Holds and manages ViewBinding inside a Fragment.

ViewBindingHolder.kt

/**
 * Holds and manages ViewBinding inside a Fragment.
 */
interface ViewBindingHolder<T : ViewBinding> {

    val binding: T?

    /**
     * Saves the binding for cleanup on onDestroy, calls the specified function [onBound] with `this` value
     * as its receiver and returns the bound view root.
     */
    fun initBinding(binding: T, fragment: Fragment, onBound: (T.() -> Unit)?): View

    /**
     * Calls the specified [block] with the binding as `this` value and returns the binding. As a consequence, this method
     * can be used with a code block lambda in [block] or to initialize a variable with the return type.
     *
     * @throws IllegalStateException if not currently holding a ViewBinding (when called outside of an active fragment's lifecycle)
     */
    fun requireBinding(block: (T.() -> Unit)? = null): T
}

class ViewBindingHolderImpl<T : ViewBinding> : ViewBindingHolder<T>, LifecycleObserver {

    override var binding: T? = null
    var lifecycle: Lifecycle? = null

    private lateinit var fragmentName: String

    /**
     * To not leak memory we nullify the binding when the view is destroyed.
     */
    @OnLifecycleEvent(Lifecycle.Event.ON_DESTROY)
    fun onDestroyView() {
        lifecycle?.removeObserver(this) // not mandatory, but preferred
        lifecycle = null
        binding = null
    }

    override fun requireBinding(block: (T.() -> Unit)?) =
        binding?.apply { block?.invoke(this) } ?: throw IllegalStateException("Accessing binding outside of Fragment lifecycle: $fragmentName")

    override fun initBinding(binding: T, fragment: Fragment, onBound: (T.() -> Unit)?): View {
        this.binding = binding
        lifecycle = fragment.viewLifecycleOwner.lifecycle
        lifecycle?.addObserver(this)
        fragmentName = fragment::class.simpleName ?: "N/A"
        onBound?.invoke(binding)
        return binding.root
    }
}

Hey, when and how is the optional onDestroyView called ? Is it called automatically by the system when onDestroy() is triggerred ? In What sequence would this be getting called, when we have an onDestroyView already overridden in our fragment?

Hey, when and how is the optional onDestroyView called ? Is it called automatically by the system when onDestroy() is triggerred ? In What sequence would this be getting called, when we have an onDestroyView already overridden in our fragment?

It is called when onDestroyView is triggered (using onDestroy would be a memory leak). It is not optional btw.

It is called just before your own (Fragment's) onDestroyView, so you cannot access the binding anymore in your custom onDestroyView.