FragmentExt.kt

/*
* Based on https://gist.github.com/jamiesanson/d1a3ed0910cd605e928572ce245bafc4
* 
* Refactored to use the preferred `DefaultLifecycleObserver` which does not rely on the annotation processor.
* See https://developer.android.com/reference/kotlin/androidx/lifecycle/Lifecycle#init
*
* Usage: 
* `private var binding: TheViewBinding by viewLifecycle()`
*/

/**
 * An extension to bind and unbind a value based on the view lifecycle of a Fragment.
 * The binding will be unbound in onDestroyView.
 *
 * @throws IllegalStateException If the getter is invoked before the binding is set, 
 *                               or after onDestroyView an exception is thrown.
 */
fun <T> Fragment.viewLifecycle(): ReadWriteProperty<Fragment, T> =
    object : ReadWriteProperty<Fragment, T>, DefaultLifecycleObserver {

        private var binding: T? = null

        init {
            // Observe the view lifecycle of the Fragment.
            // The view lifecycle owner is null before onCreateView and after onDestroyView.
            // The observer is automatically removed after the onDestroy event.
            this@viewLifecycle
                .viewLifecycleOwnerLiveData
                .observe(this@viewLifecycle, Observer { owner: LifecycleOwner? ->
                    owner?.lifecycle?.addObserver(this)
                })
        }

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

        override fun getValue(
            thisRef: Fragment,
            property: KProperty<*>
        ): T {
            return this.binding ?: error("Called before onCreateView or after onDestroyView.")
        }

        override fun setValue(
            thisRef: Fragment,
            property: KProperty<*>,
            value: T
        ) {
            this.binding = value
        }
    }

Thanks for the explanation. This code is better than the original supplied by the author of the medium article.

in onDestroy why didn't you have the line

        super.onDestroy(owner)

I was wondering because the original author made a new implmentation
https://gist.github.com/jamiesanson/478997780eb6ca93361df311058dc5c2
where he did that.

Thanks

in onDestroy why didn't you have the line

        super.onDestroy(owner)

Because DefaultLifecycleObserver is an interface, albeit with default (empty) implementations to reduce boiler plate (you don't have to implement all the methods in FullLifecycleObserver. Thus calling super.onDestroy does nothing.

Thanks frel, I really appretiate it.

thanks frel

Nice - thanks :)

Hi @frel. This is good. Can you please add instrumentation tests for this if you have time? It will be really helpful.