ViewDelegates.kt

import android.view.View
import kotlin.properties.ReadOnlyProperty
import android.app.Activity
import kotlin.properties.Delegates

public fun Activity.bindView<T: View>(id: Int): ReadOnlyProperty<Any, T> = ViewVal(id)
public fun ViewHolder.bindView<T: View>(id: Int): ReadOnlyProperty<Any, T> = ViewVal(id)

public trait ViewHolder {
  public val view: View;
}

public class ViewHolderImpl(v: View) : ViewHolder {
  override val view: View = v
}

private class ViewVal<T: View>(val id: Int) : ReadOnlyProperty<Any, T> {
  var value: T? = null

  override fun get(thisRef: Any, desc: PropertyMetadata): T {
    [suppress("UNCHECKED_CAST")]
    if (value == null) {
      value = when (thisRef) {
        is Activity -> thisRef.findViewById(id)
        is ViewHolder -> thisRef.view.findViewById(id)
        else -> throw IllegalStateException("Unsupported type.")
      } as T
    }

    return value!!
  }
}