ViewPager update fragment on swipe

Question

i have a problem that i have been struggling with for the past 2 days.

I am building an app that uses ActionBar, ViewPager & FragmentPagerAdapter. The code for

Answer 1

Surprisingly the ViewPager doesn't do this "natively" (among other things). But not all is lost.

First you have to modify your fragments so they only run the animation when you tell them that it's ok and not when they are instantiated. This way you can play with the viewpager offset (default = 3) and have 2-3 fragments preloaded but not animated.

Second step is to create an interface or similar that defines when the "fragment has become visible".

Third step would be to attach a new OnPageScrollListener to your viewpager.

Code follows (in semi-untested-code):

1) Attach the Listener:

mViewPager.setOnPageChangeListener(new ViewPager.OnPageChangeListener() {
            @Override
            public void onPageScrolled(final int i, final float v, final int i2) {
            }
            @Override
            public void onPageSelected(final int i) {
                YourFragmentInterface fragment = (YourFragmentInterface) mPagerAdapter.instantiateItem(mViewPager, i);
                if (fragment != null) {
                    fragment.fragmentBecameVisible();
                } 
            }
            @Override
            public void onPageScrollStateChanged(final int i) {
            }
        });

2) This is your Interface:

public interface YourFragmentInterface {
    void fragmentBecameVisible();
}

3) Change your fragments so they implement this:

public class YourLovelyFragment extends Fragment implements YourFragmentInterface {

4) Implement the interface in the fragment

@Override
public void fragmentBecameVisible() {
    // You can do your animation here because we are visible! (make sure onViewCreated has been called too and the Layout has been laid. Source for another question but you get the idea.
}

Where to go from here?

You might want to implement a method/listener to notify the "other" fragments that they are no longer visible (i.e. when one is visible, the others are not). But that may not be needed.