做项目的时候用到fragment，在fragment快速切换的时候发生了异常：IllegalStateException: Can not perform this action after onSaveInstanceState
这个异常是由于在使用FragmentTransition的 commit方法添加一个Fragment的时候出现的
public abstract int commit ()
Schedules a commit of this transaction. The commit does not happen immediately; it will be scheduled as work on the main thread to be done the next time that thread is ready.
A transaction can only be committed with this method prior to its containing activity saving its state. If the commit is attempted after that point, an exception will be thrown. This is because the state after the commit can be lost if the activity needs to
be restored from its state. See commitAllowingStateLoss() for
situations where it may be okay to lose the commit.
Returns
- Returns the identifier of this transaction's back stack entry, if
addToBackStack(String)had
been called. Otherwise, returns a negative number.
开始我用的是这个方法，fragment切换的时候基本上都会用这个方法。以前用的时候没出错，这次却不行了。在commit（）方法的后边还有个方法：
public abstract int commitAllowingStateLoss ()
Like commit() but
allows the commit to be executed after an activity's state is saved. This is dangerous because the commit can be lost if the activity needs to later be restored from its state, so this should only be used for cases where it is okay for the UI state to change
unexpectedly on the user.
这个方法与commit（）一样，但是允许在activity的状态信息保存后调用。用这个方法是不安全的，有可能activity还没恢复呢就不执行commit方法了。所以这个方法只能用在ui状态转换意外的时候
比较两个方法的不同就可以知道，我调用的 commit方法是在Activity的onSaveInstanceState()之后调用的。onSaveInstanceState方法是在该Activity即将被销毁前调用，来保存Activity数据的，如果在保存完状态后
再给它添加Fragment就会出错。解决办法就是把commit（）方法替换成 commitAllowingStateLoss()就行
了