ViewBinders.kt

package com.yourapp

import android.support.annotation.IdRes
import android.support.v4.app.Fragment
import android.view.View
import android.view.ViewGroup
import kotlin.properties.ReadOnlyProperty
import kotlin.reflect.KProperty

private object UNINITIALIZED

@Suppress("unused")
fun <T : View, F> F.bindView(@IdRes resId: Int) where F : Fragment, F : ViewBindingOwner = FragmentViewBinder<T, F>(resId)

class FragmentViewBinder<T : View, F>(@IdRes private val resId: Int) : ReadOnlyProperty<F, T> where F : Fragment, F : ViewBindingOwner {
    private var view: Any? = UNINITIALIZED

    override fun getValue(thisRef: F, property: KProperty<*>): T {
        if (view == UNINITIALIZED) {
            view = thisRef.view?.findViewById(resId) ?: throw IllegalStateException("Unable to find view for ${thisRef.resources.getResourceName(resId)}")
            assertCorrectType(thisRef)
            thisRef.addViewBinder(this)
        }
        @Suppress("UNCHECKED_CAST")
        return view as T
    }

    fun clear() {
        view = UNINITIALIZED
    }

    private fun assertCorrectType(thisRef: Fragment) {
        @Suppress("UNCHECKED_CAST")
        if (view as? T == null) {
            val resName = thisRef.resources.getResourceName(resId)
            throw IllegalStateException("Incorrect type for $resName")
        }
    }
}

/**
 * Contains helpers to get butterknife-like support for Kotlin Fragments.
 *
 * To use, create a property like:
 * private val myView: TextView by bindView(R.id.your_id)
 */

fun <T : View> bindView(@IdRes resId: Int) = object : ReadOnlyProperty<ViewGroup, T> {
    private lateinit var view: Any

    override fun getValue(thisRef: ViewGroup, property: KProperty<*>): T {
        if (!::view.isInitialized) {
            view = thisRef.findViewById(resId) ?: throw IllegalStateException("Unable to find view for ${thisRef.resources.getResourceName(resId)}")
            assertCorrectType(thisRef)
        }

        @Suppress("UNCHECKED_CAST")
        return view as T
    }

    private fun assertCorrectType(thisRef: ViewGroup) {
        @Suppress("UNCHECKED_CAST")
        if (view as? T == null) {
            val resName = thisRef.resources.getResourceName(resId)
            throw IllegalStateException("Incorrect type for $resName")
        }
    }
}

/**
 * A Fragment should implement this if they want to use the bindView delegate.
 *
 * The Fragment should keep track of a list of ViewBinders as they are added then call clear on each of them in onDestroyView.
 */
interface ViewBindingOwner {
    fun addViewBinder(binder: FragmentViewBinder<*, *>)
}