Activity生命周期
红色框:运行FirstActivity (运行onCreate---onStart---onResume)
蓝色框:点击FirstActivity中的Button按钮,调用SecondActivity (运行onPause---onCreate---onStart---onResume---onStop)
绿色框:点击返回键 (运行onPause---onRestart---onStart---onResume---onStop---onDestroy)
红色框:运行FirstActivity (运行onCreate---onStart---onResume)
蓝色框:点击FirstActivity中的Button按钮,调用SecondActivity (运行onPause---onCreate---onStart---onResume---onStop)
绿色框:点击返回键 (运行onPause---onRestart---onStart---onResume---onStop---onDestroy)