对Property进行RACObserve

@property (assign,nonatomic) int price;

@property (assign,nonatomic) int time;

[RACObserve(self,price) subscribNext:(id x){

}];

1.仅调用price的set方法时，才会触发rac观察，即self.price = 100触发，而_price = 100不会触发

2.sendNext传递的值，是price的get方法返回值