一、Lifecycle是什麼？

　　Lifecycle生命週期感知元件，可執行操作響應另一個元件（Activity或者Fragment）的生命週期狀態。

二、Lifecycle出現的背景

　　用於解耦系統元件與其它元件的生命週期。

三、示例

　　App中都有開屏廣告，在開屏廣告右上角一個倒計時功能。倒計時功能需要和App進入後臺暫停，進入前臺繼續的。

　　在沒有Lifecycle之前，程式碼實現：

　　RCountDownTimeView實現

class RCountDownTimeView constructor(context: Context, attrs: AttributeSet?) :
    androidx.appcompat.widget.AppCompatTextView(context, attrs) {

    
 
private var _handler: Handler? = null
    private var _countDownRunnable: Runnable? = null
    private var _seconds: Int = 5

    init {
        _handler = Handler(Looper.getMainLooper())
    }

    private fun startCountDown() {
        if (_seconds <= 0) {
            stopCountDown()
            
 
return
        }

        _countDownRunnable = Runnable {
            this.text = "${--_seconds}"
            startCountDown()
        }
        _countDownRunnable?.let {
            _handler?.postDelayed(it, 1000)
        }
    }

    private fun stopCountDown() {
        _countDownRunnable?.let {
            _handler
 
?.removeCallbacks(it)
        }
        _countDownRunnable = null
    }

    fun setStartTime(seconds: Int) {
        _seconds = seconds
    }

    fun start() {
        startCountDown()
    }

    fun pause() {
        stopCountDown()
    }

    fun cancel() {
        stopCountDown()
        _handler?.removeCallbacksAndMessages(null)

        _handler = null

} }

　　Activity實現：

class MainActivity : AppCompatActivity() {

    private val _countDownTime: RCountDownTimeView by lazy {
        countDownTime
    }

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

        _countDownTime.setStartTime(10)
    }

    override fun onResume() {
        super.onResume()

        _countDownTime.start()
    }

    override fun onPause() {
        super.onPause()

        _countDownTime.pause()
    }

    override fun onDestroy() {
        super.onDestroy()

        _countDownTime.cancel()
    }

}

　　從上面例子可以看出，倒計時元件與Activity元件生命週期耦合嚴重。

　　使用Lifecycle後，實現：

　　RCountDownTimeView實現

class RCountDownTimeView constructor(context: Context, attrs: AttributeSet?) :
    androidx.appcompat.widget.AppCompatTextView(context, attrs), LifecycleObserver {

    private var _handler: Handler? = null
    private var _countDownRunnable: Runnable? = null
    private var _seconds: Int = 5

    init {
        _handler = Handler(Looper.getMainLooper())
    }

    private fun startCountDown() {
        if (_seconds <= 0) {
            stopCountDown()
            return
        }

        _countDownRunnable = Runnable {
            this.text = "${--_seconds}"
            startCountDown()
        }
        _countDownRunnable?.let {
            _handler?.postDelayed(it, 1000)
        }
    }

    private fun stopCountDown() {
        _countDownRunnable?.let {
            _handler?.removeCallbacks(it)
        }
        _countDownRunnable = null
    }

    fun setStartTime(seconds: Int) {
        _seconds = seconds
    }

    @OnLifecycleEvent(Lifecycle.Event.ON_RESUME)
    fun start() {
        startCountDown()
    }

    @OnLifecycleEvent(Lifecycle.Event.ON_PAUSE)
    fun pause() {
        stopCountDown()
    }

    @OnLifecycleEvent(Lifecycle.Event.ON_DESTROY)
    fun cancel() {
        stopCountDown()
        _handler?.removeCallbacksAndMessages(null)
    }

}

　　Activity實現：

class MainActivity : AppCompatActivity() {

    private val _countDownTime: RCountDownTimeView by lazy {
        countDownTime
    }

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

        lifecycle.addObserver(_countDownTime)
        _countDownTime.setStartTime(10)

    }

    override fun onDestroy() {
        super.onDestroy()

        lifecycle.removeObserver(_countDownTime)
    }

}

　　在使用LifeCycle後，與系統元件Activity耦合降低，程式碼出更簡潔。

四、Lifecycle工作原理

五、LifecycleService

六、ProcessLifecycleOwner