Question

I try to write an app, that is using slide effect for activities and found that ViewPager from Compatibility Package is what I need. I successfully made the slide, it works in the way i want, but my problem is that I am not able to refer to the elements in my Activity.

This is my main.xml:

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:orientation="vertical" >

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

MainActivity.java:

public class MainActivity extends Activity {
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);

        MyPagerAdapter adapter = new MyPagerAdapter();
        ViewPager myPager = (ViewPager) findViewById(R.id.pager);

        myPager.setAdapter(adapter);
        myPager.setCurrentItem(0);

        if (Fibonacci.getInstance().number != 0)
            displayResults();

        ImageButton btnCalculate = (ImageButton) findViewById(R.id.btnCalculateDark);
        btnCalculate.setOnClickListener(new OnClickListener() {
            public void onClick(View arg0) {
                // do button click
            }
        });
    }

    private class MyPagerAdapter extends PagerAdapter {
        public int getCount() {
            return 2;
        }

        public Object instantiateItem(View collection, int position) {
            LayoutInflater inflater = (LayoutInflater) collection.getContext()
                    .getSystemService(Context.LAYOUT_INFLATER_SERVICE);
            int resId = 0;
            switch (position) {
            case 0:
                resId = R.layout.activity_dark;
                break;
            case 1:
                resId = R.layout.activity_pale;
                break;
            }
            View view = inflater.inflate(resId, null);
            ((ViewPager) collection).addView(view, 0);
            return view;
        }

        @Override
        public void destroyItem(View arg0, int arg1, Object arg2) {
            ((ViewPager) arg0).removeView((View) arg2);
        }

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

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

My problem is that I receive NullPointerEception for the following line:

ImageButton btnCalculate = (ImageButton) findViewById(R.id.btnCalculateDark);

I know it’s because in setContentView I set up main.xml, and I want to refer to an element in activity_dark.xml or activity_pale.xml. Unfortunately i am not able to figure out how to refer to the element. Any hints would be appreciated! Thank you!

Answer 1

You should better use Fragments for this
http://developer.android.com/guide/components/fragments.html

but if you want do it like this, then try to set listener after page changed:

 myPager.setOnPageChangeListener(new OnPageChangeListener(){

            @Override
            public void onPageScrollStateChanged(int arg0) {
                // TODO Auto-generated method stub

            }

            @Override
            public void onPageScrolled(int arg0, float arg1, int arg2) {
                // TODO Auto-generated method stub

            }

            @Override
            public void onPageSelected(int arg0) {
                ImageButton btnCalculate = (ImageButton) findViewById(R.id.btnCalculateDark);
                btnCalculate.setOnClickListener(new OnClickListener() {
                    public void onClick(View arg0) {
                        // do button click
                    }
                });
            }

        });