Kotlin delegates for Android View Binding with usage examples

1 ViewBindingDelegates.kt

import android.view.LayoutInflater
import android.view.View
import android.view.ViewGroup
import androidx.appcompat.app.AppCompatActivity
import androidx.fragment.app.DialogFragment
import androidx.fragment.app.Fragment
import androidx.lifecycle.DefaultLifecycleObserver
import androidx.lifecycle.Lifecycle
import androidx.lifecycle.LifecycleOwner
import androidx.viewbinding.ViewBinding
import kotlin.properties.ReadOnlyProperty
import kotlin.reflect.KProperty

/** Activity binding delegate, may be used since onCreate up to onDestroy (inclusive) */
inline fun <T : ViewBinding> AppCompatActivity.viewBinding(crossinline factory: (LayoutInflater) -> T) =
    lazy(LazyThreadSafetyMode.NONE) {
        factory(layoutInflater)
    }

/** Fragment binding delegate, may be used since onViewCreated up to onDestroyView (inclusive) */
fun <T : ViewBinding> Fragment.viewBinding(factory: (View) -> T): ReadOnlyProperty<Fragment, T> =
    object : ReadOnlyProperty<Fragment, T>, DefaultLifecycleObserver {
        private var binding: T? = null

        override fun getValue(thisRef: Fragment, property: KProperty<*>): T =
            binding ?: factory(requireView()).also {
                // if binding is accessed after Lifecycle is DESTROYED, create new instance, but don't cache it
                if (viewLifecycleOwner.lifecycle.currentState.isAtLeast(Lifecycle.State.INITIALIZED)) {
                    viewLifecycleOwner.lifecycle.addObserver(this)
                    binding = it
                }
            }

        override fun onDestroy(owner: LifecycleOwner) {
            binding = null
        }
    }

/** Binding delegate for DialogFragments implementing onCreateDialog (like Activities, they don't
 *  have a separate view lifecycle), may be used since onCreateDialog up to onDestroy (inclusive) */
inline fun <T : ViewBinding> DialogFragment.viewBinding(crossinline factory: (LayoutInflater) -> T) =
    lazy(LazyThreadSafetyMode.NONE) {
        factory(layoutInflater)
    }

/** Not really a delegate, just a small helper for RecyclerView.ViewHolders */
inline fun <T : ViewBinding> ViewGroup.viewBinding(factory: (LayoutInflater, ViewGroup, Boolean) -> T) =
    factory(LayoutInflater.from(context), this, false)

2 MainActivity.kt

class MainActivity : AppCompatActivity() {
    private val binding by viewBinding(ActivityMainBinding::inflate)

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(binding.root)
        binding.button.text = "Bound!"
    }
}

3 RegularFragment.kt

// Don't forget to pass layoutId in Fragment constructor

class RegularFragment : Fragment(R.layout.fragment) {
    private val binding by viewBinding(FragmentBinding::bind)

    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        super.onViewCreated(view, savedInstanceState)
        binding.button.text = "Bound!"
    }
}

4 DialogFragment1.kt

// DialogFragment with onCreateDialog doesn't have a view lifecycle, so we need a different delegate

class DialogFragment1 : DialogFragment() {
    private val binding by viewBinding(FragmentBinding::inflate)

    override fun onCreateDialog(savedInstanceState: Bundle?): Dialog {
        binding.button.text = "Bound!"
        return AlertDialog.Builder(requireContext()).setView(binding.root).create()
    }
}

5 DialogFragment2.kt

// For DialogFragment with full-blown view we can use a regular Fragment delegate (actually the
// whole code here is exactly the same as in RegularFragment)
// NB: Constructor with layoutId was only recently added (in Fragment 1.3.0)

class DialogFragment2 : DialogFragment(R.layout.fragment) {
    private val binding by viewBinding(FragmentBinding::bind)

    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        super.onViewCreated(view, savedInstanceState)
        binding.button.text = "Bound!"
    }
}

6 Adapter1.kt

// For RecyclerView we don't need any delegates, just a property.
// Unfortunately, here we have a name overloading: View Binding vs "binding" holder to data (onBindViewHolder).
// ViewGroup.viewBinding() helper function can reduce boilerplate a little.

class Adapter1 : ListAdapter<String, Adapter1.Holder>(Differ()) {

    override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): Holder {
        return Holder(parent.viewBinding(ListItemBinding::inflate))
    }

    override fun onBindViewHolder(holder: Holder, position: Int) {
        holder.binding.textView.text = getItem(position)
    }

    class Holder(val binding: ListItemBinding) : RecyclerView.ViewHolder(binding.root)

    private class Differ : DiffUtil.ItemCallback<String>() { ... }
}

7 Adapter2.kt

// Alternatively, we can use generic BoundHolder for all Adapters

class Adapter2 : ListAdapter<String, BoundHolder<ListItemBinding>>(Differ()) {

    override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): BoundHolder<ListItemBinding> {
        return BoundHolder(parent.viewBinding(ListItemBinding::inflate))
    }

    override fun onBindViewHolder(holder: BoundHolder<ListItemBinding>, position: Int) {
        holder.binding.textView.text = getItem(position)
    }

    private class Differ : DiffUtil.ItemCallback<String>() { ... }
}

open class BoundHolder<T : ViewBinding>(val binding: T) : RecyclerView.ViewHolder(binding.root)

8 Adapter3.kt

// Personally, I prefer to encapsulate view creation & manipulation inside ViewHolder.
// In this case BoundHolder can be used as a superclass.

class Adapter3 : ListAdapter<String, Adapter3.Holder>(Differ()) {

    override fun onCreateViewHolder(parent: ViewGroup, viewType: Int) = Holder(parent)

    override fun onBindViewHolder(holder: Holder, position: Int) = holder.bind(getItem(position))

    class Holder(parent: ViewGroup) : BoundHolder<ListItemBinding>(parent.viewBinding(ListItemBinding::inflate)) {

        fun bind(item: String) {
            binding.textView.text = item
        }
    }

    private class Differ : DiffUtil.ItemCallback<String>() { ... }
}

abstract class BoundHolder<T : ViewBinding>(protected val binding: T) : RecyclerView.ViewHolder(binding.root)

I've updated the example for DialogFragment (the one with full-blown View) using new constructor from Fragment 1.3.0 and a regular Fragment's viewBinding delegate. This allows to safely use such a DialogFragment as an embeddable fragment. AFAIK, other use cases are not affected.

helo @gmk57 can this be used on view data binding? i tried to used this on view data binding but got IllegalStateException

@KevinAngga, I personally don't use Data Binding, but I just replaced viewBinding true with dataBinding true and added <layout> tags in my sample project, and it seems to work, because ViewDataBinding implements ViewBinding. Can you share your code and/or stack trace?

lol so sorry @gmk57 i made some silly error by myself xD. yea nothing wrong with this one, its can be use to viewdatabinding too

helo @gmk57 so i already use this code and roll it to production. but im getting some intermitten crash on crashlytic saying {someFragmetnName} did not return a View from onCreateView() or this was called before onCreateView(). but i cant repoduce why this happend :v did you got same problem with this one? all i know is this are illegalStateException are from requireView() code.

and i already check all my code was always call binding on onViewCreated method

@KevinAngga, I've only seen this error when accessing ViewBinding outside the view lifecycle (onViewCreated-onDestroyView). This may happen if you register some callbacks/listeners/observers (e.g. in onViewCreated), and for some reason they get triggered when view does not exist. Coroutines, when incorrectly used, may cause this too. Can you share your code and/or stack trace?

@gmk57 i cant share the code / stact trace tho cause this one are company code. but when i tried this code and accessing some binding on destroyView it wont throw that same error ._. dunno why this happen on our user device lol, any way thank for the feedback :)

@gmk57 i have tried to implement this approach in custom views but dont work, you could show a example of this implementation in custom views, please.

@hqfranca Since a view has a same lifecycle as its child views, you don't really need a delegate. Simple property works fine:

class SomeView @JvmOverloads constructor(context: Context, attrs: AttributeSet? = null, defStyleAttr: Int = 0, defStyleRes: Int = 0) :
    FrameLayout(context, attrs, defStyleAttr, defStyleRes) {
    private val binding = SomeViewBinding.inflate(LayoutInflater.from(context), this, true)
}

My ViewGroup.viewBinding helper does not fit well here, because it passes false as a last parameter (as it should for RecyclerView.ViewHolder). You can create a second helper, but I'm not sure if it's really worth the effort: it would only save you 27 chars. ;)