为什么 instantiateItem 不调用并且 ViewPager 不工作？

Question

I try to make simple application with sliding images. I use this article http://www.androidbegin.com/tutorial/android-viewpager-gallery-images-and-texts-tutorial/ .

This is my xml with viewpager: main.xml

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="horizontal" >

    <android.support.v4.view.ViewPager
        android:id="@+id/pages"
        android:layout_height="match_parent"
        android:layout_width="match_parent">
    </android.support.v4.view.ViewPager>

</LinearLayout>

this is my xml with image: item.xml

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent" >

    <ImageView
        android:id="@+id/image"
        android:layout_gravity="center"
        android:layout_width="fill_parent"
        android:layout_height="fill_parent"
        android:src="@drawable/ic_launcher" />

</RelativeLayout>

this is my adapter: ViewerPagerAdapter.java

public class ViewerPagerAdapter extends PagerAdapter {

    private Context mContext;
    private ArrayList<CueModel> mArray;
    private LayoutInflater mInflater;
    private ImageView mImage;

    public ViewerPagerAdapter(Context _context){
        mContext = _context;

        mInflater = (LayoutInflater) mContext.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
    }


    public void setArray(ArrayList<Bitmap> arr){
        mArray = arr;
    }

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

    @Override
    public Object instantiateItem(ViewGroup container, int position) {
          View itemView = mInflater.inflate(R.layout.item, container, false);
          mImage = (ImageView)itemView.findViewById(R.id.image);
          mImage.setImageBitmap(mArray.get(position));
          container.addView(itemView);

          return itemView;
     }

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

    @Override
    public void destroyItem(ViewGroup container, int position, Object object) {
        container.removeView((View)object);
    }

@Override
public boolean isViewFromObject(View arg0, Object arg1) {
    return arg0.equals(arg1);
}

this is my activity: OneActivity.java (mArrays full of image)

public class OneActivity extends Activity {
    ArrayList<Bitmap> mArrays;
    ViewerPagerAdapter mAdapter;
    ViewPager mPager;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main.xml);
        mPager = (ViewPager) findViewById(R.id.pages);
        mAdapter = new ViewerPagerAdapter(OneActivity.this);
        mAdapter.setArrays(mArrays);
        mPager.setAdapter(mAdapter);
    }
}

I don't understand what I did wrong. instantiateItem doesn't call.

PS.Difference between my app and

article I have a few activity(and my arraylist initialized by items) Thank you

Answer 1

#instantiateItem() is called whenever the ViewPager needs a new View. Initially, it'll call #instantiateItem() twice for the View in the center and view to the right if the adapter's getCount() > 0 . That's why in your #setArray() method you need to call #notifyDataSetChanged() every time you change the array. You also need to call it every time you add or remove items from the list. If your array is empty in #onCreate() , then #getCount() == 0 so the ViewPager thinks it's empty and won't bother.

Answer 2

I know this is very old post but I am answering this becoz someone would get benefited. I got the same issue - instantiateItem () was not getting called. After trying so many thing I manage to finally found the mistake I did in my code and it was a silly one I forgot to set adapter to viewpager:

viewPager.setAdapter(adapter)

Answer 3

For modern versions (2020+) where the error message is

Binary XML file line #(lineNumber): Error inflating class android.viewpager.widget.ViewPager at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2817)

you might want to update

android.support.v4.view.ViewPager to

androidx.viewpager.widget.ViewPager

Answer 4

In my case, the size of the list was zero. Therefore, this method was not calling. When the list was not empty, it worked properly.