kotlin 在 onNext 或不应为 null 的 BehaviorSubject 中传递 mutableList

Question

Kotlin 1.3.72，
RxJava2
我有以下代码，我试图避免使用 !!运算符，但不确定为什么它认为该值为 null，我需要使用安全调用运算符。
以后我得用!!这是不好的做法。为什么这会是空的，因为我已经声明了任何可以为空的类型？

class SharedViewModel : ViewModel() {
    private val compositeDisposable = CompositeDisposable()
    private val imageSubject = BehaviorSubject.create<MutableList<Photo>>()
    private val selectedPhotos = MutableLiveData<List<Photo>>()

    init {
        imageSubject.subscribeBy {
            selectedPhotos.value = it
        }.addTo(compositeDisposable)
    }

    fun getSelectedPhotos(): LiveData<List<Photo>> {
        return selectedPhotos
    }

    fun addPhotos(photo: Photo) {
        // Not sure why I need the safe-call operator here
        imageSubject.value?.add(photo)
        // Using the !! is bad practice and would like to avoid it
        imageSubject.onNext(imageSubject.value!!)

        // This is how I am currently handling it, but just wondering why the value would be null when it is not a nullable type?
        if(imageSubject.value != null) {
            imageSubject.onNext(imageSubject.value ?: mutableListOf())
        }
    }
}    

=== 更新 =====
我做了一些更改和更新。我的最后一个使用 let 。

fun addPhotos(photo: Photo) {
        imageSubject.value?.add(photo)

        // Original
        imageSubject.onNext(imageSubject.value!!)

        // First Attempt
        if(imageSubject.value != null) {
            imageSubject.onNext(imageSubject.value ?: mutableListOf())
        }

        // Final Attempt
        imageSubject.value?.let {
            imageSubject.onNext(it)
        }
    }

还有一个问题:
将某些内容添加到 BehaviourSubject imageSubject.value?.add(photo) 中是一种好习惯吗？然后立即使用 onNext imageSubject.onNext(it) 发出该值?

Answer 1

BehaviourSubject可以有空值，因为它打算在订阅时发出默认值(这是它内部的最新值，如果不存在则为空)如果您不想要这种方法，请使用 PublishSubject .
至于使用当前类型的实现，我会做类似的事情

class SharedViewModel : ViewModel() {
    private val compositeDisposable = CompositeDisposable()
    //It is better to use singular object streams instead of list publish
    private val imageSubject = BehaviorSubject.create<Photo>()
    //It is better to use singular object livedata instead of list post. Collect it into a list in place where you subscribe to it or use approach recommended by @Franz Andel above
    private val _selectedPhotos = MutableLiveData<Photo>()
    //Use this approach instead of explicit getter function
    val selectedPhotos: LiveData<Photo>
        get() = _selectedPhotos

    init {
        imageSubject.subscribeBy {
            _selectedPhotos.postValue(it)
        }.addTo(compositeDisposable)
    }

    fun addPhotos(photo: Photo) {
        imageSubject.onNext(photo)
    }
}    

一些建议:

//Do not do it like that with subjects - do not add something to their value directly.
        imageSubject.value?.add(photo)
        // Using the !! is bad practice and would like to avoid it
        imageSubject.onNext(imageSubject.value!!)

回答如何在没有 !! 的情况下以最佳方式处理可空类型:

//This piece of code is pointless but it has needed context.
imageSubject.value?.apply{
   imageSubject.onNext(this)
}

您也可能想使用 elvis operator :

imageSubject.onNext(imageSubject.value ?: someDefaultValue)