PublishSubject 是Observable的子类
var publishSubject = PublishSubject()
publishSubject.onNext("Hello")
publishSubject.onNext("Hello1")
let publishSubscription = publishSubject.subscribe(onNext: { (str) in
print("str\(str)")
})
publishSubject.onNext("Hello2")
let publishSubscription2 = publishSubject.subscribe(onNext: { (str) in
print("str2\(str)")
})
publishSubject.onNext("Hello3")
打印：
strHello2
strHello3
str2Hello3
结论：PublishSubject 只能受（只能接受订阅之后发送的代码）
BehaviorSubject 可以介绍上一个加后面所有，所以的构造方法是BehaviorSubject(value: "Value A")
let behaviorSubject = BehaviorSubject(value: "Value A")
behaviorSubject.onNext("Value A1")
behaviorSubject.onNext("Value A2")
behaviorSubject.subscribe( onNext:{
print("订阅者1：\($0)")
})
behaviorSubject.onNext("Value B")
behaviorSubject.onNext("Value C")
let behaviorSubjectTwo = behaviorSubject.subscribe( onNext:{
print("订阅者2：\($0)")
})
behaviorSubject.onNext("Value D")
behaviorSubject.onNext("Value E")
打印
订阅者1：Value A2
订阅者1：Value B
订阅者1：Value C
订阅者2：Value C
订阅者1：Value D
订阅者2：Value D
订阅者1：Value E
订阅者2：Value E
