Fragment的管理与内核（三）：嵌套处理，ChildFrag

前言

项目中在使用Fragment中嵌套Fragment时，你可能会遇到这么一个问题，多次显示隐藏父Fragment之后子Fragment不显示。具体的表现是在父Fragment A中嵌套添加了一个子Fragment B。外部调用父Fragment A第一次显示时，FragmentA、B都正常显示；外部第二次调用父Fragment显示时，Fragment A正常显示，Fragment B显示空白。

问题回放

        FragmentManager fm = getActivity().getSupportFragmentManager();
        FragmentTransaction ft = fm.beginTransaction();
        ft.add(R.id.fl_base_info,new FragmentA());
        ft.commit();

问题:

1. Fragment放ViewPager，ViewPager里面是fragment。第一次进入没问题，再次进入ViewPager的fragment时里面内容就没了,数据丢失。
2. Fragment低频率点击切换不会发生问题，过快点击马上崩溃
3. 错误：Java.lang.IllegalArgumentException：No view found for id for fragment
4. 调用fragment的replace方法不走onDestroy()、onDestroyView()方法，无法销毁fragment
5. 在fragment中写倒计时，每次切换后倒计时越来越快的问题

getSupportFragmentManager()主要用于支持 3.0以下android系统API版本，3.0以上系统可以直接调用getFragmentManager() ，因为fragment是3.0以后才出现的组件，为了这之前的系统版本也能使用fragment,借助V4包里面的getSupportFragmentManager()方法来间接获取FragmentManager()对象，3.0版本之后，有了Fragment的api，就可以直接使用getFragmentManager()这个方法来获取对象。

解决办法

        FragmentManager fm = getChildFragmentManager();
        FragmentTransaction ft = fm.beginTransaction();
        ft.add(R.id.fl_base_info,new FragmentA());
        ft.commit();

getFragmentManager到的是activity对所包含fragment的Manager，而如果是fragment嵌套fragment，那么就需要利用getChildFragmentManager()。

源码

/**
 * Return the FragmentManager for interacting with fragments associated
 * with this fragment's activity.  Note that this will be non-null slightly
 * before {@link #getActivity()}, during the time from when the fragment is
 * placed in a {@link FragmentTransaction} until it is committed and
 * attached to its activity.
 *
 * <p>If this Fragment is a child of another Fragment, the FragmentManager
 * returned here will be the parent's {@link #getChildFragmentManager()}.
 */
final public FragmentManager getFragmentManager() {
    return mFragmentManager;
}

/**
 * Return a private FragmentManager for placing and managing Fragments
 * inside of this Fragment.
 */
final public FragmentManager getChildFragmentManager() {
    if (mChildFragmentManager == null) {
        instantiateChildFragmentManager();
        if (mState >= RESUMED) {
            mChildFragmentManager.dispatchResume();
        } else if (mState >= STARTED) {
            mChildFragmentManager.dispatchStart();
        } else if (mState >= ACTIVITY_CREATED) {
            mChildFragmentManager.dispatchActivityCreated();
        } else if (mState >= CREATED) {
            mChildFragmentManager.dispatchCreate();
        }
    }
    return mChildFragmentManager;
}