Failed to install MainActivity.apk on device 'emulator-5554': timeoutLaunch canceled!
解决方法：
window->preferences->Android->DDMS。把 ADB connection time out (ms):调大。
如把原来默认的值5000改为10000.即默认是5s改为10s。然后重启模拟器，之后用了一天没在出现上述问题。