关于“不保留活动”时 Fragment的数据还原问题

1.fragment里嵌套fragment时，最好使用getChildFragmentManager()方法来获取FragmentManager。比较没有坑

2.获取FragmentManager的方法有3中：
getChildFragmentManager()；（只用于Fragment）
getFragmentManager()；（只用于非FragmentActivity的Activity）
getSupportFragmentManager()；（只用于FragmentActivity的Activity）

3.合理使用Tag

@Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_camera);

        NativeLibrary.ndkInit(this);
        mCameraFragment = (CameraFragment) getSupportFragmentManager().findFragmentByTag(CameraFragment.TAG);
        if (mCameraFragment == null) {
            mCameraFragment = new CameraFragment();
            Bundle bundle = new Bundle();
            bundle.putParcelable(CameraFragment.ACTIVITY_INTENT, getIntent());
            mCameraFragment.setArguments(bundle);
        }
        getSupportFragmentManager().beginTransaction()
            .replace(R.id.rl_root, mCameraFragment, CameraFragment.TAG)
            .commitAllowingStateLoss();
    }

4.关于getFragmentManager()与getChildFragmentManager()：

    // Fragment.java

    // The fragment manager we are associated with.  Set as soon as the
    // fragment is used in a transaction; cleared after it has been removed
    // from all transactions.
    FragmentManagerImpl mFragmentManager;

    /**
     * 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;
    }

    // Fragment.java

    // Private fragment manager for child fragments inside of this one.
    FragmentManagerImpl mChildFragmentManager;

    /**
     * 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;
    }

    void instantiateChildFragmentManager() {
        if (mHost == null) {
            throw new IllegalStateException("Fragment has not been attached yet.");
        }
        mChildFragmentManager = new FragmentManagerImpl();
        mChildFragmentManager.attachController(mHost, new FragmentContainer() {
            @Override
            @Nullable
            public View onFindViewById(int id) {
                if (mView == null) {
                    throw new IllegalStateException("Fragment does not have a view");
                }
                return mView.findViewById(id);
            }

            @Override
            public boolean onHasView() {
                return (mView != null);
            }
        }, this);
    }