Android编程权威指南（第四版）- 第 4 章 UI状态的保存与恢复

代码：

大体是一样的，修改了一些

依赖

    implementation("androidx.lifecycle:lifecycle-extensions:2.2.0")

MainActivity

package com.example.geoquizimport androidx.appcompat.app.AppCompatActivity
import android.os.Bundle
import android.util.Log
import android.view.View
import android.widget.Button
import android.widget.TextView
import android.widget.Toast
import androidx.lifecycle.ViewModelProviders
private const val KEY_INDEX = "index"
private const val KEY_SOCRE = "socre"
private const val KEY_BUTTONSTATUS="buttonStatus"
class MainActivity : AppCompatActivity() {private lateinit var questionText: TextViewprivate lateinit var ture_button: Buttonprivate lateinit var false_button: Buttonprivate lateinit var last_button: Buttonprivate lateinit var next_button: Buttonprivate lateinit var score_button: Buttonprivate val TAG="MainActivity"private val quizViewModel: QuizViewModel by lazy {ViewModelProviders.of(this)[QuizViewModel::class.java]}override fun onCreate(savedInstanceState: Bundle?) {super.onCreate(savedInstanceState)setContentView(R.layout.activity_main)val index= savedInstanceState?.getInt(KEY_INDEX,0)?:0quizViewModel.index=indexquestionText = findViewById(R.id.question)ture_button = findViewById(R.id.ture_button)false_button = findViewById(R.id.false_button)last_button = findViewById(R.id.last_button)next_button = findViewById(R.id.next_button)next_button = findViewById(R.id.next_button)score_button = findViewById(R.id.score_button)updateQuestion()//点击问题跳转下一题questionText.setOnClickListener(View.OnClickListener {quizViewModel.nextQuestion()updateQuestion()})ture_button.setOnClickListener(View.OnClickListener {quizViewModel.buttonStatus = falsecheckAnswer(true)closeButton()})false_button.setOnClickListener(View.OnClickListener {quizViewModel.buttonStatus = falsecheckAnswer(false)closeButton()})last_button.setOnClickListener(View.OnClickListener {openButton()quizViewModel.lastQuestion()updateQuestion()})next_button.setOnClickListener(View.OnClickListener {openButton()quizViewModel.nextQuestion()updateQuestion()})score_button.setOnClickListener(View.OnClickListener {settlementScore()})}override fun onStart() {super.onStart()Log.e(TAG, "onStart")}override fun onResume() {super.onResume()Log.e(TAG, "onResume")}override fun onPause() {super.onPause()Log.e(TAG, "onPause")}override fun onSaveInstanceState(outState: Bundle) {super.onSaveInstanceState(outState)Log.e(TAG,"onSaveInstanceState")//保存问题索引值outState.putInt(KEY_INDEX,quizViewModel.index)}override fun onStop() {super.onStop()Log.e(TAG, "onStop")}override fun onDestroy() {super.onDestroy()Log.e(TAG, "onDestroy")}private fun updateQuestion() {var textId = quizViewModel.questionIndexquestionText.setText(textId)}private fun checkAnswer(answer: Boolean) {var questionanswer = quizViewModel.questionAnswerif (answer == questionanswer) {quizViewModel.score += 20Toast.makeText(this, R.string.ture_toast, Toast.LENGTH_SHORT).show()} else {Toast.makeText(this, R.string.false_toast, Toast.LENGTH_SHORT).show()}}private fun closeButton() {if (!quizViewModel.buttonStatus) {ture_button.isEnabled = quizViewModel.buttonStatusfalse_button.isEnabled = quizViewModel.buttonStatus}}private fun openButton() {if (!quizViewModel.buttonStatus) {ture_button.isEnabled = truefalse_button.isEnabled = true}}private fun settlementScore() {//答完所有题结算成绩if (quizViewModel.index == quizViewModel.listSize - 1) {Toast.makeText(this, "得分为${quizViewModel.score}", Toast.LENGTH_SHORT).show()quizViewModel.score = 0} else {Toast.makeText(this, "题目没有答完哦", Toast.LENGTH_SHORT).show()}}
}

QuizViewModel

package com.example.geoquizimport androidx.lifecycle.ViewModelclass QuizViewModel : ViewModel() {var index = 0private var questionList = listOf(Question(R.string.question, true),Question(R.string.question1, true),Question(R.string.question2, true),Question(R.string.question3, false),Question(R.string.question4, false),Question(R.string.question5, false))var buttonStatus = true//按钮可以按var score = 0val listSize = questionList.sizeval questionIndex: Intget() = questionList[index].questionIDval questionAnswer: Booleanget() = questionList[index].answerfun nextQuestion() {index = (index + 1) % questionList.size}fun lastQuestion() {//       if (index == 0) {
//            index = questionList.size - 1
//        } else {
//            index -= 1
//        }index = if (index == 0) questionList.size - 1 else index - 1}}

Question

package com.example.geoquizimport androidx.annotation.StringResdata class Question(@StringRes val questionID: Int, val answer: Boolean) {
}

知识点

ViewModelProviders类提供了ViewModelProvider实例，调用ViewModelProviders.of(this)创建并返回一个关联了MainActivity的ViewModelProvider实例，ViewModelProvider会提供ViewModel实例给MainActivity

activity被销毁的两种情况：

1. 用户结束使用activity
2. 因设备配置发送改变时系统销毁