1. 场景：
在开发过程中遇到这么一个需要，在主页点击按钮进入另一个Activity（ReadActivity），在该ReadActivity中点击一个按钮再返回主页并指定主页选中特定的Tab.主页是用FragmentTabHost + Fragment 实现。思路是通过startActivityForResult以及setResult() 以及requestCode作为标志位，是ReadActivity返回，因为还有其他的requestCode。再通过
FragmentTabHost的setCurrentTab(int index)方法切换Tab，我把该方法放置在了startActivityForResult()中，出现该问题java.lang.IllegalStateException: Can not perform this action after onSaveInstanceState。
2.为什么？
查阅源码，进入到FragmentManager的源码找到这么一段解释：
/**
* Start a series of edit operations on the Fragments associated with
* this FragmentManager.
*
* <p>Note: A fragment transaction can only be created/committed prior
* to an activity saving its state. If you try to commit a transaction
* after {@link FragmentActivity#onSaveInstanceState FragmentActivity.onSaveInstanceState()}
* (and prior to a following {@link FragmentActivity#onStart FragmentActivity.onStart}
* or {@link FragmentActivity#onResume FragmentActivity.onResume()}, you will get an error.
* This is because the framework takes care of saving your current fragments
* in the state, and if changes are made after the state is saved then they
* will be lost.</p>
*/
public abstract FragmentTransaction beginTransaction();
大意是：一个fragment事务只能在该依附的Activity正在保存状态时创建（created）或者提交（committed ），如果在调用onSaveInstanceState()之后再提交事务，就会导致问题发生—— Can not perform this action after onSaveInstanceState 。
3. 发生的时机：
先看FragmentManager中这么一段代码（只展示相关代码）：（static final boolean HONEYCOMB = android.os.Build.VERSION.SDK_INT >= 11;）
Parcelable saveAllState() {
// Make sure all pending operations have now been executed to get
// our state update-to-date.
execPendingActions();
if (HONEYCOMB) {
// As of Honeycomb, we save state after pausing. Prior to that
// it is before pausing. With fragments this is an issue, since
// there are many things you may do after pausing but before
// stopping that change the fragment state. For those older
// devices, we will not at this point say that we have saved
// the state, so we will allow them to continue doing fragment
// transactions. This retains the same semantics as Honeycomb,
// though you do have the risk of losing the very most recent state
// if the process is killed... we‘ll live with that.
mStateSaved = true;
}
}
只看if中的注释：HONEYCOMB -> 大于等于Android3.0 版本的系统，Activity在不可见状态（pausing）后保存其状态，置mStateSaved状态为true。对于3.0更早的设备，则允许在不可见状态后执行fragment事务，所以保存状态就是不确定的了，但是肯定会在stop之前保存状态。
mStateSaved（布尔值）标志是否保存状态：
private void checkStateLoss() {
if (mStateSaved) {
throw new IllegalStateException(
"Can not perform this action after onSaveInstanceState");
}
if (mNoTransactionsBecause != null) {
throw new IllegalStateException(
"Can not perform this action inside of " + mNoTransactionsBecause);
}
}
static final boolean HONEYCOMB = android.os.Build.VERSION.SDK_INT >= 11;4. 思路整理： 从主页跳转到ReadActivity，MainActivity（首页）已不可见并且保存其状态，此时已经不能再对fragment执行commit操作。而FragmentTabHost的setCurrentTab(int index)中调用了commit方法，导致问题出现。setCurrentTab() -> invokeOnTabChangeListener() -> onTabChanged() -> ft.commit(); 一目了然！！！
1 public void setCurrentTab(int index) {
2 if (index < 0 || index >= mTabSpecs.size()) {
3 return;
4 }
5
6 if (index == mCurrentTab) {
7 return;
8 }
9
10 // notify old tab content
11 if (mCurrentTab != -1) {
12 mTabSpecs.get(mCurrentTab).mContentStrategy.tabClosed();
13 }
14
15 mCurrentTab = index;
16 final TabHost.TabSpec spec = mTabSpecs.get(index);
17
18 // Call the tab widget‘s focusCurrentTab(), instead of just
19 // selecting the tab.
20 mTabWidget.focusCurrentTab(mCurrentTab);
21
22 // tab content
23 mCurrentView = spec.mContentStrategy.getContentView();
24
25 if (mCurrentView.getParent() == null) {
26 mTabContent
27 .addView(
28 mCurrentView,
29 new ViewGroup.LayoutParams(
30 ViewGroup.LayoutParams.MATCH_PARENT,
31 ViewGroup.LayoutParams.MATCH_PARENT));
32 }
33
34 if (!mTabWidget.hasFocus()) {
35 // if the tab widget didn‘t take focus (likely because we‘re in touch mode)
36 // give the current tab content view a shot
37 mCurrentView.requestFocus();
38 }
39
40 //mTabContent.requestFocus(View.FOCUS_FORWARD);
41 invokeOnTabChangeListener();
42 }
43
44 /**
45 * Register a callback to be invoked when the selected state of any of the items
46 * in this list changes
47 * @param l
48 * The callback that will run
49 */
50 public void setOnTabChangedListener(OnTabChangeListener l) {
51 mOnTabChangeListener = l;
52 }
53
54 private void invokeOnTabChangeListener() {
55 if (mOnTabChangeListener != null) {
56 mOnTabChangeListener.onTabChanged(getCurrentTabTag());
57 }
58 }
59
60 @Override
61 public void onTabChanged(String tabId) {
62 if (mAttached) {
63 FragmentTransaction ft = doTabChanged(tabId, null);
64 if (ft != null) {
65 ft.commit();
66 }
67 }
68 if (mOnTabChangeListener != null) {
69 mOnTabChangeListener.onTabChanged(tabId);
70 }
71 }
5. 解决：
知道问题所在这就好说了，只要在Activity未保存状态之前执行setCurrentTab()方法，其关键就是执行commit方法。要么在当前未保存状态之前，要么在Activity再次可见并未onPause之前都是不会出问题的，哦了。