java.lang.IllegalStateException: Can not perform this action after onSaveInstanceState

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！！！

public void setCurrentTab(int index) {
        if (index < 0 || index >= mTabSpecs.size()) {
            return;
        }

        if (index == mCurrentTab) {
            return;
        }

        // notify old tab content
        if (mCurrentTab != -1) {
            mTabSpecs.get(mCurrentTab).mContentStrategy.tabClosed();
        }

        mCurrentTab = index;
        final TabHost.TabSpec spec = mTabSpecs.get(index);

        // Call the tab widget's focusCurrentTab(), instead of just
        // selecting the tab.
        mTabWidget.focusCurrentTab(mCurrentTab);

        // tab content
        mCurrentView = spec.mContentStrategy.getContentView();

        if (mCurrentView.getParent() == null) {
            mTabContent
                    .addView(
                            mCurrentView,
                            new ViewGroup.LayoutParams(
                                    ViewGroup.LayoutParams.MATCH_PARENT,
                                    ViewGroup.LayoutParams.MATCH_PARENT));
        }

        if (!mTabWidget.hasFocus()) {
            // if the tab widget didn't take focus (likely because we're in touch mode)
            // give the current tab content view a shot
            mCurrentView.requestFocus();
        }

        //mTabContent.requestFocus(View.FOCUS_FORWARD);
        invokeOnTabChangeListener();
    }

    /**
     * Register a callback to be invoked when the selected state of any of the items
     * in this list changes
     * @param l
     * The callback that will run
     */
    public void setOnTabChangedListener(OnTabChangeListener l) {
        mOnTabChangeListener = l;
    }

    private void invokeOnTabChangeListener() {
        if (mOnTabChangeListener != null) {
            mOnTabChangeListener.onTabChanged(getCurrentTabTag());
        }
    }

@Override
    public void onTabChanged(String tabId) {
        if (mAttached) {
            FragmentTransaction ft = doTabChanged(tabId, null);
            if (ft != null) {
                ft.commit();
            }
        }
        if (mOnTabChangeListener != null) {
            mOnTabChangeListener.onTabChanged(tabId);
        }
    }

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。