LifeCycle的诞生：

为了解决代码解耦的问题，将系统组件和普通组件尽可能的解耦

LifeCycle应用：

1. 使用Lifecycle解耦页面与组件
2. 使用LifecycleService解耦Service与组件
3. 使用ProcessLifecycleOwner监听应用程序生命周期

使用Lifecycle解耦页面与组件

案例：统计页面停留时间

基础逻辑实现思路：

/**
 * 计时器，只显示页面停留时间，在页面进入后台，时间不再计算
 * 业务的实现依赖于系统组件Activity
 */
class TestActivity : AppCompatActivity() {
    private lateinit var chronometer:Chronometer
    var elapsedTime:Long= 0L
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_test)
        chronometer = findViewById<Chronometer>(R.id.chronometer)
    }

    override fun onResume() {
        super.onResume()
        chronometer.base = SystemClock.elapsedRealtime()-elapsedTime
        chronometer.start()
    }

    override fun onPause() {
        super.onPause()
        elapsedTime =  SystemClock.elapsedRealtime()-chronometer.base
        chronometer.stop()
    }
}

封装自定义组件MyChronometer，使用Lifecycle进行解耦：

class MyChronometer : Chronometer, LifecycleObserver {
    var elapsedTime: Long = 0L
    constructor(context: Context?, attrs: AttributeSet?) : super(context, attrs)
    @OnLifecycleEvent(Lifecycle.Event.ON_RESUME)
    private fun startMeter() {
        base = SystemClock.elapsedRealtime() - elapsedTime
        start()
    }
    @OnLifecycleEvent(Lifecycle.Event.ON_PAUSE)
    private fun stopMeter() {
        elapsedTime =  SystemClock.elapsedRealtime()-base
        stop()
    }
}

//控件使用
/**
 * 计时器，只显示页面停留时间，在页面进入后台，时间不再计算
 * 业务的实现依赖于系统组件Activity
 */
class TestActivity : AppCompatActivity() {
    private lateinit var chronometer:MyChronometer
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_test)
        chronometer = findViewById<MyChronometer>(R.id.chronometer)
        lifecycle.addObserver(chronometer)
    }

}

使用LifecycleService解耦Service与组件

案例：使用Service后台监听GPS改变

class LocationObserver(var context: Context) : LifecycleObserver {

    private lateinit var locationManager: LocationManager
    private lateinit var myLocationListener: MyLocationListener

    @OnLifecycleEvent(Lifecycle.Event.ON_CREATE)
    fun startLocation() {
        Log.d("Location", "startLocation")
        locationManager = context.getSystemService(Context.LOCATION_SERVICE) as LocationManager
        if (ActivityCompat.checkSelfPermission(
                context,
                Manifest.permission.ACCESS_FINE_LOCATION
            ) != PackageManager.PERMISSION_GRANTED && ActivityCompat.checkSelfPermission(
                context,
                Manifest.permission.ACCESS_COARSE_LOCATION
            ) != PackageManager.PERMISSION_GRANTED
        ) {
            return
        }
        myLocationListener = MyLocationListener()
        locationManager.requestLocationUpdates(
            LocationManager.GPS_PROVIDER,
            300,
            10F,
            myLocationListener
        )

    }

    @OnLifecycleEvent(Lifecycle.Event.ON_DESTROY)
    fun stopLocation() {
        Log.d("Location", "stopLocation")
        locationManager.removeUpdates(myLocationListener)
    }

    open class MyLocationListener : LocationListener {
        override fun onLocationChanged(location: Location) {
            Log.d("Location", "${location.latitude} --- ${location.longitude}")
        }

        override fun onLocationChanged(locations: MutableList<Location>) {
            super.onLocationChanged(locations)
        }

        override fun onFlushComplete(requestCode: Int) {
            super.onFlushComplete(requestCode)
        }

        override fun onStatusChanged(provider: String?, status: Int, extras: Bundle?) {
            super.onStatusChanged(provider, status, extras)
        }

        override fun onProviderEnabled(provider: String) {
            super.onProviderEnabled(provider)
        }

        override fun onProviderDisabled(provider: String) {
            super.onProviderDisabled(provider)
        }
    }
}

LocationService继承LifecycleService：

class LocationService: LifecycleService() {

   init {
       lifecycle.addObserver(LocationObserver(this))
   }
}

启动服务：

class Test2Activity : AppCompatActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_test2)

        startService(Intent(this,LocationService::class.java))
    }
}

使用ProcessLifecycleOwner监听应用程序生命周期

• 针对整个应用程序的监听，与Activity数量无关
• Lifecycle.Event.ON_CREATE只会被调用一次，Lifecycle.Event.ON_DESTROY永远不会被调用

class App: Application() {
    override fun onCreate() {
        super.onCreate()
        ProcessLifecycleOwner.get().lifecycle.addObserver(AppLifecycleObserver())
    }
}

class AppLifecycleObserver : LifecycleObserver {
    @OnLifecycleEvent(Lifecycle.Event.ON_CREATE)
    fun onCreate() {
        Log.d("AppLifecycleObserver", "Lifecycle.Event.ON_CREATE")
    }

    @OnLifecycleEvent(Lifecycle.Event.ON_START)
    fun onStart() {
        Log.d("AppLifecycleObserver", "Lifecycle.Event.ON_START")
    }

    @OnLifecycleEvent(Lifecycle.Event.ON_RESUME)
    fun onResume() {
        Log.d("AppLifecycleObserver", "Lifecycle.Event.ON_RESUME")
    }

    @OnLifecycleEvent(Lifecycle.Event.ON_PAUSE)
    fun onPause() {
        Log.d("AppLifecycleObserver", "Lifecycle.Event.ON_PAUSE")
    }

    @OnLifecycleEvent(Lifecycle.Event.ON_STOP)
    fun onStop() {
        Log.d("AppLifecycleObserver", "Lifecycle.Event.ON_STOP")
    }

    @OnLifecycleEvent(Lifecycle.Event.ON_DESTROY)
    fun onDestroy() {
        Log.d("AppLifecycleObserver", "Lifecycle.Event.ON_DESTROY")
    }
}

前后台切换App输出：
27761-27761 D/AppLifecycleObserver: Lifecycle.Event.ON_STOP
27994-27994 D/AppLifecycleObserver: Lifecycle.Event.ON_CREATE
27994-27994 D/AppLifecycleObserver: Lifecycle.Event.ON_START
27994-27994 D/AppLifecycleObserver: Lifecycle.Event.ON_RESUME
27994-27994 D/AppLifecycleObserver: Lifecycle.Event.ON_PAUSE
27994-27994 D/AppLifecycleObserver: Lifecycle.Event.ON_STOP