reusing fragments in a fragmentpageradapter
Solution 1:
The FragmentPagerAdapter
already caches the Fragments
for you. Each fragment is assigned a tag, and then the FragmentPagerAdapter
tries to call findFragmentByTag
. It only calls getItem
if the result from findFragmentByTag
is null
. So you shouldn't have to cache the fragments yourself.
Solution 2:
Appendix for Geoff's post:
You can get reference to your Fragment
in FragmentPagerAdapter
using findFragmentByTag()
. The name of the tag is generated this way:
private static String makeFragmentName(int viewId, int index)
{
return "android:switcher:" + viewId + ":" + index;
}
where viewId is id of ViewPager
Look at this link: http://code.google.com/p/openintents/source/browse/trunk/compatibility/AndroidSupportV2/src/android/support/v2/app/FragmentPagerAdapter.java#104
Solution 3:
Seems a lot of the people viewing this question are looking for a way to reference the Fragments
created by FragmentPagerAdapter
/FragmentStatePagerAdapter
. I would like to offer my solution to this without relying on the internally created tags
that the other answers on here use.
As a bonus this method should also work with FragmentStatePagerAdapter
. See notes below for more detail.
Problem with current solutions: relying on internal code
A lot of the solutions I've seen on this and similar questions rely on getting a reference to the existing Fragment
by calling FragmentManager.findFragmentByTag()
and mimicking the internally created tag: "android:switcher:" + viewId + ":" + id
. The problem with this is that you're relying on internal source code, which as we all know is not guaranteed to remain the same forever. The Android engineers at Google could easily decide to change the tag
structure which would break your code leaving you unable to find a reference to the existing Fragments
.
Alternate solution without relying on internal tag
Here's a simple example of how to get a reference to the Fragments
returned by FragmentPagerAdapter
that doesn't rely on the internal tags
set on the Fragments
. The key is to override instantiateItem()
and save references in there instead of in getItem()
.
public class SomeActivity extends Activity {
private FragmentA m1stFragment;
private FragmentB m2ndFragment;
// other code in your Activity...
private class CustomPagerAdapter extends FragmentPagerAdapter {
// other code in your custom FragmentPagerAdapter...
public CustomPagerAdapter(FragmentManager fm) {
super(fm);
}
@Override
public Fragment getItem(int position) {
// Do NOT try to save references to the Fragments in getItem(),
// because getItem() is not always called. If the Fragment
// was already created then it will be retrieved from the FragmentManger
// and not here (i.e. getItem() won't be called again).
switch (position) {
case 0:
return new FragmentA();
case 1:
return new FragmentB();
default:
// This should never happen. Always account for each position above
return null;
}
}
// Here we can finally safely save a reference to the created
// Fragment, no matter where it came from (either getItem() or
// FragmentManger). Simply save the returned Fragment from
// super.instantiateItem() into an appropriate reference depending
// on the ViewPager position.
@Override
public Object instantiateItem(ViewGroup container, int position) {
Fragment createdFragment = (Fragment) super.instantiateItem(container, position);
// save the appropriate reference depending on position
switch (position) {
case 0:
m1stFragment = (FragmentA) createdFragment;
break;
case 1:
m2ndFragment = (FragmentB) createdFragment;
break;
}
return createdFragment;
}
}
public void someMethod() {
// do work on the referenced Fragments, but first check if they
// even exist yet, otherwise you'll get an NPE.
if (m1stFragment != null) {
// m1stFragment.doWork();
}
if (m2ndFragment != null) {
// m2ndFragment.doSomeWorkToo();
}
}
}
or if you prefer to work with tags
instead of class member variables/references to the Fragments
you can also grab the tags
set by FragmentPagerAdapter
in the same manner:
NOTE: this doesn't apply to FragmentStatePagerAdapter
since it doesn't set tags
when creating its Fragments
.
@Override
public Object instantiateItem(ViewGroup container, int position) {
Fragment createdFragment = (Fragment) super.instantiateItem(container, position);
// get the tags set by FragmentPagerAdapter
switch (position) {
case 0:
String firstTag = createdFragment.getTag();
break;
case 1:
String secondTag = createdFragment.getTag();
break;
}
// ... save the tags somewhere so you can reference them later
return createdFragment;
}
Note that this method does NOT rely on mimicking the internal tag
set by FragmentPagerAdapter
and instead uses proper APIs for retrieving them. This way even if the tag
changes in future versions of the SupportLibrary
you'll still be safe.
Don't forget that depending on the design of your Activity
, the Fragments
you're trying to work on may or may not exist yet, so you have to account for that by doing null
checks before using your references.
Also, if instead you're working with FragmentStatePagerAdapter
, then you don't want to keep hard references to your Fragments
because you might have many of them and hard references would unnecessarily keep them in memory. Instead save the Fragment
references in WeakReference
variables instead of standard ones. Like this:
WeakReference<Fragment> m1stFragment = new WeakReference<Fragment>(createdFragment);
// ...and access them like so
Fragment firstFragment = m1stFragment.get();
if (firstFragment != null) {
// reference hasn't been cleared yet; do work...
}
Solution 4:
If the fragment still in memory you can find it with this function.
public Fragment findFragmentByPosition(int position) {
FragmentPagerAdapter fragmentPagerAdapter = getFragmentPagerAdapter();
return getSupportFragmentManager().findFragmentByTag(
"android:switcher:" + getViewPager().getId() + ":"
+ fragmentPagerAdapter.getItemId(position));
}
Sample code for v4 support api.