RxSwift(二)----Subjects

在RxSwift中有一种东西既能当做observable,又能当做observer,这种就是Subject.在RxSwift中有四种类型的Subject:

PublishSubject,初始元素为空,只把新元素发送给其订阅者.
BehaviorSubject,有初始元素,把最近的元素发送器订阅者.
ReplaySubject,会缓存一定数量的元素,并将发送给器新订阅者.
Variable,对BehaviorSubject的封装,将初始值或最近的值发送给其订阅者.

PublishSubject

Snip20181127_14.png

如果只是将新元素发送给订阅者直到订阅者取消订阅或者subject发出.completed或者.error事件为止.

let subject = PublishSubject<String>()
subject.onNext("hello world!")
let subscriptionOne = subject.subscribe { event in
    print(event)
}
subject.on(.next("1"))
subject.onNext("2")
    
let subscriptionTwo = subject.subscribe { event in
    print("2) ", event.element ?? event)
}
subject.onNext("3")
    
subscriptionOne.dispose()
subject.onNext("4")
    
subject.onCompleted()
subject.onNext("5")
subscriptionTwo.dispose()
    
let bag = DisposeBag()
subject.subscribe {
    print("3) ",$0.element ?? $0)
}
    .disposed(by: bag)

打印如下:

next(1)
next(2)
next(3)
2)  3
2)  4
2)  completed
3)  completed

如果subject发出.completed或者.error,这些都称为完成事件,它将发送这些完成事件给它将来的订阅者,不会发出.next()事件.这样不仅仅可以知道它什么时候终止,还可以知道你可能订阅了一个已经终止了的subject.

BehaviorSubject

Snip20181127_15.png

BehaviorSubject它初始化一个元素,它将最近的元素发送其订阅者.

let subject = BehaviorSubject(value: "hello world!")
let bag = DisposeBag()
// 初始值
subject.subscribe { event in
    print("1) ",(event.element ?? event.error) ?? event)
}
    .disposed(by: bag)
    
subject.onError(MyError.anError)
    
subject.subscribe {
     print("2) ",($0.element ?? $0.error) ?? $0)
}
    .disposed(by: bag)

打印如下:

1)  hello world!
1)  anError
2)  anError

因为BehaviorSubject它将最近的元素发送给订阅者,所以它必须初始化一个元素,如果你不带初始化元素,你可以用PublishSubject代替.

ReplaySubject

Snip20181127_16.png

ReplaySubject会缓存最近的元素,并将缓存的元素发送给其订阅者.

let subject = ReplaySubject<String>.create(bufferSize: 2)
let bag = DisposeBag()
    
subject.onNext("1")
subject.onNext("2")
subject.onNext("3")
    
subject.subscribe { event in
    print("1) ",(event.element ?? event.error) ?? event)
}
    .disposed(by: bag)
    
subject.subscribe {
    print("2) ",($0.element ?? $0.error) ?? $0)
}
    .disposed(by: bag)
    
subject.onNext("4")
subject.onError(MyError.anError)
subject.subscribe {
    print("3) ",($0.element ?? $0.error) ?? $0)
}
    .disposed(by: bag)

打印如下:

1)  2
1)  3
2)  2
2)  3
1)  4
2)  4
1)  anError
2)  anError
3)  3
3)  4
3)  anError

如果你缓存的元素太多的话,且是图片对象,这样对内存造成压力,或者是或者的元素是数组,这样也会有很大的缓存压力.

Variables

Variable是对BehaviorSubject的包装,并储存当前的值作为状态,你可以使用value属性获取或者设置值,这样可以不用onNext(_:)方法,

let variable = Variable("hello world!")
let bag = DisposeBag()
variable.value = "hello swift!"
variable.asObservable().subscribe {
     print("1) ",($0.element ?? $0.error) ?? $0)
}
    .disposed(by: bag)
    
variable.value = "1"
variable.asObservable().subscribe {
     print("2) ",($0.element ?? $0.error) ?? $0)
}
    .disposed(by: bag)
variable.value = "2"

打印如下:

1)  hello swift!
1)  1
2)  1
1)  2
2)  2

Variable不能添加.error和.completed事件,否则编译器会产生错误.