RecyclerView with databinding and onClick listener

I'm starting using Kotlin (i'm a web dev) to maintain the mobile app of my current job. To practice my learning, I'm creating a basic app which is displaying a list of France departments (using a REST Api), and I need to allow the user to click on a list item to get more info on the selected item.
I'm trying to build this with databinding, Koin as dependency injection, and Room as db layer.

My issue is that I created a RecyclerView custom Adapter, and used the databinding to give it the datas. But now I want to implement the onClick behaviour, which should launch another activity to display item details. My problem is: I don't know how to do this in a clean way.

I was thinking about creating a viewModel to link to my Adapter, but can't really find how to do it well. And even if I did, how to start another activity in a viewModel ? (don't have access to the context and startActivity function). So I finally dropped that solution that doesn't seems to fit.
So I'm currently thinking of passing directly from my adapter the onClick function, but can't find a way to bind this function in my xml file. Here is my files:

MainActivity:

class MainActivity : AppCompatActivity() {

    private val mViewModel: DepartmentsViewModel by viewModel()
    private lateinit var binding: ActivityMainBinding

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

        binding = ActivityMainBinding.inflate(layoutInflater)
        setContentView(binding.root)

        binding.toolbar.title = "Liste des départements"

        val adapter = DepartmentListAdaptater()
        binding.recyclerview.adapter = adapter
        binding.recyclerview.layoutManager = LinearLayoutManager(this)
        mViewModel.allDepartments.observe(this, Observer { data -> adapter.submitList(data) })
    }
}

RecyclerView.Adapter:

class DepartmentListAdaptater : RecyclerView.Adapter<DepartmentListAdaptater.ViewHolder>() {
    private var dataSet: List<Department>? = null

    inner class ViewHolder(private val binding: DepartmentListRowBinding) : RecyclerView.ViewHolder(binding.root) {
        fun bind(department: Department?) {
            binding.department = department
        }
    }

    fun submitList(list: List<Department>) {
        dataSet = list
        notifyDataSetChanged()
    }

    override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): ViewHolder {
        val binding = DepartmentListRowBinding.inflate(LayoutInflater.from(parent.context), parent, false)

        return ViewHolder(binding)
    }

    override fun getItemCount(): Int = dataSet?.size ?: 0

    override fun onBindViewHolder(holder: ViewHolder, position: Int) {
        holder.bind(dataSet?.get(position))
    }
}

The XML View:

<?xml version="1.0" encoding="utf-8"?>
<layout xmlns:tools="http://schemas.android.com/tools"
        xmlns:android="http://schemas.android.com/apk/res/android">

    <data>
        <variable name="department" type="com.navalex.francemap.data.entity.Department" />
    </data>

    <RelativeLayout
            android:layout_width="match_parent"
            android:layout_height="72dp">
        <LinearLayout
                android:orientation="vertical"
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                android:gravity="center_vertical"
                android:background="@drawable/list_item_bg"
                android:layout_alignParentEnd="true"
                android:layout_alignParentTop="true"
                android:layout_alignParentBottom="true"
                android:clickable="true"
                tools:ignore="UselessParent">

            <TextView
                    android:id="@+id/textView"
                    android:layout_width="fill_parent"
                    android:layout_height="wrap_content"
                    android:paddingRight="16dp"
                    android:text="@{department.nom}"
                    android:paddingLeft="16dp"/>

        </LinearLayout>
    </RelativeLayout>
</layout>

I don't know about data binding specifically, but a typical way to do it is to let the Activity handle details like app navigation, and let the Adapter trigger that logic. A listener function is an easy way to do this:

// in your Adapter
var clickListener: ((YourData) -> ())? = null

// in your ViewHolder (make it an inner class so it can access the Adapter's
// fields, like the listener object and the stored data)
init {
    clickableView.setOnClickListener {
        // pass back whatever data here, if the listener needs to know
        // what's been clicked. I'm just doing a lookup and passing
        // the data item currently being displayed
        clickListener?.invoke(
            adapterData[bindingAdapterPosition]
        )
    }
}

// in your Activity, when setting up the adapter
adapter.clickListener = { whateverData ->
    // do what you need to do in response to the click
}

So the Activity itself is defining that logic about actions that should be taken when a click happens - it's basically wiring up different parts of the app, so the Adapter doesn't need to be concerned with anything except taking data, displaying it, and informing a listener when specific interactions take place. That listener code (defined by the Activity) could navigate somewhere else, or update a database, or pass it to a networking component... the adapter doesn't need to know about that.

(The non-Kotlin way to do this would be to create an interface and have the Activity implement that, and pass itself as the listener/callback object, that kind of thing)

First I want to say that it's really impressive that you are a web developer and you already have a lot of knowledge about things like dependency injection and keep the state of the view on ViewModel, congrats. Now, let's talk about your problem... I'll start with some suggestions that will improve the code clarity and performance.

1. For the Adapter implementation, always prefer to use ListAdapter, because this implementation have a more efficient way to compare the current list with the new list and update it. You can follow this example:

class MyAdapter: ListAdapter<ItemModel, MyAdapter.MyViewHolder>(DIFF_CALLBACK) {

    override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): MyViewHolder {
        val binding = FragmentFirstBinding.inflate(LayoutInflater.from(parent.context), parent, false)
        return MyViewHolder(binding)
    }

    override fun onBindViewHolder(holder: MyViewHolder, position: Int) {
        holder.bind(getItem(position))
    }

    class MyViewHolder(
        private val binding: FragmentFirstBinding
    ): RecyclerView.ViewHolder(binding.root) {

        fun bind(item: ItemModel) {
            // Here you can get the item values to put this values on your view
        }

    }

    companion object {
        private val DIFF_CALLBACK = object : DiffUtil.ItemCallback<ItemModel>() {
            override fun areItemsTheSame(oldItem: ItemModel, newItem: ItemModel): Boolean {
                // need a unique identifier to have sure they are the same item. could be a comparison of ids. In this case, that is just a list of strings just compare like this below
                return oldItem.id == newItem.id
            }

            override fun areContentsTheSame(oldItem: ItemModel, newItem: ItemModel): Boolean {
                // compare the objects
                return oldItem == newItem
            }

        }
    }
}

In your fragment, you have a observer, that observe the value you want to sent to the adapter, right? When a update happen, you call the submitList sending the updated list and when the adapter receive this new list, the adapter will be responsible to update just the items that changed, because of your DIFF_CALLBACK implementation.

2. About the onClick item, you can wait for a callback on your adapter. Doing this:

class MyAdapter(
    private val onItemClicked: (item: ItemModel) -> Unit
): ListAdapter<ItemModel, MyAdapter.MyViewHolder>(DIFF_CALLBACK) {

    override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): MyViewHolder {
        val binding = FragmentFirstBinding.inflate(LayoutInflater.from(parent.context), parent, false)
        return MyViewHolder(binding, onItemClicked)
    }

    // ...

    class MyViewHolder(
        private val binding: FragmentFirstBinding,
        private val onItemClicked: (item: ItemModel) -> Unit
    ): RecyclerView.ViewHolder(binding.root) {

        fun bind(item: ItemModel) {
            // ...
            // Here you set the callback to a listener
            binding.root.setOnClickListener {
                onItemClicked.invoke(item)
            }
        }

    }

    // ...
}

As you can see, we will receive the callback on the Adapter constructor, then we send to the ViewHolder by constructor too. And on the ViewHolder bind we set the callback to a click listener.

On you fragment, you will have something like this:

class MyFragment: Fragment()  {

    private lateinit var adapter: MyAdapter
    
    private val onItemClicked: (itemModel: ItemModel) -> Unit = { itemModel ->
        // do something here when the item is clicked, like redirect to another activity
    }

    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        super.onViewCreated(view, savedInstanceState)

        adapter = MyAdapter(onItemClicked)
    }
}

I hope it helps you. Please, let me know if you need something more. I really appreciate helping.