Making Android View Binding convenient with Kotlin

Hello! My name is Kirill Rozov. I am the author of the Telegram channel Android Broadcast . I love Kotlin very much and I like using its features to simplify development. I recently faced this problem when they started using View Binding on a new Android project .

This feature appeared in Android Studio 3.6, but in fact it is not entirely new, but a lightweight version of Android Data Binding . Why so many complications? The problem was speed - many developers used Android Data Bindingonly to generate code with links to View and ignored other library features. To speed up code generation, created View Binding. However, the standard way to work with it is to duplicate code that you want to get rid of.

The standard way to work with View Binding

Let's look at View Binding using the Fragment example. We have a layout resource with a name profile.xml(its contents do not matter). If we want to use ViewBinding, then in the standard version it will look like this:

class ProfileFragment : Fragment(R.layout.profile) {

    private var viewBinding: ProfileBinding? = null

    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        super.onViewCreated(view, savedInstanceState)
        viewBinding = ProfileBinding.bind(view)
        //   viewBinding
    }

    override fun onDestroyView() {
        super.onDestroyView()
        viewBinding = null
    }
}

There are several problems here:

• A lot of extra code
• : Fragment
• Property viewBinding nullable .

C Kotlin

Kotlin Delegated Property

property Kotlin . , ViewBinding. , ViewBinding :

class FragmentViewBindingProperty<T : ViewBinding>(
    private val viewBinder: ViewBinder<T>
) : ReadOnlyProperty<Fragment, T> {

    internal var viewBinding: T? = null
    private val lifecycleObserver = BindingLifecycleObserver()

    @MainThread
    override fun getValue(thisRef: Fragment, property: KProperty<*>): T {
        checkIsMainThread()
        this.viewBinding?.let { return it }

        val view = thisRef.requireView()
        thisRef.viewLifecycleOwner.lifecycle.addObserver(lifecycleObserver)
        return viewBinder.bind(view).also { vb -> this.viewBinding = vb }
    }

    private inner class BindingLifecycleObserver : DefaultLifecycleObserver {

        @MainThread
        override fun onDestroy(owner: LifecycleOwner) {
            owner.lifecycle.removeObserver(this)
            viewBinding = null
        }
    }
}

-, :

inline fun <reified T : ViewBinding> Fragment.viewBinding(): ReadOnlyProperty<Fragment, T> {
    return FragmentViewBindingProperty(DefaultViewBinder(T::class.java))
}

:

class ProfileFragment() : Fragment(R.layout.profile) {

    private val viewBinding: ProfileBinding by viewBinding()

    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        super.onViewCreated(view, savedInstanceState)
        //   viewBinding
    }
}

, , . ?

, - ...

- View :

class ProfileFragment() : Fragment(R.layout.profile) {

    private val viewBinding: ProfileBinding by viewBinding()

    override fun onDestroyView() {
        super.onDestroyView()
        //  View  viewBinding
    }
}

, ViewBinding property . super.onDestroyView() . Fragment.viewLifecycleOwner.

ON_DESTROY Fragment.viewLifecycleOwner Fragment.onDestroyView(), FragmentViewBindingProperty , . . , Handler:

class FragmentViewBindingProperty<T : ViewBinding>(...) : ReadOnlyProperty<Fragment, T> {

    internal var viewBinding: T? = null

    private inner class BindingLifecycleObserver : DefaultLifecycleObserver {

        private val mainHandler = Handler(Looper.getMainLooper())

        @MainThread
        override fun onDestroy(owner: LifecycleOwner) {
            owner.lifecycle.removeObserver(this)
            mainHandler.post { viewBinding = null }
        }
    }
}

.