为什么在Android Architecture Components的Observer中，值param可以为空? [英] Why is the value param nullable in Observer from Android Architecture Components?

问题描述

来自架构组件的LiveData定义了一个具有可为空的值的Observer，用于接收者回调:

LiveData from architecture components defines an Observer with nullable value for the receiver callback:

public interface Observer<T> {
    /**
     * Called when the data is changed.
     * @param t  The new data
     */
    void onChanged(@Nullable T t);
}

为什么有显式可为空的注释?

LiveData.observe()的文档还显示:

如果LiveData已经设置了数据，它将被发送给观察者.

If LiveData already has data set, it will be delivered to the observer.

例如观察者会等待不可为空的更新，或立即收到以前的不可为空的值，该值在Kotlin中尤其如此，直到我将T定义为可为空.

E.g. Observer waits for non-nullable updates or immediately receive previous non-nullable value, that should hold especially in Kotlin, until I define T as nullable.

代码似乎可以这样工作.我了解为什么LiveData.getValue()不能满足要求，在发送第一个数据之前可以手动调用LiveData.getValue()(并因此检查mData != NOT_SET返回空值).

The code seem to be working like that. I understand why this doesn't hold for LiveData.getValue(), which may be called manually before first data is delivered (and checks therefore for mData != NOT_SET to return a null).

第二个问题是:当T为不可为空时，可以安全地假设Kotlin中的值为非为空吗?

推荐答案

已修复 androix.lifecycle 2.0.0-beta01.

如果遇到任何问题，请报告 Android团队.

Please report android Team if you encounter any issues.

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