public class ViewPager2 extends ViewGroup {
    
	private void initialize(Context context, AttributeSet attrs) {
            mRecyclerView = new RecyclerView(context) {
               ...
            };

            mLayoutManager = new LinearLayoutManager(context);
            mRecyclerView.setLayoutManager(mLayoutManager);
            setOrientation(context, attrs);

            new PagerSnapHelper().attachToRecyclerView(mRecyclerView);
            ...
        }
}

implementation 'androidx.viewpager2:viewpager2:1.0.0-alpha01'

<androidx.viewpager2.widget.ViewPager2
        android:id="@+id/viewPager2"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:orientation="vertical" />

public class ViewPager2Adapter extends FragmentStateAdapter {

    public ViewPager2Adapter(@NonNull FragmentManager fragmentManager) {
        super(fragmentManager);
    }

    @NonNull
    @Override
    public Fragment getItem(int position) {
        return new SimpleFragment();
    }

    @Override
    public int getItemCount() {
        return 10;
    }
}

viewPager2 = findViewById(R.id.viewpager2);
ViewPager2Adapter adapter = new ViewPager2Adapter(getSupportFragmentManager());
viewPager2.setAdapter(adapter);

private static final int DEFAULT_VALUE = 1;

/**
 * Return the number of views available.
 */
public abstract int getCount();


/**
 * Create the page for the given position.  The adapter is responsible
 * for adding the view to the container given here, although it only
 * must ensure this is done by the time it returns from
 * {@link #finishUpdate(ViewGroup)}.
 *
 * @param container The containing View in which the page will be shown.
 * @param position The page position to be instantiated.
 * @return Returns an Object representing the new page.  This does not
 * need to be a View, but can be some other container of the page.
 */
public Object instantiateItem(ViewGroup container, int position) {
    return instantiateItem((View) container, position);
}

 	@Override
   public Object instantiateItem(ViewGroup container, int position) {
       ...
       final long itemId = getItemId(position);

       // Do we already have this fragment?
       String name = makeFragmentName(container.getId(), itemId);
       Fragment fragment = mFragmentManager.findFragmentByTag(name);
       if (fragment != null) {
           if (DEBUG) Log.v(TAG, "Attaching item #" + itemId + ": f=" + fragment);
           mCurTransaction.attach(fragment);
       } else {
           fragment = getItem(position);
           if (DEBUG) Log.v(TAG, "Adding item #" + itemId + ": f=" + fragment);
           mCurTransaction.add(container.getId(), fragment,
                   makeFragmentName(container.getId(), itemId));
       }
      ...
       return fragment;
   }


   @Override
   public void destroyItem(ViewGroup container, int position, Object object) {
       if (mCurTransaction == null) {
           mCurTransaction = mFragmentManager.beginTransaction();
       }
       if (DEBUG) Log.v(TAG, "Detaching item #" + getItemId(position) + ": f=" + object
               + " v=" + ((Fragment)object).getView());
       mCurTransaction.detach((Fragment)object);
   }

public long getItemId(int position) {
       return position;
   }


   private static String makeFragmentName(int viewId, long id) {
       return "android:switcher:" + viewId + ":" + id;
   }

mCurTransaction.add(container.getId(), fragment, makeFragmentName(container.getId(), itemId));

mFragmentManager.findFragmentByTag(name)

/**
 * Return the Fragment associated with a specified position.
 */
public abstract Fragment getItem(int position);

@Override
public Object instantiateItem(ViewGroup container, int position) {
    ...
    
    if (mCurTransaction == null) {
        mCurTransaction = mFragmentManager.beginTransaction();
    }

    Fragment fragment = getItem(position);
    while (mFragments.size() <= position) {
        mFragments.add(null);
    }
    
 	...
    mCurTransaction.add(container.getId(), fragment);

    return fragment;
}

@Override
public void destroyItem(ViewGroup container, int position, Object object) {
    Fragment fragment = (Fragment) object;
    ...
    mCurTransaction.remove(fragment);
}

void dataSetChanged() {
    // This method only gets called if our observer is attached, so mAdapter is non-null.

  	...
    for (int i = 0; i < mItems.size(); i++) {
        final ItemInfo ii = mItems.get(i);
        final int newPos = mAdapter.getItemPosition(ii.object);

        if (newPos == PagerAdapter.POSITION_UNCHANGED) {
            continue;
        }

        if (newPos == PagerAdapter.POSITION_NONE) {
            mItems.remove(i);
            i--;

            if (!isUpdating) {
                mAdapter.startUpdate(this);
                isUpdating = true;
            }

            mAdapter.destroyItem(this, ii.position, ii.object);
            needPopulate = true;

            if (mCurItem == ii.position) {
                // Keep the current item in the valid range
                newCurrItem = Math.max(0, Math.min(mCurItem, adapterCount - 1));
                needPopulate = true;
            }
            continue;
        }

     ...
    }

   ...
}

 /**
 * Called when the host view is attempting to determine if an item's position
 * has changed. Returns {@link #POSITION_UNCHANGED} if the position of the given
 * item has not changed or {@link #POSITION_NONE} if the item is no longer present
 * in the adapter.
 *
 * <p>The default implementation assumes that items will never
 * change position and always returns {@link #POSITION_UNCHANGED}.
 *
 * @param object Object representing an item, previously returned by a call to
 *               {@link #instantiateItem(View, int)}.
 * @return object's new position index from [0, {@link #getCount()}),
 *         {@link #POSITION_UNCHANGED} if the object's position has not changed,
 *         or {@link #POSITION_NONE} if the item is no longer present.
 */
public int getItemPosition(Object object) {
    return POSITION_UNCHANGED;
}

@Override
public int getItemPosition(Object object) {
    return POSITION_NONE;
}

  private ViewPager.OnPageChangeListener onPageChangeListener = new ViewPager.OnPageChangeListener() {
      @Override
      public void onPageScrolled(int position, float positionOffset, int positionOffsetPixels) {

      }

      @Override
      public void onPageSelected(int position) {

      }

      @Override
      public void onPageScrollStateChanged(int state) {

      }
  };

  public void OnCreate(){
      ...
      mViewPager.setOnPageChangeListener(onPageChangeListener);
onPageChangeListener.onPageSelected(0);  // 手动调用第一页
      ...
  }

mViewPager.post(new Runnable(){
@Override
    public void run() {
        onPageChangeListener.onPageSelected(0);  // 手动调用第一页
    }
});

public class MyPagerAdapter extends FragmentStatePagerAdapter {
    ...
        
    private Object lastPrimaryItem;
    
    private OnPageSwitchListener onPageSwitchListener;
    public MyPagerAdapter(FragmentManager fm, OnPageSwitchListener onPageSwitchListener) {
        super(fm);
        this.onPageSwitchListener = onPageSwitchListener;
    }
    
    @Override
    public void setPrimaryItem(ViewGroup container, int position, Object object) {
        super.finishUpdate(container);
        super.setPrimaryItem(container, position, object);
        if (object == lastPrimaryItem) {
            return;  // setPrimaryItem会多次调用，过滤掉多余的调用
        }
        lastPrimaryItem = object;
        MyFragment fragment = (MyFragment)object;
        if (onPageSwitchListener != null && fragment != null) {
            onPageSwitchListener.onPageSwitch(position, fragment);
        }
    }

    @Override
    public void finishUpdate(ViewGroup container) {
		// 这里将finishUpdate放到setPrimaryItem之前执行，见上面的方法👆
        // 原因是finishUpdate后的Fragment才算正式可以用了
    }
    
    public interface OnPageSwitchListener{
        void onPageSwitch(int position, MyFragment fragment);
    }
}

class MyActivity extends Activity implements MyPagerAdapter.OnPageSwitchListener{

    @Override
    public void onPageSwitch(int position, MyFragment fragment) {
    	// Do whatever you like here. :)
    }
}

1 android.text.StaticLayout.getLineTop(StaticLayout.java:878)
2 android.widget.TextView.getExtendedPaddingTop(TextView.java:1966)
3 android.widget.TextView.bringTextIntoView(TextView.java:7291)
4 android.widget.TextView.onPreDraw(TextView.java:5107)
5 android.view.ViewTreeObserver.dispatchOnPreDraw(ViewTreeObserver.java:944)
6 android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:2348)
7 android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1292)
8 android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:6598)
9 android.view.Choreographer$CallbackRecord.run(Choreographer.java:800)
10 android.view.Choreographer.doCallbacks(Choreographer.java:603)
11 android.view.Choreographer.doFrame(Choreographer.java:572)
12 android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:786)
13 android.os.Handler.handleCallback(Handler.java:815)
14 android.os.Handler.dispatchMessage(Handler.java:104)
15 android.os.Looper.loop(Looper.java:194)
16 android.app.ActivityThread.main(ActivityThread.java:5682)
17 java.lang.reflect.Method.invoke(Native Method)
18 java.lang.reflect.Method.invoke(Method.java:372)