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更改（来回）活动时 LiveData 不更新

[英]LiveData not updating when changing (back and forth) activities

原文 2022-09-20 03:59:58 5 1 android/ android-livedata/ android-viewmodel

Before I post detailed code, I want to make sure I do understand LiveData correctly.在发布详细代码之前，我想确保我确实正确理解LiveData 。 I have a MainActivity (A) and then go to Activity (B).我有一个 MainActivity (A)，然后是 go 到 Activity (B)。 I did implement a ViewModel for 'B' and observe (and display) the MutableLiveData for a Counter (residing in the ViewModel) in Activity 'B'.我确实为“B”实现了一个 ViewModel，并在活动“B”中观察（并显示）计数器（位于 ViewModel 中）的 MutableLiveData。

This works perfect, I do see the countdown in 'B', but when I do the back-button to return to MainActivity 'A' and then go back to 'B', the counter is not updated anymore.这很完美，我确实在“B”中看到了倒计时，但是当我按后退按钮返回 MainActivity“A”然后 go 返回“B”时，计数器不再更新。

I do see the counter pick up nicely when I switch apps ie send it to the background and bring it back.当我切换应用程序时，我确实看到计数器很好地恢复了，即将它发送到后台并带回来。

I might not understand the whole Lifecycle properly, but I thought the beauty of LiveData is that I don't have to worry about all this and once I return to 'B' it should just pick up the observation seamlessly?我可能无法正确理解整个生命周期，但我认为LiveData的美妙之处在于我不必担心这一切，一旦我回到“B”，它应该无缝地接受观察吗？

PS> Forgot to mention that I do see the counter counting down uninterrupted while I'm in 'A' (via LOG.i) PS> 忘了提，当我在“A”时，我确实看到计数器不间断地倒计时（通过 LOG.i）

1 个解决方案

When you go from A > B -- A is alive B is alive Back button to go to A -> A is alive B is destroyed hence the ViewModel associated with B is destroyed as well.当您从 A > B 开始 go - A 还活着 B 还活着后退按钮到 go 到 A -> A 还活着 B 被破坏，因此与 B 关联的 ViewModel 也被破坏。 Button to go from A > B -> A is alive and a new instance of ViewModel B is created.从 A > B -> A 到 go 的按钮处于活动状态，并创建了 ViewModel B 的新实例。 This new ViewModel of B has no idea the previous ViewModel existed. B 的这个新 ViewModel 不知道之前的 ViewModel 存在。