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)

@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. ;)