RxJava风格的LiveData TestObserver和测试扩展失败，并出现NullPointerException

问题描述

我创建了一个TestObserver类，例如RxJava对应类和LiveData扩展函数，以测试LiveData的多个值，例如LOADING之类的状态，以及操作ERROR或{{1 }}。

SUCCESS

并在样本测试中使用它来检查其是否有效

class TestObserver<T>(private val liveData: LiveData<T>) : Observer<T> {

    init {
        liveData.observeForever(this)
    }

    private val testValues = mutablelistof<T>()

    override fun onChanged(t: T) {
        if (t != null) testValues.add(t)
    }

    fun testAssertNovalues(): TestObserver<T> {
        if (testValues.isNotEmpty()) throw AssertionException("Assertion error with actual size ${testValues.size}")
        return this
    }

    fun testAssertValueCount(count: Int): TestObserver<T> {
        if (count < 0) throw AssertionException("Assert count cannot be smaller than zero")
        if (count != testValues.size) throw AssertionException("Assertion error with expected $count while actual ${testValues.size}")
        return this
    }

    fun assertValues(vararg predicates: T): TestObserver<T> {
        predicates.forEach { predicate ->
            testValues.forEach { testValue ->
                if (predicate != testValue) throw  Exception("Assertion error")
            }
        }
        return this
    }

    fun assertValues(predicate: List<T>.() -> Boolean): TestObserver<T> {
        testValues.predicate()
        return this
    }

    fun values(predicate: List<T>.() -> Unit): TestObserver<T> {
        testValues.predicate()
        return this
    }

    fun values(): List<T> {
        return testValues
    }


    fun dispose() {
        testValues.clear()
        liveData.removeObserver(this)
    }
}


fun <T> LiveData<T>.test(): TestObserver<T> {

    val testObserver = TestObserver(this)

    observeForever(testObserver)

    return testObserver
}

class AssertionException(message: String) : Exception(message)

但是mutableList为null我不知道为什么吗？

解决方法

在观察之前调用liveData.removeObserver(this)解决了该问题。并在完全实现上进行了一些更改。

/**
 * RxJava style [Observer] for [LiveData] to test multiple values or states in container.
 *
 * This class is useful for testing view or action states or order of states if you are using
 * stateful machine.
 *
 * * Use with `InstantTaskExecutorRule` or a similar mechanism to execute tasks synchronously.
 *
 */

class LiveDataTestObserver<T> constructor(
    private val liveData: LiveData<T>
) : Observer<T> {

    init {
        liveData.observeForever(this)
    }

    private val testValues = mutableListOf<T>()

    override fun onChanged(t: T) {
        if (t != null) testValues.add(t)
    }

    fun assertNoValues(): LiveDataTestObserver<T> {
        if (testValues.isNotEmpty()) throw AssertionError(
            "Assertion error with actual size ${testValues.size}"
        )
        return this
    }

    fun assertValueCount(count: Int): LiveDataTestObserver<T> {
        if (count < 0) throw AssertionError(
            "Assertion error! value count cannot be smaller than zero"
        )
        if (count != testValues.size) throw AssertionError(
            "Assertion error! with expected $count while actual ${testValues.size}"
        )
        return this
    }

    fun assertValues(vararg predicates: T): LiveDataTestObserver<T> {
        if (!testValues.containsAll(predicates.asList())) throw AssertionError("Assertion error!")
        return this
    }

    fun assertValues(predicate: (List<T>) -> Boolean): LiveDataTestObserver<T> {
        predicate(testValues)
        return this
    }

    fun values(predicate: (List<T>) -> Unit): LiveDataTestObserver<T> {
        predicate(testValues)
        return this
    }

    fun values(): List<T> {
        return testValues
    }

    /**
     * Removes this observer from the [LiveData] which was observing
     */
    fun dispose() {
        liveData.removeObserver(this)
    }

    /**
     * Clears data available in this observer and removes this observer from the [LiveData] which was observing
     */
    fun clear() {
        testValues.clear()
        dispose()
    }
}

fun <T> LiveData<T>.test(): LiveDataTestObserver<T> {

    val testObserver = LiveDataTestObserver(this)

    // Remove this testObserver that is added in init block of TestObserver,and clears previous data
    testObserver.clear()
    observeForever(testObserver)

    return testObserver
}