ZXing CaptureActivity黑屏问题
ZXing的CaptureActivity在其他activity切换回来时会出现黑屏现象，跟踪发现surfaceDestroyed并不一定能调用到。
在ZXing项目找到zxing不支持tabhost的答复。
Project Member #1 srowen The app doesn't use a TabHost. If you are writing your own app, indeed, you may need to write different code! this part is not a library that is supposed to work in other contexts.
虽然项目采用tabactivity搭建框架，但CaptureActivity是单独的activity。
针对surfaceDestroyed在activity切换时不会总是调用的问题，https://github.com/commonsguy/cw-advandroid/tree/master/Camera/Preview/ 的解决方法值的参考。
也即是把打开摄像头的动作延迟到surfaceChanged时调用。