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Question

I'm currently developing an android app and I've encountered a trouble that I'm not able to resolve myself.

Introduction

I've created a new project and picked "Tabbed Activity" as a template for the project. As you may know, the project created with this template, has 2 .xml files: activity_main.xml (that contains AppBarLayout and ViewPager2 ) and fragment_main.xml . It also has MainActivity.kt set up to make tabs work, and ui.main package with 3 .kt files in it that are responsible for displaying tabs content eg "Fragment #1" etc.

What do I want to have

I need the application to have top action bar with title, logo and tabs navigation. In total, I need to have 3 different tabs (fragments) with their own layout and logic.

What did I do and what happend

So, I've customized the activity_main.xml layout, then created a new layout fragment_dashboard.xml for the one of the fragments that I want to have in the application.
I've deleted auto generated code and wrote my own. Since I'm mostly like as a beginner in android development, I've used google to learn how to bind tabs , fragments and main activity together. I found several articles that I considered suitable for me.

After I finished the code, I wanted to check how my customized action bar with tabs and the half-finished fragment_dashboard.xml layout look together.

So I tried to run the app and encoutered the problem: when app starts in the emulated phone there is just a white screen and nothing else... (before deleting the auto-generated code for tabs it run without any problems)

What I tried to do

First of all I tried to debug MainActivity.kt . I put a breakpoint at the first line of the function onCreate() :

super.onCreate(savedInstanceState, persistentState)

But when I run app in debug mode, debuger does not stop at this breakpoint. Thus, I came to a conclusion: execution does not even get to the function onCreate() .

So, the question is: what am I doing wrong and how can I fix it to be able to see tabs and their fragments?

Code

DashboardFragment.kt

class DashboardFragment() : Fragment() {
    override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle?): View? {
        return inflater.inflate(R.layout.fragment_dashboard, container, false);
    }

    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        Log.i("DashboardFragment","onViewCreated")
        super.onViewCreated(view, savedInstanceState)
    }
}

ViewPagerFragmentStateAdapter.kt

class ViewPagerFragmentStateAdapter(fa: FragmentActivity) : FragmentStateAdapter(fa) {
    var positionToPageName = mapOf(
        0 to "Dashboard"
    )

    private var _pageNameToFragment = mapOf<String, Fragment>(
        "Dashboard" to DashboardFragment()
    )


    override fun getItemCount(): Int = _pageNameToFragment.size

    override fun createFragment(position: Int): Fragment {
        val pageName = positionToPageName[position]
        val page = _pageNameToFragment[pageName]

        return page ?: DashboardFragment() as Fragment
    }
}

ViewPagerFragmentStateAdapter.kt

class MainActivity : AppCompatActivity() {
    private lateinit var tabLayout: TabLayout
    private lateinit var viewPager: ViewPager2

    override fun onCreate(savedInstanceState: Bundle?, persistentState: PersistableBundle?) {
        super.onCreate(savedInstanceState, persistentState)

        setContentView(R.layout.activity_main)

        val adapter = ViewPagerFragmentStateAdapter(this)

        viewPager = findViewById(R.id.view_pager)
        viewPager.adapter = adapter

        tabLayout = findViewById(R.id.tabs)
        TabLayoutMediator(tabLayout, viewPager) {
            tab, position -> tab.text = adapter.positionToPageName[position]
        }.attach()
    }
}

Answer 1

I've tried to change some things in the MainActivity.kt . It seems that the problem was just in the definition of the onCreate() function:

override fun onCreate(savedInstanceState: Bundle?, persistentState: PersistableBundle?)   {
    super.onCreate(savedInstanceState, persistentState)

I've created a test project with "Tabbed Activity" template again and noticed that in the new project the persistentState argument is missing. So, I just removed it from definition in my project and the project started working well.

Now the definition of the onCreate() looks like this:

override fun onCreate(savedInstanceState: Bundle?)   {
    super.onCreate(savedInstanceState)