解析 IllegalStateException--"Fragment already active"

大家都知道Fragment的数据传递是用setArgument来完成的，但是，实际使用的时候需要注意一些问题，也就是调用setArgument的时机。

比如下面的情况：
首先，在Activity的xml布局文件中引用fragment：

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/answer_question_fragment_root"
    android:layout_width="match_parent"
    android:layout_height="match_parent" >

    <fragment
        android:id="@+id/answer_question_fragmentyyy"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:name="com.emiage.bank.assistant.answer.AnswerQuestionFragment"
        tools:layout="@layout/fragment_answer_question" />
</RelativeLayout>

然后，我在Activity中试图通过setArgument来传递一些必要的参数：

@Override
protected void initViews() {
    FragmentManager fragmentManager = getSupportFragmentManager();
    AnswerQuestionFragment fragment = (AnswerQuestionFragment) fragmentManager.findFragmentById(R.id.answer_question_fragment);
    String categoryName = getIntent().getStringExtra(ExtraName.CATEGORYNAME);
    Bundle bundle = new Bundle();
    bundle.putString(ExtraName.CATEGORYNAME, categoryName);
    fragment.setArguments(bundle);
    FragmentTransaction transaction = fragmentManager.beginTransaction();
    transaction.add(R.id.answer_question_fragment_root, fragment);
    transaction.commitAllowingStateLoss();
}

这时候就会抛出上面的异常了：”Fragment already active”。异常是在setArgument的时候抛出的，我们看看setArgument方法：

    /**
     * Supply the construction arguments for this fragment.  This can only
     * be called before the fragment has been attached to its activity; that
     * is, you should call it immediately after constructing the fragment.  The
     * arguments supplied here will be retained across fragment destroy and
     * creation.
     */
    public void setArguments(Bundle args) {
        if (mIndex >= 0) {
            throw new IllegalStateException("Fragment already active");
        }
        mArguments = args;
    }

方法注释上说的很清楚了，setArgument方法要在“attach to activity”之前、构造fragment后立即调用，或许你还在疑惑，我就是在构造后立即调用的啊，怎么回事？其实真正的构造发生在解析xml文件时，通过findFragmentById得到的fragment已经是初始化过了，再调用setArgument就触发了上面的异常。

那么，如何纠正错误？很简单，把xml中的fragment标签去掉，只留下用来放fragment的容器。然后在Activity中commit：

    @Override
    protected void initViews() {
        FragmentManager fragmentManager = getSupportFragmentManager();
        AnswerQuestionFragment fragment = new AnswerQuestionFragment();
        String categoryName = getIntent().getStringExtra(ExtraName.CATEGORYNAME);
        Bundle bundle = new Bundle();
        bundle.putString(ExtraName.CATEGORYNAME, categoryName);
        fragment.setArguments(bundle);

        FragmentTransaction transaction = fragmentManager.beginTransaction();
        transaction.add(R.id.answer_question_fragment_root, fragment);
        transaction.commitAllowingStateLoss();
    }

所以，如果要给fragment传递参数，最好不要使用xml布局引用的方式，而是通过构造new出来，然后设置要传递的参数，再add-commit。

注意
我注意到setArgument方法下面还有一个方法也会同样的方式抛出上面的异常：

    /**
     * Set the initial saved state that this Fragment should restore itself
     * from when first being constructed, as returned by
     * {@link FragmentManager#saveFragmentInstanceState(Fragment)
     * FragmentManager.saveFragmentInstanceState}.
     *
     * @param state The state the fragment should be restored from.
     */
    public void setInitialSavedState(SavedState state) {
        if (mIndex >= 0) {
            throw new IllegalStateException("Fragment already active");
        }
        mSavedFragmentState = state != null && state.mState != null
                ? state.mState : null;
    }

用来保存fragment状态的方法，用来恢复最后保存状态，从代码上来看，这里触发异常的条件是跟setArgument方法一样的，所以，使用上也要注意同样情况。