Android5:活动生命周期

创建项目Stopwatch

activity_main.xml

strings.xml

    Stopwatch
    Start
    Pause
    Reset

MainActivity.kt

package com.demo.stopwatch

import androidx.appcompat.app.AppCompatActivity
import android.os.Bundle
import android.os.SystemClock
import android.widget.Button
import android.widget.Chronometer

class MainActivity : AppCompatActivity() {

    lateinit var stopwatch: Chronometer  //The stopwatch
    var running = false  //Is the stopwatch running?
    var offset: Long = 0  //The base offset for the stopwatch

    
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        //Get a reference to the stopwatch
        stopwatch = findViewById(R.id.stopwatch)

        //The start button starts the stopwatch if it's not running
        val startButton = findViewById(R.id.start_button)
        startButton.setOnClickListener {
            if (!running) {
                setBaseTime()
                stopwatch.start()
                running = true
            }
        }

        //The pause button pauses the stopwatch if it’s running
        val pauseButton = findViewById(R.id.pause_button)
        pauseButton.setOnClickListener {
            if (running) {
                saveOffset()
                stopwatch.stop()
                running = false
            }
        }

        //The reset button sets the offset and stopwatch to 0
        val resetButton = findViewById(R.id.reset_button)
        resetButton.setOnClickListener {
            offset = 0
            setBaseTime()
        }
    }
    

    //Update the stopwatch base time, allowing for any offset
    fun setBaseTime() {
        stopwatch.base = SystemClock.elapsedRealtime() - offset
    }

    //Record the offset
    fun saveOffset() {
        offset = SystemClock.elapsedRealtime() - stopwatch.base
    }
}

运行查看效果

代码说明
1.应用运行，MainActivity启动
初始化running和offset属性
2.调用MainActivity的onCreate方法
将activity_main.xml布局链接到活动，为stopwatch属性指定视图的一个引用
3.点击“Start”按键
秒表开始运行
4.点击“Pause”按钮
更新offset属性，并调用秒表的stop方法，running更新为false，秒表暂停
5.再次点击“Start”按钮
使用offset的值来调整stopwatch.base属性，调用start方法，更新running为true，表秒再次运行
6.点击“Reset”按钮
offset更新为0，stopwatch.base属性更新为SystemClock.elapsedRealtime()

但是这里有个问题，当旋转屏幕时，秒表会重置为0，并停止运行

旋转时发生了什么？
当屏幕方向有变化时，Android会撤销MainActivity，所以MainActivity的属性值会丢失
然后MainActivity会重启，它的所有属性都会重新初始化，并且再次运行onCreate()方法

活动从启动状态变成撤销状态时，会触发一些活动生命周期的方法：onCreate()和onDestroy()。这些是活动继承的生命周期方法，不过可以覆盖这些方法。

解决方案：使用Bundle

Bundle是一种保存键值对的对象。在活动撤销之前，Android允许你把键值对放在Bundle里面，然后在活动重建时，活动的新实例通过Bundle恢复属性值

活动撤销前都会调用onSaveInstanceState方法，所以这里需要覆盖onSaveInstanceState方法

活动重建时会调用onCreate()方法，这里通过检查savedInstanceState != null来判断是否需要恢复属性值

MainActivity.kt最终代码

package com.demo.stopwatch

import androidx.appcompat.app.AppCompatActivity
import android.os.Bundle
import android.os.SystemClock
import android.widget.Button
import android.widget.Chronometer

class MainActivity : AppCompatActivity() {

    lateinit var stopwatch: Chronometer  //The stopwatch
    var running = false  //Is the stopwatch running?
    var offset: Long = 0  //The base offset for the stopwatch

    //Add key Strings for use with the Bundle
    val OFFSET_KEY = "offset"
    val RUNNING_KEY = "running"
    val BASE_KEY = "base"

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        //Get a reference to the stopwatch
        stopwatch = findViewById(R.id.stopwatch)

        //Restore the previous state
        if (savedInstanceState != null) {
            offset = savedInstanceState.getLong(OFFSET_KEY)
            running = savedInstanceState.getBoolean(RUNNING_KEY)
            if (running) {
                stopwatch.base = savedInstanceState.getLong(BASE_KEY)
                stopwatch.start()
            } else setBaseTime()
        }

        //The start button starts the stopwatch if it's not running
        val startButton = findViewById(R.id.start_button)
        startButton.setOnClickListener {
            if (!running) {
                setBaseTime()
                stopwatch.start()
                running = true
            }
        }

        //The pause button pauses the stopwatch if it’s running
        val pauseButton = findViewById(R.id.pause_button)
        pauseButton.setOnClickListener {
            if (running) {
                saveOffset()
                stopwatch.stop()
                running = false
            }
        }

        //The reset button sets the offset and stopwatch to 0
        val resetButton = findViewById(R.id.reset_button)
        resetButton.setOnClickListener {
            offset = 0
            setBaseTime()
        }
    }

    override fun onPause() {
        super.onPause()
        if (running) {
            saveOffset()
            stopwatch.stop()
        }
    }

    override fun onResume() {
        super.onResume()
        if (running) {
            setBaseTime()
            stopwatch.start()
            offset = 0
        }
    }

    override fun onSaveInstanceState(savedInstanceState: Bundle) {
        savedInstanceState.putLong(OFFSET_KEY, offset)
        savedInstanceState.putBoolean(RUNNING_KEY, running)
        savedInstanceState.putLong(BASE_KEY, stopwatch.base)
        super.onSaveInstanceState(savedInstanceState)
    }

    //Update the stopwatch base time, allowing for any offset
    fun setBaseTime() {
        stopwatch.base = SystemClock.elapsedRealtime() - offset
    }

    //Record the offset
    fun saveOffset() {
        offset = SystemClock.elapsedRealtime() - stopwatch.base
    }
}