让多个Fragment 切换时不重新实例化

    public void switchContent(Fragment fragment) {
        if(mContent != fragment) {
            mContent = fragment;
            mFragmentMan.beginTransaction()
                .setCustomAnimations(android.R.anim.fade_in, R.anim.slide_out)
                .replace(R.id.content_frame, fragment) // 替换Fragment，实现切换
                .commit();
        }
    }

    public void switchContent(Fragment from, Fragment to) {
        if (mContent != to) {
            mContent = to;
            FragmentTransaction transaction = mFragmentMan.beginTransaction().setCustomAnimations(
                    android.R.anim.fade_in, R.anim.slide_out);
            if (!to.isAdded()) {    // 先判断是否被add过
                transaction.hide(from).add(R.id.content_frame, to).commit(); // 隐藏当前的fragment，add下一个到Activity中
            } else {
                transaction.hide(from).show(to).commit(); // 隐藏当前的fragment，显示下一个
            }
        }
    }