Passing a value from Activity to Fragment in Kotlin

Question

I created a bottom navigation activity in my project, which contains one activity and two fragments. In Main Activity I have value stored in a variable but if I pass the value to the fragments then I am getting NullPointer Exception error. I am using kotlin in my project and any help is appreciated. Expectation

Get Value into Fragment from MainActivity. MainActivity--->TestOneFragment

Language Used

Kotlin

Main Activity

class Test : AppCompatActivity(), BottomNavigationView.OnNavigationItemSelectedListener
{
    private val KEY_POSITION = "keyPosition"
    private var navPosition: BottomNavigationPosition = BottomNavigationPosition.ONE
    private lateinit var toolbar: Toolbar
    private lateinit var bottomNavigation: BottomNavigationView

    override fun onCreate(savedInstanceState: Bundle?)
    {
        super.onCreate(savedInstanceState)
        restoreSaveInstanceState(savedInstanceState)
        setContentView(R.layout.activity_test)
        toolbar = findViewById(R.id.toolbar)
        bottomNavigation = findViewById(R.id.bottom_navigation)
        setSupportActionBar(toolbar)
        initBottomNavigation()
        initFragment(savedInstanceState)
        var Name:String=intent.getStringExtra("name")
        println("Test CLLicked: $Name")


        //This code is to pass the value to Fragment
        var bundle=Bundle()
        bundle.putString("name",Name)
        var frag=TestFragment()
        frag.arguments=bundle
    }

    override fun onSaveInstanceState(outState: Bundle?)
    {
        outState?.putInt(KEY_POSITION, navPosition.id)
        super.onSaveInstanceState(outState)
    }

    override fun onNavigationItemSelected(item: MenuItem): Boolean
    {
        navPosition = findNavigationPositionById(item.itemId)
        return switchFragment(navPosition)
    }

    private fun restoreSaveInstanceState(savedInstanceState: Bundle?)
    {
        savedInstanceState?.also {
            val id = it.getInt(KEY_POSITION, BottomNavigationPosition.ONE.id)
            navPosition = findNavigationPositionById(id)
        }
    }

    private fun initBottomNavigation()
    {
        bottomNavigation.active(navPosition.position)
        bottomNavigation.setOnNavigationItemSelectedListener(this)
    }

    private fun initFragment(savedInstanceState: Bundle?)
    {
        savedInstanceState ?: switchFragment(BottomNavigationPosition.ONE)
    private fun switchFragment(navPosition: BottomNavigationPosition): Boolean {
        return supportFragmentManager.findFragment(navPosition).let {
            if (it.isAdded) return false
            supportFragmentManager.detach() // Extension function
            supportFragmentManager.attach(it, navPosition.getTag()) // Extension function
            supportFragmentManager.executePendingTransactions()
        }
    }
    private fun FragmentManager.findFragment(position: BottomNavigationPosition): Fragment
    {
        return findFragmentByTag(position.getTag()) ?: position.createFragment()
    }
}

TestOneFragment

 override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?,savedInstanceState: Bundle?): View? {
    val testName= arguments!!.getString("name")
....
}

Error

kotlin.KotlinNullPointerException
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2778)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2856)
        at android.app.ActivityThread.-wrap11(Unknown Source:0)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1589)

Solution

Here is an example of the newInstance pattern for creating Fragments.

This is within a companion object, which is pretty much just a way to say "these things are Static."

First, you should define constants for your Bundle names, this will help keep everything aligned. Next, define a newInstance method that takes your parameters, such as the name.

And within there, you will create your Fragment and return it. This way, your Activity doesn't have to worry about the Bundle or anything. All your logic is within one place, for storing/retrieving, all within your Fragment.

class TestOneFragment {

    companion object {
        const val ARG_NAME = "name"


        fun newInstance(name: String): TestOneFragment {
            val fragment = TestOneFragment()

            val bundle = Bundle().apply {
                putString(ARG_NAME, name)
            }

            fragment.arguments = bundle

            return fragment
        }
    }

    override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle?): View? {
        val name = arguments?.getString(ARG_NAME)
        // ...
    }
}

And now, you can easily get your Fragment by doing the following.

class Test : AppCompatActivity(), BottomNavigationView.OnNavigationItemSelectedListener {

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

        val name = intent.getStringExtra("name")

        // Creating the new Fragment with the name passed in.
        val fragment = TestFragment.newInstance(name)
    }

}

Hopefully that helps!

Comments

[Shubham AgaRwal]

You can share proper logs like where you are getting NPE and try to add more details of your code like in which function you are passing/accessing the data etc.

[Adrian Coman]

when are you calling arguments!!.getString("name")? As well, try to avoid force unwrapping (using !!) in your code.

[Admin]

@AdrianComan thanks for your comment. If I do then I am getting null in my variable output. val testName= arguments?.getString("name")

[Adrian Coman]

correct, because arguments is null, but when or where in the code are you calling it? can you post a sample of your fragment class?

[Admin]

I am calling this inside onCreateView function.

[Admin]

Thanks for your comment. Can you explain little bit more about MyActivity code. I am not sure where to write that piece of code like either in activity or in fragment

[advice]

@JoshJo - The MyActivity code goes into whatever Activity you're in. Say you're on your MainActivity and you want to show your TestOneFragment, your MainActivity can call the TestOneFragment.newInstance("name"), get the Fragment all prepared, and just display it however you want to.

[advice]

If you edit your question to include how you're going to display your Fragment in your MainActivty, I'll edit my answer to be more specific.

[Admin]

I am looking otherway round. I need to get Activity Value into Fragment. MainActivity---->TestOneFragment

[advice]

Yes, I understand. MainActivty has some data, and you want to create a new Fragment with that data. Right?

[advice]

@JoshJo Okay, edit your question to include more of your MainActivity code, to include what you want to do with the new Fragment, and I'll edit my answer to help more.

[advice]

@JoshJo, Ok, I updated my code to kind of use your code for an example of how you'd use that in your actual code. Hopefully that helps.

[Admin]

still getting null value after the above changes.

[advice]

@JoshJo I think it may be related to how you're using the Fragment. In the MainActivity class, you're not actually using that instance. You're just creating it, and ignoring it. How are you using your Fragments? Are you creating them some other way?

[advice]

Add the entire callstack, it should point to the exact line that has a KotlinNullPointerException.

[Zorayr]

What if I am using NavigationHost which then automatically creates the Fragments?