Have misunderstanding with PagerAdapter for ViewPager. InstantianteItem method never gets called

Question

Here is my simple PagerAdapter and onCreate() of activity for paging.

public class MPagerAdapter extends PagerAdapter{

    List<ViewGroup> pages = null;

    public MPagerAdapter(List<ViewGroup> pages){
        this.pages = pages;
    }

    @Override
    public Object instantiateItem(ViewGroup collection, int position){
        ViewGroup v = pages.get(position);
        ((ViewPager) collection).addView(v, 0);
        return v;
    }

    @Override
    public void destroyItem(ViewGroup collection, int position, Object view){
        ((ViewPager) collection).removeView((View) view);
    }

    @Override
    public int getCount(){
        return pages.size();
    }

    @Override
    public boolean isViewFromObject(View view, Object object){
        return view.equals(object);
    }

    @Override
    public void finishUpdate(View arg0){
    }

    @Override
    public void restoreState(Parcelable arg0, ClassLoader arg1){
    }

    @Override
    public Parcelable saveState(){
        return null;
    }

    @Override
    public void startUpdate(View arg0){
    }
}

and here is onCreate() of activity in which pager will live.

    protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_pager);
    getActionBar().setDisplayHomeAsUpEnabled(true);

    Intent intent = getIntent();
    String message = intent.getStringExtra(MainActivity.EXTRA_MESSAGE);
    amount=Integer.parseInt(message);

    Resources res = getResources();
    Drawable background =  res.getDrawable(R.drawable.page_back1);

    BackgroudImage bImage = new BackgroudImage(x, y, background);

    List<MyLayout> pages = new ArrayList<MyLayout>();
    int a = amount/bImage.mPointsList.size();
    for (int i=0; i<=a; i++){
        MyLayout page = new MyLayout(getBaseContext(), bImage);     
        page.setLayoutParams(new ViewGroup.LayoutParams(LayoutParams.MATCH_PARENT, LayoutParams.MATCH_PARENT));
        pages.add(page);
    }
    MPagerAdapter pagerAdapter = new MPagerAdapter(pages);
    ViewPager viewPager = (ViewPager)findViewById(R.id.pager);//new ViewPager(this);
    viewPager.setAdapter(pagerAdapter);
    viewPager.setCurrentItem(1); 
}

OnLayout() method of my custom layout which is used as a control for pages of pagerView

protected void onLayout(boolean changed, int l, int t, int r, int b) {
    if (PagerActivity.amount<=points.size()) {

        for (int i=0; i<PagerActivity.amount; i++){
            ImageView childV = new ImageView(super.getContext());
            childV.setBackgroundResource(R.drawable.lesson_frame_closed);
            ViewGroup.LayoutParams imageParams = new ViewGroup.LayoutParams(LayoutParams.WRAP_CONTENT, LayoutParams.WRAP_CONTENT);
            childV.setOnClickListener(new View.OnClickListener() {

                  @Override
                  public void onClick(View view) {

                  }

                });
            this.addView(childV, imageParams);
        }
        PagerActivity.amount=0;
    } 

    if ((float)(bgrImage.getBackground().getIntrinsicWidth())/(float)(bgrImage.getBackground().getIntrinsicHeight())<(float)this.getMeasuredWidth()/(float)this.getMeasuredHeight()) {
        scaleFX=(float)this.getMeasuredHeight()/(float)bgrImage.getBackground().getIntrinsicWidth();
        scaleFY=(float)this.getMeasuredHeight()/(float)bgrImage.getBackground().getIntrinsicHeight();
    } else {
        scaleFX=(float)this.getMeasuredWidth()/(float)bgrImage.getBackground().getIntrinsicWidth();
        scaleFY=(float)this.getMeasuredWidth()/(float)bgrImage.getBackground().getIntrinsicHeight();
    }

    for (int i = 0; i < getChildCount(); i++) {
        final View childV = getChildAt(i);
        if (childV.getVisibility() != View.VISIBLE)
        continue;
        childV.layout((int)(points.get(i).getOriginX()*scaleFX), (int)(points.get(i).getOriginY()*scaleFY), childV.getWidth(), childV.getHeight());
    }

}

Can't understand why InstantiateItem() isn't being called. Any helpful idea will be appreciated.

Answer 1

The ViewPager is a View and should be added to the UI. typically by declaring it in a layout file.

Extracted from the SDK samples :

• the layout file :

  <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
      android:layout_width="match_parent"
      android:layout_height="match_parent"
      android:gravity="center_horizontal"
      android:orientation="vertical"
      android:padding="4dip" >
  
      <android.support.v4.view.ViewPager
          android:id="@+id/pager"
          android:layout_width="match_parent"
          android:layout_height="0px"
          android:layout_weight="1" >
      </android.support.v4.view.ViewPager>
      ...
  

• the activity class

  @Override
  protected void onCreate(Bundle savedInstanceState) {
      super.onCreate(savedInstanceState);
      setContentView(R.layout.fragment_pager);
  
      mAdapter = new MyAdapter(getSupportFragmentManager());
  
      mPager = (ViewPager)findViewById(R.id.pager);
      mPager.setAdapter(mAdapter);
      ...
  

The complete sources are in the extras/android/support/samples/Support4Demos directory of the SDK