I have a ViewPager that loads three pages at a time. If I swipe from page 1 to page 2 then to 3, the first page(fragment) goes to onPause()
. Then, if I swipe to the second page, 1st page comes to onResume()
even though the page 1 is still not visible to the user. So, my question is: how to distinguish between the first and second page in code? For example, if I have to run a piece of code when the fragment is visible, how is that done?
ViewPager is a layout manager that allows the user to flip left and right through pages of data. It is mostly found in apps like Youtube, Snapchat where the user shifts right – left to switch to a screen. Instead of using activities fragments are used.
Each Fragment instance has its own lifecycle. When a user navigates and interacts with your app, your fragments transition through various states in their lifecycle as they are added, removed, and enter or exit the screen.
onCreate is called on initial creation of the fragment. You do your non graphical initializations here. It finishes even before the layout is inflated and the fragment is visible. onCreateView is called to inflate the layout of the fragment i.e graphical initialization usually takes place here.
The FragmentPagerAdapter keeps additional fragments, besides the one shown, in resumed state. The solution is to implement a custom OnPageChangeListener and create a new method for when the fragment is shown.
1) Create LifecycleManager Interface The interface will have two methods and each ViewPager’s Fragment will implement it. These methods Are as follows:
public interface FragmentLifecycle { public void onPauseFragment(); public void onResumeFragment(); }
2) Let each Fragment implement the interface Add iplements statement for each class declaration:
public class FragmentBlue extends Fragment implements FragmentLifecycle public class FragmentGreen extends Fragment implements FragmentLifecycle public class FragmentPink extends Fragment implements FragmentLifecycle
3) Implement interface methods in each fragment In order to check that it really works as expected, I will just log the method call and show Toast:
@Override public void onPauseFragment() { Log.i(TAG, "onPauseFragment()"); Toast.makeText(getActivity(), "onPauseFragment():" + TAG, Toast.LENGTH_SHORT).show(); } @Override public void onResumeFragment() { Log.i(TAG, "onResumeFragment()"); Toast.makeText(getActivity(), "onResumeFragment():" + TAG, Toast.LENGTH_SHORT).show(); }
4) Call interface methods on ViewPager page change You can set OnPageChangeListener on ViewPager and get callback each time when ViewPager shows another page:
pager.setOnPageChangeListener(pageChangeListener);
5) Implement OnPageChangeListener to call your custom Lifecycle methods
Listener knows the new position and can call the interface method on new Fragment with the help of PagerAdapter. I can here call onResumeFragment() for new fragment and onPauseFragment() on the current one.
I need to store also the current fragment’s position (initially the current position is equal to 0), since I don’t know whether the user scrolled from left to right or from right to left. See what I mean in code:
private OnPageChangeListener pageChangeListener = new OnPageChangeListener() { int currentPosition = 0; @Override public void onPageSelected(int newPosition) { FragmentLifecycle fragmentToShow = (FragmentLifecycle)pageAdapter.getItem(newPosition); fragmentToShow.onResumeFragment(); FragmentLifecycle fragmentToHide = (FragmentLifecycle)pageAdapter.getItem(currentPosition); fragmentToHide.onPauseFragment(); currentPosition = newPosition; } @Override public void onPageScrolled(int arg0, float arg1, int arg2) { } public void onPageScrollStateChanged(int arg0) { } };
I didn't write the code. Full tutorial here: http://looksok.wordpress.com/2013/11/02/viewpager-with-detailed-fragment-lifecycle-onresumefragment-including-source-code/
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