How to determine fragment restored from backstack

无人久伴 提交于 2019-11-28 16:29:44

I think that most simple way is do this for example in onViewCreated() method:

if (savedInstanceState == null && !mAlreadyLoaded) {
    mAlreadyLoaded = true;

    // Do this code only first time, not after rotation or reuse fragment from backstack
}

Because when android put fragment on backstack, it only destroy its view, but don't kill instance itself, so mAlreadyLoaded will be still true when fragment will be restored from backstack.

getSupportFragmentManager().addOnBackStackChangedListener(new OnBackStackChangedListener() {    
            public void onBackStackChanged() {
                Log.i(TAG, "back stack changed ");
                int backCount = getSupportFragmentManager().getBackStackEntryCount();
                if (backCount == 0){
                                   // block where back has been pressed. since backstack is zero.
                }
            }
        });

use this addOnBackStackChangedListener.

When a fragment goes to back-stack onDestroyView() called. Not onDestroy().

And when a fragment pops from back-stack onCreateView() called. Not onCreate().

So add a boolean mIsRestoredFromBackstack to fragment and follow as below:

@Override
public void onCreate(Bundle savedInstanceState)
{
    super.onCreate(savedInstanceState);
    mIsRestoredFromBackstack = false;
}

@Override
public void onResume()
{
    super.onResume();
    if(mIsRestoredFromBackstack)
    {
        // The fragment restored from backstack, do some work here!
    }
}

@Override
public void onDestroyView()
{
    super.onDestroyView();
    mIsRestoredFromBackstack = true;
}

MAJOR EDIT: Oct 15 2013

The previous explanation (kept below for reference) fails when the application is put to the background and brought back to the foreground.

Instead, it is better to compare the current size of the backstack with the one when the fragment was created & put into the backstack.


Take a good look at Figure 2 in http://developer.android.com/guide/components/fragments.html#Creating

What this figure tells you is that when a fragment is restored from the backstack, its onCreate() is not called, while its onCreateView() is.


So, you may want to do something like this:

public class MyFragment extends Fragment {
    int mBackStackSize = 0;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        mBackStackSize = getFragmentManager().getBackStackEntryCount();
    }

    public boolean isRestoredFromBackstack() {
        return mBackStackSize > getFragmentManager().getBackStackEntryCount();
    }
}
Inc

If you added fragment to backstack, and after some manipulation you hide it using fragmentTransaction.hide(fragment) and then restore it from backstack like fragmentTransaction.show(fragmentManager.findFragmentByTag(fragment.getName())); you can override onHiddenChanged(boolean hidden)

@Override
public void onHiddenChanged(boolean hidden) {
    // TODO Auto-generated method stub
    super.onHiddenChanged(hidden);
    if (!hidden) {
        //fragment became visible
        //your code here
    }
}

In some cases you can use isVisible method to understand is it first showing of a fragment or is it restored from the backstack.

Fragment.getFragmentManager() == null 

before added and after popped

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