How can I switch between two fragments, without re

问题:

I'm working on an android application, that uses a navigation drawer to switch between two fragments. However, each time I switch, the fragment is completely recreated.

Here is the code from my main activity.

/* The click listener for ListView in the navigation drawer */
private class DrawerItemClickListener implements ListView.OnItemClickListener {
    @Override
    public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
        selectItem(position);
    }
}

private void selectItem(int position) {
    android.support.v4.app.Fragment fragment;
    String tag;
    android.support.v4.app.FragmentManager; fragmentManager = getSupportFragmentManager();

    switch(position) {
        case 0:
            if(fragmentManager.findFragmentByTag("one") != null) {
                fragment = fragmentManager.findFragmentByTag("one");
            } else {
                fragment = new OneFragment();
            }
            tag = "one";
            break;
        case 1:
            if(fragmentManager.findFragmentByTag("two") != null) {
                fragment = fragmentManager.findFragmentByTag("two");
            } else {
                fragment = new TwoFragment();
            }
            tag = "two";
            break;
    }

    fragment.setRetainInstance(true);
    fragmentManager.beginTransaction().replace(R.id.container, fragment, tag).commit();

    // update selected item and title, then close the drawer
    mDrawerList.setItemChecked(position, true);
    setTitle(mNavTitles[position]);
    mDrawerLayout.closeDrawer(mDrawerList);
}

I've set up some debug logging, and every time selectItem is called, one fragment is destroyed, while the other is created.

Is there any way to prevent the fragments from being recreated, and just reuse them instead?

回答1:

After @meredrica pointed out that replace() destroys the fragments, I went back through the FragmentManager documentation. This is the solution I've come up with, that seems to be working.

/* The click listener for ListView in the navigation drawer */
private class DrawerItemClickListener implements ListView.OnItemClickListener {
    @Override
    public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
        selectItem(position);
    }
}

private void selectItem(int position) {
    android.support.v4.app.FragmentManager; fragmentManager = getSupportFragmentManager();

    switch(position) {
        case 0:
            if(fragmentManager.findFragmentByTag("one") != null) {
                //if the fragment exists, show it.
                fragmentManager.beginTransaction().show(fragmentManager.findFragmentByTag("one")).commit();
            } else {
                //if the fragment does not exist, add it to fragment manager.
                fragmentManager.beginTransaction().add(R.id.container, new OneFragment(), "one").commit();
            }
            if(fragmentManager.findFragmentByTag("two") != null){
                //if the other fragment is visible, hide it.
                fragmentManager.beginTransaction().hide(fragmentManager.findFragmentByTag("two")).commit();
            }
            break;
        case 1:
            if(fragmentManager.findFragmentByTag("two") != null) {
                //if the fragment exists, show it.
                fragmentManager.beginTransaction().show(fragmentManager.findFragmentByTag("two")).commit();
            } else {
                //if the fragment does not exist, add it to fragment manager.
                fragmentManager.beginTransaction().add(R.id.container, new TwoFragment(), "two").commit();
            }
            if(fragmentManager.findFragmentByTag("one") != null){
                //if the other fragment is visible, hide it.
                fragmentManager.beginTransaction().hide(fragmentManager.findFragmentByTag("one")).commit();
            }
            break;
    }

    // update selected item and title, then close the drawer
    mDrawerList.setItemChecked(position, true);
    setTitle(mNavTitles[position]);
    mDrawerLayout.closeDrawer(mDrawerList);
}

I also added this bit, but I'm not sure if it's necessary or not.

@Override
public void onDestroy() {
    super.onDestroy();
    FragmentManager fragmentManager = getSupportFragmentManager();
    if(fragmentManager.findFragmentByTag("one") != null){
        fragmentManager.beginTransaction().remove(fragmentManager.findFragmentByTag("one")).commit();
    }
    if(fragmentManager.findFragmentByTag("two") != null){
        fragmentManager.beginTransaction().remove(fragmentManager.findFragmentByTag("two")).commit();
    }
}

回答2:

Use the attach/detach method with tags:

Detach will destroy the view hirachy but keeps the state, like if on the backstack; this will let the "not-visible" fragment have a smaller memory footprint. But mind you that you need to correctly implement the fragment lifecycle (which you should do in the first place)

Detach the given fragment from the UI. This is the same state as when it is put on the back stack: the fragment is removed from the UI, however its state is still being actively managed by the fragment manager. When going into this state its view hierarchy is destroyed.

The first time you add the fragment

FragmentTransaction t = getSupportFragmentManager().beginTransaction();
t.add(android.R.id.content, new MyFragment(),MyFragment.class.getSimpleName());
t.commit();

then you detach it

FragmentTransaction t = getSupportFragmentManager().beginTransaction();
t.detach(MyFragment.class.getSimpleName());
t.commit();

and attach it again if switched back, state will be kept

FragmentTransaction t = getSupportFragmentManager().beginTransaction();
t.attach(getSupportFragmentManager().findFragmentByTag(MyFragment.class.getSimpleName()));
t.commit();

But you always have to check if the fragment was added yet, if not then add it, else just attach it:

if (getSupportFragmentManager().findFragmentByTag(MyFragment.class.getSimpleName()) == null) {
    FragmentTransaction t = getSupportFragmentManager().beginTransaction();
    t.add(android.R.id.content, new MyFragment(), MyFragment.class.getSimpleName());
    t.commit();
} else {
    FragmentTransaction t = getSupportFragmentManager().beginTransaction();
    t.attach(getSupportFragmentManager().findFragmentByTag(MyFragment.class.getSimpleName()));
    t.commit();
}

回答3:

The replace method destroys your fragments. One workaround is to set them to Visibility.GONE, another (less easy) method is to hold them in a variable. If you do that, make sure you don't leak memory left and right.

回答4:

I did this before like this:

        if (mPrevFrag != fragment) {

            // Change
            FragmentTransaction ft = fragmentManager.beginTransaction();
            if (mPrevFrag != null){
                ft.hide(mPrevFrag);
            }
            ft.show(fragment);
            ft.commit();
            mPrevFrag = fragment;

        }

(you will need to track your pervious fragment in this solution)

回答5:

I guess you can not directly manipulate the lifecycle mechanisms of your Fragments. The very fact that you can findFragmentByTag is not very bad. It means that the Fragment object is not recreated fully, if it is already commited. The existing Fragment just passes all the lifecycle steps each Fragment has - that means that only UI is "recreated".

It is a very convenient and useful memory management strategy - and appropriate, in most cases. Fragment which is gone, has the resources which have to be utilized in order to de-allocate memory.

If you just cease using this strategy, the memory usage of your application could increase badly.

Nonetheless, there are retained fragments, which lifecycle is a bit different and do not correspond to the Activity they are attached to. Typically, they are used to retain some things you want to save, for example, to manage configuration changes

However, the fragment [re]creation strategy depends on the context - that is, what you would like to solve, and what are the trade-offs that you are willing to accept.

回答6:

Same idea as Tester101 but this is what I ended up using.

    FragmentManager fragmentManager = getSupportFragmentManager();
    FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();

    Fragment oldFragment = fragmentManager.findFragmentByTag( "" + m_lastDrawerSelectPosition );
    if ( oldFragment != null )
        fragmentTransaction.hide( oldFragment );

    Fragment newFragment = fragmentManager.findFragmentByTag( "" + position );
    if ( newFragment == null )
    {
        newFragment = getFragment( position );
        fragmentTransaction.add( R.id.home_content_frame, newFragment, "" + position );
    }

    fragmentTransaction.show( newFragment );
    fragmentTransaction.commit();

回答7:

Just find the current fragment calling getFragmentById("id of your container") and then hide it and show needed fragment.

private void openFragment(Fragment fragment, String tag) {
        FragmentManager fragmentManager = getSupportFragmentManager();
        FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();
        Fragment existingFragment = fragmentManager.findFragmentByTag(tag);
        if (existingFragment != null) {
            Fragment currentFragment = fragmentManager.findFragmentById(R.id.container);
            fragmentTransaction.hide(currentFragment);
            fragmentTransaction.show(existingFragment);
        }
        else {
            fragmentTransaction.add(R.id.container, fragment, tag);
        }
        fragmentTransaction.commit();
    }

回答8:

How about playing with the Visible attribute?

回答9:

this is a little late response. if you're using view pager for fragments, set the off screen page limit of the fragment to the number of fragments created.

mViewPager.setOffscreenPageLimit(3); // number of fragments here is 3