKeyPath:@"books.name"];
//计数
NSLog(@"%@", [p.books valueForKeyPath:@"@count"]);
//计数
NSLog(@"%@", [p valueForKeyPath:@"books.@count"]);
//最大价格
NSLog(@"%@", [p.books valueForKeyPath:@"@max.price"]);
//平均值
NSLog(@"%@", [p.books valueForKeyPath:@"@avg.price"]);
//平均值
NSLog(@"%@", [p valueForKeyPath:@"books.@avg.price"]);