Get Backstack status using android navigation component

问题

I want to implement backpress behaviour such that it prompts a confirmation popup when you press back with the backstack empty, otherwise it pops the next fragment in the stack.

I'm trying to get the backstack count, but i always get 0 from both fragment managers

 getSupportFragmentManager().getBackStackEntryCount();
 getFragmentManager().getBackStackEntryCount();

I guess it should work since i checked the google code of fragment navigator and it adds to backstack through the canonical fragment transaction:

FragmentNavigator.java:

 if (initialNavigation || isClearTask) {
        backStackEffect = BACK_STACK_DESTINATION_ADDED;
    } else if (isSingleTopReplacement) {
        // Single Top means we only want one instance on the back stack
        if (mBackStack.size() > 1) {
            // If the Fragment to be replaced is on the FragmentManager's
            // back stack, a simple replace() isn't enough so we
            // remove it from the back stack and put our replacement
            // on the back stack in its place
            mFragmentManager.popBackStack();
            ft.addToBackStack(Integer.toString(destId));
            mIsPendingBackStackOperation = true;
        }
        backStackEffect = BACK_STACK_UNCHANGED;
    } else {
        ft.addToBackStack(Integer.toString(destId));
        mIsPendingBackStackOperation = true;
        backStackEffect = BACK_STACK_DESTINATION_ADDED;
    }
    ft.setReorderingAllowed(true);
    ft.commit();

I didn't find any API to retrieve this information through NavController or Navigator neither.

Thanks

回答1:

May be help to someone too:

<fragment
                android:id="@+id/YOUR_NAVIGATION_HOST_ID"
                android:name="androidx.navigation.fragment.NavHostFragment"
                android:layout_width="match_parent"
                android:layout_height="match_parent"
                app:defaultNavHost="true"
                app:navGraph="@navigation/YOUR_NAVIGATION_HOST" />

val navHostFragment = supportFragmentManager.findFragmentById(R.id.YOUR_NAVIGATION_HOST_ID)
val backStackEntryCount = navHostFragment?.childFragmentManager?.backStackEntryCount

回答2:

I understand that the answer is late. But may be it will help someone else.

val navHostFragment = supportFragmentManager.findFragmentById(R.id.navDrawerActivity)
val backStackEntryCount = navHostFragment?.childFragmentManager?.fragments?.size

<fragment
    android:id="@+id/navDrawerActivity"
    android:name="androidx.navigation.fragment.NavHostFragment"
    app:navGraph="@navigation/nav_item"
    app:defaultNavHost="true"
/>

回答3:

using addToBackStack() method during transaction you can solve your problem,

FragmentTransaction tx = fragmentManager.beginTransation();
tx.replace( R.id.fragment, new MyFragment() ).addToBackStack( "tag" ).commit();


//You need to add the following line for this solution to work; 
fragment.getView().setFocusableInTouchMode(true);
fragment.getView().requestFocus();
fragment.getView().setOnKeyListener( new OnKeyListener(){
@Override
public boolean onKey( View v, int keyCode, KeyEvent event )
  {
    if( keyCode == KeyEvent.KEYCODE_BACK )
    {
        return true;
    }
    return false;
   }
 } );

来源：https://stackoverflow.com/questions/52481786/get-backstack-status-using-android-navigation-component