片段Backstack

Question

Hi I have an activity and three fragments. When I replace my first fragment I am adding it to backstack.

FragmentTransaction fragmentTransaction = getFragmentManager()
            .beginTransaction();
fragmentTransaction.replace(R.id.authentication_parent0_linear,
                new LoginFragment(), LOGINTAG);
fragmentTransaction.addToBackStack(null);
    fragmentTransaction.commit();

But when I move between the other two fragments I am not adding them to backstack.So whenever I press back from the other two fragments I need to go back to first fragment.It works fine when I remove first fragment and add second fragment or remove first and add third ie when I press back I am coming back to first fragment. But when I navigate between those two fragments that is remove second and add third or remove third and add second etc . At that instance when I press back I am exiting the app and backstackcount is shown as zero but for the first time when I removed the first fragment I stored it in the backstack.

getFragmentManager()
            .beginTransaction()
            .replace(R.id.authentication_parent0_linear,
                    new SignupFragment(), SIGNUPTAG).commit();

So my question is will the fragment backstack resets between commits. If so how can I get the stored fragment in the backstack for the first time

Answer 1

Best way to achieve what you want is not to use fragment's backstack.

1. Do not use addToBackStack() anywhere

2. Keep your first fragment LoginFragment reference in the instance field mLoginFragment. Addition of your first fragment will look like this (probably in activity's onCreate() :

     FragmentTransaction fragmentTransaction = getFragmentManager().beginTransaction(); mLoginFragment = new LoginFragment(); fragmentTransaction.replace(R.id.authentication_parent0_linear, mLoginFragment, LOGINTAG); fragmentTransaction.commit(); 

3. in your activity override onBackPressed() method as following:

    @Override public void onBackPressed() { Fragment fr = getFragmentManager().findFragmentById(R.id.authentication_parent0_linear); // chek that mLoginFragment!=null, basically this should never happen! if (fr == mLoginFragment) { super.onBackPressed(); } else { FragmentTransaction fragmentTransaction = getFragmentManager().beginTransaction(); fragmentTransaction.replace(R.id.authentication_parent0_linear, mLoginFragment, LOGINTAG); fragmentTransaction.commit(); } } 

There is a cumbersome way to achieve the desired effect with the use of backstack. Here is how:

1. do not call addToBAckStack() when creating fragment A
2. When navigating from fragment A to B or to C, or to other fragments call addToBackStack()

3. When navigating deeper from B to C, etc. call popBackStack() before the transaction. Explanation of it can be found on SO. Like this:

    getFragmentManager().popBackStack(); FragmentTransaction fragmentTransaction = getFragmentManager().beginTransaction(); fragmentTransaction.replace(R.id.authentication_parent0_linear, new FragmentC(), "whatever"); fragmentTransaction.addToBackStack(null); fragmentTransaction.commit(); 

Answer 2

The Fragment manager allows you to put a Name (or TAG) on a given state. If you pass that same name to popBackStack it will pop states until it reaches that state. For example

// Add initial fragment
getFragmentManager()
    .beginTransaction();
    .replace(R.id.authentication_parent0_linear, new LoginFragment());
    .addToBackStack(TAG_FIRST_FRAGMENT_STATE) // The key is not passing null here
    .commit();

// Add other fragments like such
getFragmentManager()
    .beginTransaction();
    .replace(R.id.authentication_parent0_linear, new SignupFragment());
    .addToBackStack(null)
    .commit();


// On back press
@Override
public void onBackPressed() {
    FragmentManager fragmentManager = getFragmentManager();
    if (fragmentManager.getBackStackEntryCount() > 1) {
        fragmentManager.popBackStack(TAG_FIRST_FRAGMENT_STATE, 0);
    } else {
        super.onBackPress();
    }
}

Answer 3

I fnally figured it out. based on Kirill K answer

    public class MainActivity extends Activity implements OnClickAuthentication,
    LoginInterface, SignupInterface {

FragmentManager fragmentManager;
FragmentTransaction fragmentTransaction;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    fragmentManager = getFragmentManager();
    Fragment fragment = fragmentManager.findFragmentByTag("one");
    if (fragment == null) {
        fragmentTransaction = fragmentManager.beginTransaction();
        fragmentTransaction
                .replace(R.id.relative, new OneFragment(), "one");
        fragmentTransaction.commit();
    }
}

@Override
public void onClickAuthButton(int flag) {
    if (flag == 2) {
        fragmentTransaction = fragmentManager.beginTransaction();
        fragmentTransaction
                .replace(R.id.relative, new TwoFragment(), "two");
        fragmentTransaction.commit();
    } else {
        fragmentTransaction = fragmentManager.beginTransaction();
        fragmentTransaction.replace(R.id.relative, new ThreeFragment(),
                "three");
        fragmentTransaction.commit();

    }

}

@Override
public void switchToSignup() {
    fragmentTransaction = fragmentManager.beginTransaction();
    fragmentTransaction
            .replace(R.id.relative, new ThreeFragment(), "three");
    fragmentTransaction.commit();

}

@Override
public void switchToLogin() {
    fragmentTransaction = fragmentManager.beginTransaction();
    fragmentTransaction.replace(R.id.relative, new TwoFragment(), "two");
    fragmentTransaction.commit();

}

@Override
public void onBackPressed() {
    Fragment twoFragment = fragmentManager.findFragmentByTag("two");
    Fragment threeFragment = fragmentManager.findFragmentByTag("three");
    if ((twoFragment != null && twoFragment.isVisible())
            || (threeFragment != null && threeFragment.isVisible())) {
        fragmentTransaction = fragmentManager.beginTransaction();
        fragmentTransaction
                .replace(R.id.relative, new OneFragment(), "one");
        fragmentTransaction.commit();
    } else {
        super.onBackPressed();
    }
}

}

Answer 4

I have done it like this

1. Globally declare fragmentManager variable :

FragmentManager fm = getFragmentManager();

2.onCreate method load your 1st fragment without using backstack

fm.beginTransaction().replace(R.id.content_frame , new main_fragment()).commit();

3. now when you will add new fragment then create a method which will check if the fragment is already in backstack or not and also it will handle clicking the same fragment events(if you are using frag in navigation drawer )

void replaceFragment (Fragment fragment) {
    Fragment current = fm.findFragmentById(R.id.content_frame);
    //Check if that frag is already opened
    if (!current.getClass().equals(fragment.getClass())) {


        String backStateName = fragment.getClass().getName();

        boolean fragmentPopped = fm.popBackStackImmediate(backStateName, 0);

        if (!fragmentPopped) {

            //fragment is not in back stack,so add it.
            FragmentTransaction ft = fm.beginTransaction();
            ft.replace(R.id.content_frame, fragment);
            ft.addToBackStack(backStateName);
            ft.commit();

        }
    }
    else {
        //if fragmentPopped is true then it will auto pop up the fragment
        //so we dnt have to do anything here     
    }
}

4. now just pass your fragment class to

replaceFragment(new yourfragment());

5. now handle backpress

public void onBackPressed() {

    int backStackEntryCount = fm.getBackStackEntryCount();


       if (backStackEntryCount == 0) {
            new AlertDialog.Builder(this)
                    .setIcon(android.R.drawable.ic_dialog_alert)
                    .setTitle("Closing Activity")
                    .setMessage("Are you sure you want to close this Application?")
                    .setPositiveButton("Yes", new DialogInterface.OnClickListener() {
                        @Override
                        public void onClick(DialogInterface dialog, int which) {
                            finish();
                            System.exit(0);
                        }

                    })
                    .setNegativeButton("No", null)
                    .setOnCancelListener(new DialogInterface.OnCancelListener() {
                        @Override
                        public void onCancel(DialogInterface dialogInterface) {
                               //backpress over alert dialog
                                finish();
                                System.exit(0);
                        }
                    })
                    .show();

        } else {


            super.onBackPressed();
        }


    }

hope this will help you