FragmentのViewBinding用拡張関数

Extensions.kt

fun <T : ViewBinding> Fragment.viewBinding(viewBindingFactory: (View) -> T) =
    FragmentViewBindingDelegate(this, viewBindingFactory)

class FragmentViewBindingDelegate<T : ViewBinding>(
    val fragment: Fragment,
    val viewBindingFactory: (View) -> T
) : ReadOnlyProperty<Fragment, T> {
    private var binding: T? = null

    private val viewLifecycleOwnerObserver = Observer<LifecycleOwner?> {
        if (it == null) binding = null
    }

    private val observer = object : DefaultLifecycleObserver {

        override fun onCreate(owner: LifecycleOwner) {
            fragment.viewLifecycleOwnerLiveData.observeForever(viewLifecycleOwnerObserver)
        }

        override fun onDestroy(owner: LifecycleOwner) {
            fragment.viewLifecycleOwnerLiveData.removeObserver(viewLifecycleOwnerObserver)
            fragment.lifecycle.removeObserver(this)
        }
    }

    init {
        if (fragment.lifecycle.currentState != Lifecycle.State.DESTROYED) {
            fragment.lifecycle.addObserver(observer)
        }
    }

    override fun getValue(thisRef: Fragment, property: KProperty<*>): T {
        val binding = binding
        if (binding != null) return binding

        val view = thisRef.view
        checkNotNull(view) {
            "Should get bindings when the view is not null."
        }

        return viewBindingFactory(view).also { this.binding = it }
    }
}