How do you replace the current top item in the backstack? about simple-stack HOT 6 CLOSED

The original Flow did have a replaceTop() operator but for whatever reason I never had a need for it even though we've used it in multiple projects, so it's not a "first-class" method in simple-stack.

But the following would do exactly that:

Backstack backstack = backstackDelegate.getBackstack(); // lifecycle integration
backstack.setHistory(HistoryBuilder.from(backstack)
                                    .removeLast()
                                    .add([key of fragmentA(new)])
                                    .build(), 
                                    StateChange.REPLACE);

You might also find removeUntil() helpful on the HistoryBuilder.

Then again, this use-case:

fragmentB, finishes or closes and passes data to another instance of fragmentA

fragmentA(old) will be closed and replaced with fragmentA(new) to imitate that the view has been updated through the use of fragmentB

I did run into this and unless you want to update the arguments bundle, you can use a MessageQueue of sorts as you can see in the mvvm-sample which you can see is used here

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Thank you very much! I'll try the MessageQueue solution when I have more time on my hands

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Sorry another question, how do I get the currently displayed fragment using BackStack?

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uh, that is the responsibility of the StateChanger. Which in my case tends to be the Activity itself.

So to make the Backstack show a given fragment, what you need to do is handle the transition between the previous state and the new state, in this case using the FragmentTransaction's API. Without calling addToBackStack() on it, because that changes fragment behavior.

For example, if you only want to show the top fragment, then it is a good choice to detach the ones that are not visible, attach the top, and add it if it's not even in the fragment manager yet.

        for(Key oldKey : stateChange.<Key>getPreviousState()) {
            Fragment fragment = fragmentManager.findFragmentByTag(oldKey.getFragmentTag());
            if(fragment != null) { // already added
                if(!stateChange.getNewState().contains(oldKey)) {
                    fragmentTransaction.remove(fragment); // no longer exists
                } else if(!fragment.isDetached()) {
                    fragmentTransaction.detach(fragment); // it will not be the new top so it can be detached
                }
            }
        }

and

        for(Key newKey : stateChange.<Key>getNewState()) {
            Fragment fragment = fragmentManager.findFragmentByTag(newKey.getFragmentTag());
            if(newKey.equals(stateChange.topNewState())) { // if is top
                if(fragment != null) {
                    if(fragment.isDetached()) {
                        fragmentTransaction.attach(fragment); // then if exists and is detached, attach
                    }
                } else {
                    fragment = newKey.newFragment(); // is not added, so add it
                    fragmentTransaction.add(containerId, fragment, newKey.getFragmentTag());
                }
            } else {
                if(fragment != null && !fragment.isDetached()) { // not top - should be detached
                    fragmentTransaction.detach(fragment);
                }
            }
        }

It might seem a bit clunky at first but it's actually surprisingly easy to customize, we had a bit more custom behavior to it to define what should be added, hidden, attached, etc.

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Thank you very much for your help

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@dev-JE02 I've just checked this question again and I think I might have answered the wrong question?

The answer to checking the current top is just backstack.top(), then you can access the fragment tag and use findFragmentByTag() to get the fragment.

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