先搬一下官方示例，定位功能。

    internal class MyLocationListener(
            private val context: Context,
            private val callback: (Location) -> Unit
    ) {

        fun start() {
            // connect to system location service
        }

        fun stop() {
            // disconnect from system location service
        }
    }

    class MyActivity : AppCompatActivity() {
        private lateinit var myLocationListener: MyLocationListener

        override fun onCreate(...) {
            myLocationListener = MyLocationListener(this) { location ->
                // update UI
            }
        }

        public override fun onStart() {
            super.onStart()
            myLocationListener.start()
            // manage other components that need to respond
            // to the activity lifecycle
        }

        public override fun onStop() {
            super.onStop()
            myLocationListener.stop()
            // manage other components that need to respond
            // to the activity lifecycle
        }
    }

类似的一些组件需要感知Activity/Fragment生命周期时，需要写对应回调的样板代码。下面来看看引入LifeCycle后有什么不同，这里以MVP中Presenter为例。

IPresenter实现LifecycleObserver接口

interface IPresenter : LifecycleObserver {

    @OnLifecycleEvent(Lifecycle.Event.ON_CREATE)
    fun onCreate(@NotNull owner: LifecycleOwner)

    @OnLifecycleEvent(Lifecycle.Event.ON_DESTROY)
    fun onDestroy(@NotNull owner: LifecycleOwner)

    @OnLifecycleEvent(Lifecycle.Event.ON_ANY)
    fun onLifecycleChanged(@NotNull owner: LifecycleOwner, @NotNull event: Lifecycle.Event)
}

BasePresenter在对应的生命周期处理View

package com.chenxuan.common.base

import androidx.lifecycle.Lifecycle
import androidx.lifecycle.LifecycleOwner
import java.lang.ref.WeakReference

abstract class BasePresenter<V : IView>(view: IView) : IPresenter {
    @Suppress("UNCHECKED_CAST")
    protected var mViewRef: WeakReference<V>? = WeakReference(view as V)

    override fun onCreate(owner: LifecycleOwner) {

    }

    override fun onDestroy(owner: LifecycleOwner) {
        mViewRef?.clear()
    }

    override fun onLifecycleChanged(owner: LifecycleOwner, event: Lifecycle.Event) {

    }
}

BaseActivity

override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        presenter = createPresenter()
        lifecycle.addObserver(presenter)
    }

简单来说Activity/Fragment实现了LifecycleOwner接口，也就是生命周期拥有者；BasePresenter实现了LifecycleObserver接口观察生命周期变更；通过lifecycle.addObserver(presenter)关联，owner生命周期发生变化时会分发到已经订阅的observer。当然我们自己的类也可以通过LifeCycleRegistry成为LifeCycleOwner。

官方示例

    class MyActivity : Activity(), LifecycleOwner {

        private lateinit var lifecycleRegistry: LifecycleRegistry

        override fun onCreate(savedInstanceState: Bundle?) {
            super.onCreate(savedInstanceState)

            lifecycleRegistry = LifecycleRegistry(this)
            lifecycleRegistry.markState(Lifecycle.State.CREATED)
        }

        public override fun onStart() {
            super.onStart()
            lifecycleRegistry.markState(Lifecycle.State.STARTED)
        }

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