View Binding Delegates

FragmentA.kt

class WifiNetworksFragment : TonalFragment(R.layout.wifi_networks_fragment) {
    // This automatically creates and clears the binding in a lifecycle-aware way.
    private val binding: WifiNetworksFragmentBinding by viewBinding()
  
  ...
}

class WifiNetworkView @JvmOverloads constructor(
    context: Context,
    attrs: AttributeSet? = null,
    defStyleAttr: Int = 0,
    defStyleRes: Int = 0
) : FrameLayout(context, attrs, defStyleAttr, defStyleRes) {
    // This does the inflate too.
    private val binding: WifiNetworkViewBinding by viewBinding()
  
  ...
}

FragmentViewBindingDelegate.kt

import android.os.Handler
import android.os.Looper
import android.view.View
import androidx.fragment.app.Fragment
import androidx.lifecycle.Lifecycle
import androidx.lifecycle.LifecycleObserver
import androidx.lifecycle.OnLifecycleEvent
import androidx.viewbinding.ViewBinding
import com.tonal.trainer.utils.cast
import kotlin.properties.ReadOnlyProperty
import kotlin.reflect.KProperty

/**
 * Create bindings for a view similar to bindView.
 *
 * To use, just call
 * private val binding: FHomeWorkoutDetailsBinding by viewBinding()
 * with your binding class and access it as you normally would.
 */
inline fun <reified T : ViewBinding> Fragment.viewBinding() = FragmentViewBindingDelegate(T::class.java, this)

class FragmentViewBindingDelegate<T : ViewBinding>(
    private val bindingClass: Class<T>,
    val fragment: Fragment
) : ReadOnlyProperty<Fragment, T> {
    private val clearBindingHandler by lazy(LazyThreadSafetyMode.NONE) { Handler(Looper.getMainLooper()) }
    private var binding: T? = null

    private val bindMethod = bindingClass.getMethod("bind", View::class.java)

    init {
        fragment.viewLifecycleOwnerLiveData.observe(fragment) { viewLifecycleOwner ->
            viewLifecycleOwner.lifecycle.addObserver(object : LifecycleObserver {
                @OnLifecycleEvent(Lifecycle.Event.ON_DESTROY)
                fun onDestroy() {
                    // Lifecycle listeners are called before onDestroyView in a Fragment.
                    // However, we want views to be able to use bindings in onDestroyView
                    // to do cleanup so we clear the reference one frame later.
                    clearBindingHandler.post { binding = null }
                }
            })
        }
    }

    override fun getValue(thisRef: Fragment, property: KProperty<*>): T {
        // onCreateView may be called between onDestroyView and next Main thread cycle.
        // In this case [binding] refers to the previous fragment view. Check that binding's root view matches current fragment view
        if (binding != null && binding?.root !== thisRef.view) {
            binding = null
        }
        binding?.let { return it }

        val lifecycle = fragment.viewLifecycleOwner.lifecycle
        if (!lifecycle.currentState.isAtLeast(Lifecycle.State.INITIALIZED)) {
            error("Cannot access view bindings. View lifecycle is ${lifecycle.currentState}!")
        }

        binding = bindMethod.invoke(null, thisRef.requireView()).cast<T>()
        return binding!!
    }
}

@Suppress("UNCHECKED_CAST")
private fun <T> Any.cast(): T = this as T

ViewViewBindingDelegate.kt

/**
 * Create bindings for a view similar to bindView.
 *
 * To use, just call:
 * private val binding: FHomeWorkoutDetailsBinding by viewBinding()
 * with your binding class and access it as you normally would.
 */
inline fun <reified T : ViewBinding> ViewGroup.viewBinding() = ViewBindingDelegate(T::class.java, this)

class ViewBindingDelegate<T : ViewBinding>(
    private val bindingClass: Class<T>,
    private val view: ViewGroup
) : ReadOnlyProperty<ViewGroup, T> {
    private val binding: T = try {
        val inflateMethod = bindingClass.getMethod("inflate", LayoutInflater::class.java, ViewGroup::class.java, Boolean::class.javaPrimitiveType)
        inflateMethod.invoke(null, LayoutInflater.from(view.context), view, true).cast<T>()
    } catch (e: NoSuchMethodException) {
        // <merge> tags don't have the boolean parameter.
        val inflateMethod = bindingClass.getMethod("inflate", LayoutInflater::class.java, ViewGroup::class.java)
        inflateMethod.invoke(null, LayoutInflater.from(view.context), view).cast<T>()
    }

    override fun getValue(thisRef: ViewGroup, property: KProperty<*>): T = binding
}

@Suppress("UNCHECKED_CAST")
private fun <T> Any.cast(): T = this as T

Could you provide the cast method? Also, how do you handle this for Activity?

@zeusalmighty717 updated both gists

Thanks for sharing this, very helpful.

Hey! I love this, however I tried to implement it on my project and I am having issues with ProGuard/R8:
NoSuchMethodException: ... .bind [class android.view.View]

Did you @gpeal add any special ProGuard rules for this?

I faced the similar issue with @sjaramillo10. Could anyone find a solution ?

@oiyio I guess you could add a ProGuard keep rule to keep the bind() method of all classes that extend ViewBinding. We ended up using a different approach that does not require reflection.

Ok i solved the issue by adding the following code to proguard file :
-keep class com.example.mypackage.databinding.* {
public static ** inflate( ** );
public static ** bind( ** );
}

Any demo code for activity ?