View pager和fragment生命周期

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View pager that i created loads three pages at a time. now if i swipe from page 1 to 2 then to 3.when the third page is the current item,the first page(fragment) goes to onPause().now if i swipe to 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?

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.

Solution：

  1. Create LifecycleManager Interface The interface will have two methods and each ViewPager’s Fragment will implement it. These methods Are as follows:
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public interface FragmentLifecycle {
        public void onPauseFragment();
        public void onResumeFragment();
    }
  1. Let each Fragment implement the interface Add iplements statement for each class declaration:
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public class FragmentBlue extends Fragment implements FragmentLifecycle
public class FragmentGreen extends Fragment implements FragmentLifecycle
public class FragmentPink extends Fragment implements FragmentLifecycle
  1. 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:
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@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(); 
}
  1. Call interface methods on ViewPager page change You can set OnPageChangeListener on ViewPager and get callback each time when ViewPager shows another page:
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pager.setOnPageChangeListener(pageChangeListener);

  1. 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() {

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        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) { }

    };