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=<