RxJava - 即使调用 onNext() 如何阻止 PublishSubject 发布

Question

我查看了我正在调用以下内容：

// class member
var myPublishSubject = PublishSubject.create<SomeObservable>()

// later on in the class somewhere:    
while(true){
   myPublishSubject.onNext(someObservable)
}

我想停止发射，但让 while 循环永远继续下去。所以我希望 onNext 调用什么都不做。但我担心如果我调用 myPublishSubject.onComplete() 最终主题将为空，我将获得 NPE。即使 onNext() 被反复调用，是否还有只是为了让它静音。退订是最好的方法吗？

Answer 1

几点说明

这是一个非常罕见的案例，但如果您可以通过 Observable 向我们展示您的真实意图，我们可能会帮助您构建它，如果不是最好的，那就更好了。

你能做什么

对于我的示例，我只使用了一个非常简单的标志变量，可以根据项目的任何触发器进行更改。

选项 1

您可以直接在主题发布者上调用onComplete

val maxEmittedItemCount = 10
var currentEmittedItemCount = 0
val someStringValue = "Some observable" // Create whatever observable you have
val publishSubject = PublishSubject.create<String>()

publishSubject.subscribe({
    currentEmittedItemCount++
    println(it)
}, {
    println(it)
})

while (currentEmittedItemCount != maxEmittedItemCount) {
    // Print indication that the loop is still running
    println("Still looping")

    // Publish value on the subject
    publishSubject.onNext(someStringValue)

    // Test flag for trigger
    if (currentEmittedItemCount == maxEmittedItemCount) publishSubject.onComplete()
}

选项 2

您还可以持有对订阅的引用，然后在之后释放它，这比前一个更语义化，因为它会在资源释放时执行代码块而不调用onNext(t)。

lateinit var disposable: Disposable // Will hold reference to the subscription
var maxEmittedItemCount = 10
var currentEmittedItemCount = 0
var someStringValue = "Some observable" // Create whatever observable you have
var publishSubject = PublishSubject.create<String>()

disposable = publishSubject.subscribeWith(object : DisposableObserver<String>() {
    override fun onComplete() {
        // Print indication of completion for the subject publisher
        System.out.println("Complete")
    }

    override fun onNext(t: String) {
        // Test flag count synchonizer
        currentEmittedItemCount++

        // Print out current emitted item count
        System.out.println(currentEmittedItemCount)

        // Print current string
        System.out.println(t)
    }

    override fun onError(e: Throwable) {
        // Print error
        System.out.println(e)
    }
})

while (currentEmittedItemCount != maxEmittedItemCount) {
    // Publish value on the subject
    if (!disposable.isDisposed) publishSubject.onNext(someStringValue)

    // Test flag for trigger
    if (currentEmittedItemCount == maxEmittedItemCount) {
        publishSubject.onComplete() // optional if you need to invoke `onComplete()` block on the subject
        disposable.dispose()
    }

    // Print indication that the loop is still running
    System.out.println("Still looping")
}

了解更多

• https://medium.com/@vanniktech/rxjava-2-disposable-under-the-hood-f842d2373e64
• https://medium.com/@nazarivanchuk/types-of-subjects-in-rxjava-96f3a0c068e4
• http://reactivex.io/RxJava/javadoc/io/reactivex/disposables/Disposable.html

Answer 2

由于观察者已订阅，我们必须调用 unsubscribe 以避免多次 onNext 调用。

我建议在subject.onNext()的工作完成后调用onComplete：

这是一个例子

PublishSubject<Integer> source = PublishSubject.create();

source.onNext(1);
source.onComplete();

source.subscribe(getObserver());

然后在观察者中，我们重新创建另一个 PublishSubject 实例

 source.subscribe(new Observer<Boolean>() {
                                        @Override
                                        public void onSubscribe(Disposable d) {

                                    }

                                    @Override
                                    public void onNext(Integer value) {

                                    }

                                    @Override
                                    public void onError(Throwable e) {

                                    }

                                    @Override
                                    public void onComplete() {
                                        source = PublishSubject.create();
                                    }
                                });

希望这种方法对您的要求有所帮助