用这种方法切换，Fragment在隐藏时并不会走onDestoryView，所以显示时也不会走onCreateView，所有View都一直保存在内存中。
用replace方法：
载入Fragment1生命周期与上面相同：
Fragment1 onCreate Fragment1 onCreateView Fragment1 onStart Fragment1 onResume
切到Fragment2:
FragmentTransaction ft = getFragmentManager().beginTransaction(); ft.replace(R.id.simple_fragment, Fragment2); ft.setTransition(FragmentTransaction.TRANSIT_FRAGMENT_OPEN); ft.commit();
这次的Fragment1走生命周期了
Fragment1 onPause Fragment1 onStop Fragment1 onDestoryView Fragment1 onDestory Fragment2 onCreate Fragment2 onCreateView Fragment2 onStart Fragment2 onResume
真实打印出来可能是Fragment1和Fragment2混在一起的，可以看到，Fragment1走了onDestory,被完全回收了!
再切回到Fragment1
FragmentTransaction ft = getFragmentManager().beginTransaction(); ft.replace(R.id.simple_fragment, Fragment1); ft.setTransition(FragmentTransaction.TRANSIT_FRAGMENT_OPEN); ft.commit();
Fragment1 onCreate Fragment1 onCreateView Fragment1 onStart Fragment1 onResume Fragment2 onPause Fragment2 onStop Fragment2 onDestoryView Fragment2 onDestory
