1. R.java 文件消失： xml文件中有错误或者是资源命名中有大写字母，修改正确之后R文件就自动出现了~
2. 错误：'Thread updates not enabled for selected client (use toolbar button to enable)'.
解决方式：Windows->Preferences->Android->DDMS to check the 'Thread Updates
Enabled by Default' and then thread data was available (after
restarting the emulator).