在fragment中使用viewpager嵌套fragment

acvitity_dynamic.xml

 <Button
        android:id="@+id/btn_change"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="切换" />

    <FrameLayout
        android:id="@+id/fl_fragment"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:background="@android:color/holo_green_light"></FrameLayout>

DynamicAcvitity.java

public class DynamicActivity extends AppCompatActivity {
    private Button btn_change;
    private BlankFragment blank;
    private SecFragment sec;
    private Fragment fragment;//用该变量表示现在展示的是哪一个fragment
    private FragmentManager manager;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_dynamic);
        blank = new BlankFragment();
        sec = new SecFragment();
        fragment=blank;
        manager=getSupportFragmentManager();
        //开始一个事物
        FragmentTransaction transaction=manager.beginTransaction();
        transaction.add(R.id.fl_fragment,blank);
        transaction.commit();//事物要提交过后才会有效
        //实例化按钮
        btn_change= (Button) findViewById(R.id.btn_change);
        btn_change.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                //一个事物提交后结束不能再用，要新建一个事物
                FragmentTransaction transaction1=manager.beginTransaction();
                if(fragment instanceof BlankFragment){
                        fragment=sec;
                }else {
                    fragment=blank;
                }
                transaction1.replace(R.id.fl_fragment,fragment);
                transaction1.commit();
            }
        });