Android：FragmentTransaction的commit 和addToBackStack的方法随记

1）commit():The commit does not happen immediately  ft.add(id,fragment,tag).commit();  Fragment f=fm.findFragmentByTag(tag);// 此时找不到Fragment，fragment为null

2）addToBackStack(null):用了此法方法，按返回键，可以返回上一个添加的Fragment
英文官方解释：

Add this transaction to the back stack.  This means that the transactionwill be remembered after it is committed, and will reverse its operationwhen later popped off the stack.         
stack的特点:后进先出