Jetpack 之 Lifecycle开启掘金成长之旅！这是我参与「掘金日新计划 · 12 月更文挑战」的第1天，点击查

开启掘金成长之旅！这是我参与「掘金日新计划 · 12 月更文挑战」的第1天，点击查看活动详情

Lifecycle简介

Lifecycle是一个生命周期感知组件，一般用来响应Activity、Fragment等组件的生命周期变化，并将变化通知到已注册的观察者。有助于更好地组织代码，让代码逻辑符合生命周期规范，减少内存泄漏，增强稳定性。

可以和Lifecycle一起工作的类我们称之为有生命周期意识的控件。我们建议那些提供了需要和Android生命周期打交道的类的库最好提供有生命周期意识的控件，这样它们的用户就能轻松的集成这些类，而不是在客户端手动管理生命周期。

LiveData 就是一个有生命周期意识的控件的例子。使用LiveData 和 ViewModel 可以让数据部署到UI更简单。

Lifecycle已经纳入新版本的AppCompatActivity和Fragment中了，并且Lifecycle还是Android Jetpack中其他两个组件LiveData和ViewModel的基础

Lifecycle最佳实践

尽可能保持UI控制器（Activity和Fragment）的简洁。它们不应该去获取数据，而是使用ViewModel 来做这个工作，然后观察LiveData 把变化反应给view。

尝试写数据驱动的UI，UI controller的职责是在数据改变的时候更新view，或者把用户的操作通知给ViewModel。

把数据逻辑放在ViewModel 类中。ViewModel的角色是UI控制器与app其余部分的桥梁。不过要注意，ViewModel的职责并不是获取数据（比如，从网络）。相反 ViewModel应该调用合适的控件去做这件事情，然后把结果提供给UI控制器。

使用 Data Binding来让view和UI控制器之间的接口保持干净。这可以让你的view更加声明式同时最小化Activity和Fragment中的代码。如果你更喜欢用Java代码做这件事情，使用 Butter Knife来避免繁琐的代码。

如果你的UI非常复杂，考虑创建一个Presenter类来处理UI的变动。通常这会有点多余，但可能会让UI的测试更加简单。

绝对不要在 ViewModel中引用View 或者 Activity 的context。如果ViewModel活的比Activity更长，Activity可能会泄漏，无法正常回收。

使用Lifecycle解耦页面与组件

1.倒计时

class CountDownTimerLifecycle(btnCount:Button) : LifecycleObserver {

    //总时间和倒计时间隔时间
    val dSecond: Long = 1000
    private var countDownTimer: CountDownTimer = object : CountDownTimer(10 * dSecond, 1 * dSecond) {
            override fun onFinish() {
                Log.i("zx", "倒计时完成")
                btnCount.text = "倒计时完成"
            }

            override fun onTick(millisUntilFinished: Long) {
                Log.i("zx", "倒计时:${millisUntilFinished / 1000}")
                btnCount.text = "${millisUntilFinished / 1000}"
            }
        }

    @OnLifecycleEvent(Lifecycle.Event.ON_RESUME)
    fun startCount() {
        countDownTimer.start()
        Log.i("zx", "on_resume")
    }


    @OnLifecycleEvent(Lifecycle.Event.ON_PAUSE)
    fun stopCount() {
        countDownTimer.cancel()
        Log.i("zx", "on_pause")
    }
}

2.广播

class BroadCastLifecycle(private val activity: AppCompatActivity) : LifecycleObserver {

    private val myBroadcastReceiver by lazy { MyBroadcastReceiver() }


    @OnLifecycleEvent(Lifecycle.Event.ON_RESUME)
    fun lifeOnResume() {
        Log.v("zx", "注册广播")
        val intentFilter = IntentFilter("zxmsg")
        activity.registerReceiver(myBroadcastReceiver, intentFilter)
    }

    @OnLifecycleEvent(Lifecycle.Event.ON_PAUSE)
    fun lifeOnPause() {
        Log.v("zx", "取消注册广播")
        activity.unregisterReceiver(myBroadcastReceiver)
    }
}

使用LifecycleService解耦Service与组件

class LocationService : LifecycleService() {

    override fun onCreate() {
        super.onCreate()
        lifecycle.addObserver(LocationObserver(this))
    }

}

class LocationObserver(val context: Context) : LifecycleObserver {


    @OnLifecycleEvent(Lifecycle.Event.ON_CREATE)
    fun startLocation() {
        Log.v("zx","startLocation")
    }

    @OnLifecycleEvent(Lifecycle.Event.ON_DESTROY)
    fun stopLocation() {
        Log.v("zx","stopLocation")
    }
}

        binding.btnStartService.setOnClickListener {
            startService(Intent(this, LocationService::class.java))
        }
        binding.btnStopService.setOnClickListener {
            stopService(Intent(this, LocationService::class.java))
        }

    <service android:name=".lifecycle.LocationService"/>

使用ProcessLifecycleOwner监听应用程序生命周期

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

class ApplicationObserver:LifecycleObserver {


    @OnLifecycleEvent(Lifecycle.Event.ON_CREATE)
    fun onCreate(){
        Log.v("zx","Application  onCreate")
    }

}

新版本AppCompatActivity和Fragment已经实现了LifecycleOwner，如果没实现需要怎么做呢下面使用Activity实现LifecycleOwner

class TestActivity : Activity(), LifecycleOwner {

    private val mLifecycleRegistry = LifecycleRegistry(this)

    override fun onCreate(savedInstanceState: Bundle?) {
        mLifecycleRegistry.handleLifecycleEvent(Lifecycle.Event.ON_CREATE)
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_test)

        initLifecycle()
    }

    private fun initLifecycle() {
        lifecycle.addObserver(MyLifecycle())
    }


    override fun onStart() {
        super.onStart()
        mLifecycleRegistry.handleLifecycleEvent(Lifecycle.Event.ON_START)
    }

    override fun onResume() {
        super.onResume()
        mLifecycleRegistry.handleLifecycleEvent(Lifecycle.Event.ON_RESUME)
    }

    override fun onPause() {
        mLifecycleRegistry.handleLifecycleEvent(Lifecycle.Event.ON_PAUSE)
        super.onPause()
    }

    override fun onStop() {
        mLifecycleRegistry.handleLifecycleEvent(Lifecycle.Event.ON_STOP)
        super.onStop()
    }

    override fun onDestroy() {
        mLifecycleRegistry.handleLifecycleEvent(Lifecycle.Event.ON_DESTROY)
        super.onDestroy()
    }

    override fun getLifecycle(): Lifecycle {
        return mLifecycleRegistry
    }
}

class MyLifecycle : LifecycleObserver {

    @OnLifecycleEvent(Lifecycle.Event.ON_CREATE)
    fun myOnCreate() {
        Log.v("zx", "myOnCreate")
    }

    @OnLifecycleEvent(Lifecycle.Event.ON_RESUME)
    fun myOnResume() {
        Log.v("zx", "myOnResume")

    }

    @OnLifecycleEvent(Lifecycle.Event.ON_PAUSE)
    fun myOnPause() {
        Log.v("zx", "myOnPause")
    }
}

这样TestActivity 里面的生命执行的时候MyLifecycle 也会执行

Lifecycle官方库

lifecycle-extensions 中的 API 已弃用。您可以为特定 Lifecycle 工件添加所需的依赖项。

    dependencies {
        def lifecycle_version = "2.4.0"
        def arch_version = "2.1.0"

        // ViewModel
        implementation "androidx.lifecycle:lifecycle-viewmodel:$lifecycle_version"
        // LiveData
        implementation "androidx.lifecycle:lifecycle-livedata:$lifecycle_version"
        // Lifecycles only (without ViewModel or LiveData)
        implementation "androidx.lifecycle:lifecycle-runtime:$lifecycle_version"

        // Saved state module for ViewModel
        implementation "androidx.lifecycle:lifecycle-viewmodel-savedstate:$lifecycle_version"

        // Annotation processor
        annotationProcessor "androidx.lifecycle:lifecycle-compiler:$lifecycle_version"
        // alternately - if using Java8, use the following instead of lifecycle-compiler
        implementation "androidx.lifecycle:lifecycle-common-java8:$lifecycle_version"

        // optional - helpers for implementing LifecycleOwner in a Service
        implementation "androidx.lifecycle:lifecycle-service:$lifecycle_version"

        // optional - ProcessLifecycleOwner provides a lifecycle for the whole application process
        implementation "androidx.lifecycle:lifecycle-process:$lifecycle_version"

        // optional - ReactiveStreams support for LiveData
        implementation "androidx.lifecycle:lifecycle-reactivestreams:$lifecycle_version"

        // optional - Test helpers for LiveData
        testImplementation "androidx.arch.core:core-testing:$arch_version"
    }

知识点1：



    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
      
        lifecycleScope.launch {
            //单次触发
            whenCreated {}
            whenStarted {}
            whenResumed {}
            lifecycleScope.launchWhenCreated {}
            lifecycleScope.launchWhenStarted {}
            lifecycleScope.launchWhenResumed {}

            //多次onResume触发
            viewLifecycleOwner.lifecycle.repeatOnLifecycle(Lifecycle.State.RESUMED) {
                val id = BWallet.get().getCurrentWallet()?.id ?: 0
                val pWallet = LitePal.find(PWallet::class.java, id, true)
                refreshWallet(pWallet)
            }
        
 }