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Android Lifecycle优化

厨师小p 人气:0

前言

你是否在onStart()启动过某项任务却忘记在onStop()中取消呢?人不是机器,难免会有错漏。就算老手不会犯错,也不能保证新人不会。学会下面的小技巧,让这种粗心成为不可能。

关于Lifecycle的源码,已经有很多大佬分析过。这篇文章的主旨是让读者对Lifecycle的使用场景有更多的体会,这样也能更好地理解源码。先来看一个场景,然后一步一步优化。

场景

假设我们有一个界面,模拟一个厨房。里面有灶台和餐桌。要求每秒钟翻炒一下,总共10秒。一种常规的实现如下:

class KitchenFragment : Fragment() {
    private var timer: CountDownTimer? = null
    override fun onResume() {
        ...
        timer = object : CountDownTimer(COOKING_TIME_IN_MILLIS, SECOND_IN_MILLIS) {
            override fun onTick(millisUntilFinished: Long) {
                // 翻炒
            }
            override fun onFinish() { 
                // 出锅 
            }
        }
        timer.start()
    }
    
    override fun onPause() {
        timer?.cancel()
        ...
    }
    
    compaion object {
        private const val COOKING_TIME_IN_MILLIS = 10000L
    }
}

潜在问题:

优化版本1

先解决第一个问题,把CountDownTimer放到一个单独的class。

class KitchenFragment : Fragment() {
    private val timer: CountDownTimer? = null
    override fun onResume() {
        ...
        timer = MyCountDownTimer(onTickAction = { 翻炒 },onFinishAction = { 出锅 })
        timer.start()
    }
    
    override fun onPause() {
        timer?.cancel()
        ...
    }
}

// MyCountDownTimer.kt
class MyCountDownTimer@JvmOverloads constuctor(
    millisUntilFinished: Long = DEFAULT_DURATION_IN_MILLIS,
    countDownInterval: LONG = SECOND_IN_MILLIS,
    private val onTickAction: () -> Unit,
    private val onFinishAction: () -> Unit = {}
) : CountDownTimer(millisUntilFinished, countDownInterval) {

    override fun onTick(millisUntilFinished: Long) {
        onTickAction.invoke()
    }
    
    override fun onFinish() {
        onFinishAction.invoke()
    }
    
    compaion object {
        private const val DEFAULT_DURATION_IN_MILLIS = 10000L
    }
}

需要复用时,只需传入需要改动的参数/方法:

// NeighbourKitchenFragment.kt
class NeighbourKitchenFragment : Fragment() {
    private val timer: CountDownTimer? = null
    override fun onResume() {
        ...
        timer = MyCountDownTimer(onTickAction = { 翻炒 },onFinishAction = { 甩锅 })
        timer.start()
    }
    
    override fun onPause() {
        timer?.cancel()
        ...
    }
}

复用起来好像方便了一点,但是当上面提到过的的问题3出现时,代码会变成:

class KitchenFragment : Fragment() {
    private val cookTimer1: CountDownTimer? = null
    private val cookTimer2: CountDownTimer? = null
    private val sweepTableTimer: CountDownTimer? = null
    override fun onResume() {
        ...
        cookTimer1 = MyCountDownTimer(onTickAction = { 翻炒 },onFinishAction = { 出锅 })
        cookTimer1.start()
        
        cookTimer2 = MyCountDownTimer(millisUntilFinished = BRITAIN_SPOON_DURATION_IN_MILLIS,onTickAction = { 颠勺 })
        cookTimer2.start()
        
        sweepTableTimer = MyCountDownTimer(millisUntilFinished = SWEEP_TABLE_DURATION_IN_MILLIS,onTickAction = { 擦桌子 })
        sweepTableTimer.start()
    }
    
    override fun onPause() {
        cookTimer1?.cancel()
        cookTimer2?.cancel()
        sweepTableTimer?.cancel()
        ...
    }
    
    compaion object {
        private const val BRITAIN_SPOON_DURATION_IN_MILLIS = 5000L
        private const val SWEEP_TABLE_DURATION_IN_MILLIS = 20000L
    }
}

随着需求增加,Fragment变得越来越长,也更难维护。同时,当在onResume中添加timer时被同事打断,之后就有可能会忘记在onPause中cancel()。有没有办法解决这些问题呢?

接下来切入正题,让我们看看Lifecycle能做什么。

优化版本2

首先让MyCountDownTimer实现DefaultLifecycleObserver,这样它就是lifecycle-aware的了。这有什么用呢?有了这个,MyCountDownTimer就能在fragment/activity生命周期发生变化的时候得到通知并在内部处理cancel()等操作。

// MyCountDownTimer.kt
// Lifecycle-aware CountDownTimer
class MyCountDownTimer@JvmOverloads constuctor(
    millisUntilFinished: Long = DEFAULT_DURATION_IN_MILLIS,
    countDownInterval: LONG = SECOND_IN_MILLIS,
    private val onTickAction: () -> Unit,
    private val onFinishAction: () -> Unit = {}
) : CountDownTimer(millisUntilFinished, countDownInterval), DefaultLifecycleObserver {
    override fun onTick(millisUntilFinished: Long) {
        onTickAction.invoke()
    }
    
    override fun onFinish() {
        onFinishAction.invoke()
    }
    
    // onResume时自动开始
    override fun onResume(owner: LifecycleOwner) {
        start()
    }
    
    // onPause时自动取消
    override fun onPause(owner: LifecycleOwner) {
        cancel()
    }
    
    // onDestroy时停止观察
    override fun onDestroy(owner: LifecycleOwner) {
        owner.lifecycle.removeObserver(this)
    }
    
    compaion object {
        private const val DEFAULT_DURATION_IN_MILLIS = 10000L
    }
}

上面例子中的KitchenFragment将会变成这样:

class KitchenFragment : Fragment() {
    override fun onCreate() {
        ...
        initTimer()
    }
    
    private fun initTimer() {
        // 翻炒任务
        val timer1 = MyCountDownTimer(onTickAction = { 翻炒 },onFinishAction = { 出锅 })
        // 颠勺任务
        val timer2 = MyCountDownTimer(millisUntilFinished = BRITAIN_SPOON_DURATION_IN_MILLIS,onTickAction = { 颠勺 })
        // 擦桌任务
        val timer3 = MyCountDownTimer(millisUntilFinished = SWEEP_TABLE_DURATION_IN_MILLIS,onTickAction = { 擦桌子 })
        
        viewLifecycleOwner.lifecycle.apply {
            addObserver(timer1)
            addObserver(timer2)
            addObserver(timer3)
        }
    }
    
    compaion object {
        private const val BRITAIN_SPOON_DURATION_IN_MILLIS = 5000L
        private const val SWEEP_TABLE_DURATION_IN_MILLIS = 20000L
    }
}

在Fragment中只需要专注于添加需要的功能,不用操心取消任务与停止观察。既清爽又不容易犯错。

单元测试

因为逻辑代码都封装在MyCountDownTimer,主要测试这个class就可以了。不需要给每一个使用MyCountDownTimer的Fragment都写详细的测试。

只需要mock一个LifecycleOwner就足够,也不需要启动一个mock Fragment。

class MyCountDownTimerTest {
    private lateinit var timer: MyCountDownTimer
    private lateinit var lifeCycle: LifecycleRegistry
    @Before
    fun setUp() {
        val lifeCycleOwner: LifecycleOwner = mock(LifecycleOwner::class.java)
        lifeCycle = LifecycleRegistry(lifeCycleOwner)
        timer = MyCountDownTimer(onTickAction = { 翻炒 },onFinishAction = { 出锅 })
        lifeCycle.addObserver(timer)

        lifeCycle.handleLifecycleEvent(Lifecycle.Event.ON_CREATE)
    }

    @Test
    fun timerActionExecuted() {
        lifeCycle.markState(Lifecycle.State.RESUMED)
        // 检测是否开始翻炒,出锅
        ...
    }
}

总结

通过把重复的代码和逻辑封装在自定义的LifecycleObserver内部,不仅可以给Activity/Fragment“瘦身”,防止忘记在onStop()/onDestroy()中收拾,还可以使复用代码更加方便。同时也遵循设计模式,降低了Fragment与Timer之间的耦合度,让代码更好维护。

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