In my app, I have a TopActionBar fragment that is loaded on the MainActivity that loads a MaterialToolbar, along with my navigation drawer. I have a FrameLayout in this fragment that I replace with fragments to navigate between pages. When I replace a fragment (using a function I have defined in a utils.kt file), I am tracking the fragments that are loaded for the first time and adding them to the BackStack so that I can pop them and prevent duplicates of that fragment from being added to the BackStack. Here is the relevant logic for how that is being managed in my Utils.kt file:

fun replaceFragment(destinationFragment : Fragment,
                    currentFragment: String,
                    title : String,
                    initialLaunch: Boolean = false
) {
    val destinationFragmentName = destinationFragment.javaClass.simpleName
    val fragmentTag : Fragment? = fragmentManager.findFragmentByTag(destinationFragmentName)
    if(destinationFragmentName !== currentFragment || initialLaunch) {
        val fragmentTransaction = fragmentManager.beginTransaction()
        // some logic to determine animations depending on the fragment being replaced
        if (fragmentTag == null) {
            fragmentTransaction.replace(R.id.frame_layout, destinationFragment, destinationFragmentName)
            fragmentTransaction.addToBackStack(destinationFragmentName)
        } else { // re-use the old fragment
            fragmentTransaction.replace(R.id.frame_layout, destinationFragment, destinationFragmentName)
        }

        fragmentTransaction.commit()
    }
}

And then this is how I have overwritten the onBackPressed function in my MainActivity:

override fun onBackPressed() {
    if (fragmentManager.backStackEntryCount > 0) {
        fragmentManager.popBackStackImmediate()
    } else {
        super.onBackPressed()
    }
}

A couple of things aren't working properly. Take this example flow of fragments below:

A -> B -> C -> B -> C

When I press back I get this flow:

BC -> AC -> App Close

Here multiple Fragments are being displayed at the same time.

What I expect to happen is:

C -> B -> A -> App Close

Can someone maybe offer some insights into why this is occurring and what I can do to fix this? If I don't conditionally addToBackStack, and just addToBackStack for every single Fragment I replace, it works fine, but I don't want the multiple copies in the BackStack. I need to keep the most recent instance of each Fragment in the BackStack. So in my example:

A -> B -> C -> B -> C

The BackStack would no longer have the first C, just the most recent one.

CodePudding user response：

As per the FragmentManager guide:

When you call addToBackStack() on a transaction, note that the transaction can include any number of operations, such as adding multiple fragments, replacing fragments in multiple containers, and so on. When the back stack is popped, all of these operations are reversed as a single atomic action. If you've committed additional transactions prior to the popBackStack() call, and if you did not use addToBackStack() for the transaction, these operations are not reversed. Therefore, within a single FragmentManager, avoid interleaving transactions that affect the back stack with those that do not.

So what you are experiencing is the operation you did with addToBackStack being reversed (causing your original copy of B to reappear) while not touching the new C you did not use addToBackStack.

The FragmentManager's back stack is just that: a stack. That means you can't remove B from the stack unless it is at the top of the back stack. That means there's no way to remove B from the middle of the stack without using something like the support for multiple back stacks to completely swap between independent back stacks.

If you just want to make sure there is only one copy of B on the top of the stack, you'll want to popBackStack() to remove the topmost if the names are the same before unconditionally using addToBackStack() on your new instance.