Lifecycle Aware Lazy and Find View

fragment_main.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    tools:context="com.bignerdranch.android.lazyforlifecycle.MainActivity">

    <TextView
        android:id="@+id/questionTextView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:textAppearance="@style/TextAppearance.AppCompat.Medium"
        tools:text="Civil Rights Leader"/>

    <TextView
        android:id="@+id/nameTextView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:textAppearance="@style/TextAppearance.AppCompat.Medium"
        tools:text="Martin Luther King Jr."/>

    <TextView
        android:id="@+id/cityTextView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:textAppearance="@style/TextAppearance.AppCompat.Medium"
        tools:text="Atlanta"/>

    <TextView
        android:id="@+id/stateTextView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:textAppearance="@style/TextAppearance.AppCompat.Medium"
        tools:text="Georgia"/>

    <Button
        android:id="@+id/showAnswerButton"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Show answer"/>

</LinearLayout>

LifecycleAware.kt

package com.bignerdranch.android.kotleta.ext

import android.app.Activity
import android.arch.lifecycle.GenericLifecycleObserver
import android.arch.lifecycle.Lifecycle
import android.arch.lifecycle.LifecycleOwner
import android.support.v4.app.Fragment
import android.view.View
import kotlin.reflect.KProperty

/**
 * Represents a value with lifecycleAwareLazy initialization.
 *
 * To create an instance of [LifecycleAwareLazy] use the [lifecycleAwareLazy] function.
 */
public interface LifecycleAwareLazy<out T> {
    /**
     * Gets the lazily initialized value of the current LifecycleAwareLazy instance.
     * Once the value was initialized it must not change during the rest of lifetime of this LifecycleAwareLazy instance.
     */
    public val value: T
    /**
     * Returns `true` if a value for this LifecycleAwareLazy instance has been already initialized, and `false` otherwise.
     * Once this function has returned `true` it stays `true` for the rest of lifetime of this LifecycleAwareLazy instance.
     */
    public fun isInitialized(): Boolean
}

/**
 * Lifecycle aware
 */
public fun <T> lifecycleAwareLazy(lifecycle: Lifecycle, initializer: () -> T): LifecycleAwareLazy<T>
= LifecycleAwareLazyImpl(lifecycle, initializer)

public fun <T, U> findViewLazy(fragment: U, viewId: Int): LifecycleAwareLazy<T>
        where T: View, U: Fragment, U: LifecycleOwner {
    return lifecycleAwareLazy(fragment.lifecycle) {
        fragment.view!!.findViewById<T>(viewId)
    }
}

public fun <T, U> findViewLazy(activity: U, viewId: Int): LifecycleAwareLazy<T>
        where T: View, U: Activity, U: LifecycleOwner {
    return lifecycleAwareLazy(activity.lifecycle) {
        activity.findViewById<T>(viewId)
    }
}

/**
 * An extension to delegate a read-only property of type [T] to an instance of [LifecycleAwareLazy].
 *
 * This extension allows to use instances of LifecycleAwareLazy for property delegation:
 * `val property: TextView by lifecycleAwareLazy(lifecycle) { initializer }`
 */
public inline operator fun <T> LifecycleAwareLazy<T>.getValue(thisRef: Any?, property: KProperty<*>): T = value


private object UNINITIALIZED_VALUE

private class LifecycleAwareLazyImpl<out T>(lifecycle: Lifecycle, initializer: () -> T)
    : LifecycleAwareLazy<T>, GenericLifecycleObserver {
    override fun getReceiver() = this

    override fun onStateChanged(source: LifecycleOwner?, event: Lifecycle.Event?) {
        when(event) {
            Lifecycle.Event.ON_STOP -> _value = UNINITIALIZED_VALUE
//            Lifecycle.Event.ON_DESTROY -> initializer = null
            else -> return
        }
    }

    init {
        lifecycle.addObserver(this)
    }

    private var initializer: (() -> T)? = initializer
    private var _value: Any? = UNINITIALIZED_VALUE

    override val value: T
        get() {
            if (_value === UNINITIALIZED_VALUE) {
                _value = initializer!!()
            }
            @Suppress("UNCHECKED_CAST")
            return _value as T
        }

    override fun isInitialized(): Boolean = _value !== UNINITIALIZED_VALUE

    override fun toString(): String = if (isInitialized()) value.toString() else "LifecycleAwareLazy value not initialized yet."
}

public interface LifecycleAwareFindView<out T: View> {
    /**
     * Gets the lazily initialized value of the current LifecycleAwareLazy instance.
     * Once the value was initialized it must not change during the rest of lifetime of this LifecycleAwareLazy instance.
     */
    public val value: T
    /**
     * Returns `true` if a value for this LifecycleAwareLazy instance has been already initialized, and `false` otherwise.
     * Once this function has returned `true` it stays `true` for the rest of lifetime of this LifecycleAwareLazy instance.
     */
    public fun isInitialized(): Boolean
}

public fun <T, U> findView(fragment: U, viewId: Int): LifecycleAwareFindView<T>
        where T: View, U: Fragment, U: LifecycleOwner = FindFragmentViewImpl(fragment, viewId)

private class FindFragmentViewImpl<out T, out U>(fragment: U, val viewId: Int)
    : LifecycleAwareFindView<T>, GenericLifecycleObserver where T: View, U: Fragment, U: LifecycleOwner {
    override fun getReceiver() = this

    override fun onStateChanged(source: LifecycleOwner?, event: Lifecycle.Event?) {
        when(event) {
            Lifecycle.Event.ON_STOP -> _value = UNINITIALIZED_VALUE
            Lifecycle.Event.ON_DESTROY -> if (fragment?.retainInstance == false) fragment = null
            else -> return
        }
    }

    init {
        fragment.lifecycle.addObserver(this)
    }

    private var fragment: U? = fragment
    private var _value: Any? = UNINITIALIZED_VALUE

    override val value: T
        get() {
            if (_value === UNINITIALIZED_VALUE) {
                _value = fragment!!.view!!.findViewById(viewId)
            }
            @Suppress("UNCHECKED_CAST")
            return _value as T
        }

    override fun isInitialized(): Boolean = _value !== UNINITIALIZED_VALUE

    override fun toString(): String = if (isInitialized()) value.toString() else "LifecycleAwareLazy value not initialized yet."
}

/**
 * An extension to delegate a read-only property of type [T] to an instance of [LifecycleAwareLazy].
 *
 * This extension allows to use instances of LifecycleAwareLazy for property delegation:
 * `val property: TextView by lifecycleAwareLazy(lifecycle) { initializer }`
 */
public inline operator fun <T: View> LifecycleAwareFindView<T>.getValue(thisRef: Any?, property: KProperty<*>): T = value

MainFragment.kt

package com.bignerdranch.android.lazyforlifecycle

import android.arch.lifecycle.LifecycleRegistry
import android.arch.lifecycle.LifecycleRegistryOwner
import android.os.Bundle
import android.support.v4.app.Fragment
import android.view.LayoutInflater
import android.view.View
import android.view.ViewGroup
import android.widget.Button
import android.widget.TextView
import com.bignerdranch.android.kotleta.ext.findView
import com.bignerdranch.android.kotleta.ext.getValue
import com.bignerdranch.android.kotleta.ext.lifecycleAwareLazy

class MainFragment: Fragment(), LifecycleRegistryOwner {
    val registry = LifecycleRegistry(this)
    override fun getLifecycle() = registry

    var showAnswerButton: Button? = null
    lateinit var questionTextView: TextView
    val nameTextView by lazy { view!!.findViewById<TextView>(R.id.nameTextView) }
    val cityTextView by lifecycleAwareLazy(lifecycle) { view!!.findViewById<TextView>(R.id.cityTextView) }
    val stateTextView: TextView by findView(this, R.id.stateTextView)

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        retainInstance = true
    }

    override fun onCreateView(inflater: LayoutInflater?, container: ViewGroup?, savedInstanceState: Bundle?): View? {
        val view = layoutInflater.inflate(R.layout.fragment_main, container, false)
        showAnswerButton = view.findViewById(R.id.showAnswerButton)
        questionTextView = view.findViewById(R.id.questionTextView)
        questionTextView.text = getString(R.string.question1)
        showAnswerButton?.setOnClickListener {
            nameTextView.setText(R.string.answer1_name)
            stateTextView.setText(R.string.answer1_state)
            cityTextView.setText(R.string.answer1_city)
        }
        return view
    }
}