为什么 Android 9 应用锁屏或切后台后采集音视频无效？

问题现象：Android 9 设备锁屏 1 分钟内，音频无声或看不到视频。

问题原因：从 Android 官网来看，这是系统强制限制。原文如下：

Limited access to sensors in background
Android 9 limits the ability for background apps to access user input and sensor data. If your app is running in the background on a device running Android 9, the system applies the following restrictions to your app:

• Your app cannot access the microphone or camera.
• Sensors that use the continuous reporting mode, such as accelerometers and gyroscopes, don't receive events.
• Sensors that use the on-change or one-shot reporting modes don't receive events.
  If your app needs to detect sensor events on devices running Android 9, use a foreground service.

详见 Android 行为变更。

解决方案： 目前 Android 官网没有明确说明后台采集声音或视频应如何处理，但使用前台服务可以让应用正常工作。

如果 Android 9 设备用户有锁屏后采集音频或视频的需求，可以在锁屏或退至后台前起一个 Service，并在退出锁屏或返回前台前终止 Service。
关于如何起 Service，请参考 https://developer.android.com/reference/android/app/Service 。