Android Jetpack - Lifecycles

Posted 2022-02-02 teletian

◾︎简介

Lifecycles 是一个持有组件生命周期状态（如 Activity、Fragment）信息的类，并允许其他对象观察此状态。可以帮助我们方便的管理 Activity 和 Fragment 的生命周期。通过使用 Lifecycles 组件，可以将依赖组件的代码从生命周期方法移入组件本身中。有助于写出更有条理且往往更精简的代码，这样的代码更易于维护。

生命周期的状态和事件如下图

本文代码使用 Kotlin 讲解，若需查看 Java 代码写法，请参考文末 Sample

◾︎添加依赖

def lifecycle_version = "2.3.1"

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

◾︎简单例子

Lifecycles 是通过观察者模式实现的。
先定义一个观察者类继承自 LifecycleObserver，然后在 Activity 中注册这个观察者。
Activity 必须继承自 androidx.activity.ComponentActivity 或它的子类（AppCompatActivity 就是这样的类），因为 ComponentActivity 实现了 LifecycleOwner 接口，否则要自己实现并添加相关代码

class MainActivity : AppCompatActivity() 
    override fun onCreate(savedInstanceState: Bundle?) 
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        Log.i(javaClass.name, "onCreate")

        lifecycle.addObserver(MyObserver())
    

    override fun onStart() 
        super.onStart()
        Log.i(javaClass.name, "onStart")
    

    override fun onResume() 
        super.onResume()
        Log.i(javaClass.name, "onResume")
    

    override fun onPause() 
        super.onPause()
        Log.i(javaClass.name, "onPause")
    

    override fun onStop() 
        super.onStop()
        Log.i(javaClass.name, "onStop")
    

    override fun onDestroy() 
        super.onDestroy()
        Log.i(javaClass.name, "onDestroy")
    

    class MyObserver : LifecycleObserver 
        @OnLifecycleEvent(Lifecycle.Event.ON_CREATE)
        fun onCreate() 
            Log.i(javaClass.name, "onCreate")
        

        @OnLifecycleEvent(Lifecycle.Event.ON_START)
        fun onStart() 
            Log.i(javaClass.name, "onStart")
        

        @OnLifecycleEvent(Lifecycle.Event.ON_RESUME)
        fun onResume() 
            Log.i(javaClass.name, "onResume")
        

        @OnLifecycleEvent(Lifecycle.Event.ON_PAUSE)
        fun onPause() 
            Log.i(javaClass.name, "onPause")
        

        @OnLifecycleEvent(Lifecycle.Event.ON_STOP)
        fun onStop() 
            Log.i(javaClass.name, "onStop")
        

        @OnLifecycleEvent(Lifecycle.Event.ON_DESTROY)
        fun onDestroy() 
            Log.i(javaClass.name, "onDestroy")
        
    

◾︎Presenter 例子

如果不使用 Lifecycles 管理，必须在 Activity 的每个生命周期里面调用 Presenter 的方法，代码如下：

interface BasePresenter 
    fun onCreate()
    fun onStart()
    fun onResume()
    fun onStop()
    fun onDestroy()

class MainPresenter : BasePresenter 

    override fun onCreate() 
        Log.i(javaClass.name, "onCreate")
    

    override fun onStart() 
        Log.i(javaClass.name, "onStart")
    

    override fun onResume() 
        Log.i(javaClass.name, "onResume")
    

    override fun onStop() 
        Log.i(javaClass.name, "onStop")
    

    override fun onDestroy() 
        Log.i(javaClass.name, "onDestroy")
    

class MainActivity : AppCompatActivity() 
    private val presenter: MainPresenter = MainPresenter()

    override fun onCreate(savedInstanceState: Bundle?) 
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        Log.i(javaClass.name, "onCreate")
        presenter.onCreate()
    

    override fun onStart() 
        super.onStart()
        Log.i(javaClass.name, "onStart")
        presenter.onCreate()
    

    override fun onResume() 
        super.onResume()
        Log.i(javaClass.name, "onResume")
        presenter.onCreate()
    

    override fun onPause() 
        super.onPause()
        Log.i(javaClass.name, "onPause")
        presenter.onCreate()
    

    override fun onStop() 
        super.onStop()
        Log.i(javaClass.name, "onStop")
        presenter.onCreate()
    

    override fun onDestroy() 
        super.onDestroy()
        Log.i(javaClass.name, "onDestroy")
        presenter.onCreate()
    

那么怎么使用 Lifecycles 管理 Presenter 周期呢？
改造 MainPresenter，让它实现 LifecycleObserver 接口，并为每个生命周期函数添加 @OnLifecycleEvent 事件

class MainPresenter : BasePresenter, LifecycleObserver 

    @OnLifecycleEvent(Lifecycle.Event.ON_CREATE)
    override fun onCreate() 
        Log.i(javaClass.name, "onCreate")
    

    @OnLifecycleEvent(Lifecycle.Event.ON_START)
    override fun onStart() 
        Log.i(javaClass.name, "onStart")
    

    @OnLifecycleEvent(Lifecycle.Event.ON_RESUME)
    override fun onResume() 
        Log.i(javaClass.name, "onResume")
    

    @OnLifecycleEvent(Lifecycle.Event.ON_STOP)
    override fun onStop() 
        Log.i(javaClass.name, "onStop")
    

    @OnLifecycleEvent(Lifecycle.Event.ON_DESTROY)
    override fun onDestroy() 
        Log.i(javaClass.name, "onDestroy")
    

MainActivity 中不需要每个生命周期函数调用 Presenter 了，直接注册 LifecycleObserver 即可

class MainActivity : AppCompatActivity() 

    private val presenter: MainPresenter = MainPresenter()

    override fun onCreate(savedInstanceState: Bundle?) 
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        Log.i(javaClass.name, "onCreate")
        lifecycle.addObserver(presenter)
    

    override fun onStart() 
        super.onStart()
        Log.i(javaClass.name, "onStart")
    

    override fun onResume() 
        super.onResume()
        Log.i(javaClass.name, "onResume")
    

    override fun onPause() 
        super.onPause()
        Log.i(javaClass.name, "onPause")
    

    override fun onStop() 
        super.onStop()
        Log.i(javaClass.name, "onStop")
    

    override fun onDestroy() 
        super.onDestroy()
        Log.i(javaClass.name, "onDestroy")
    

◾︎监听 Application 生命周期

Android 里面没有提供应用进入后台以及回到前台的回调方法。一般的做法是通过registerActivityLifecycleCallbacks 方法，然后再 callback 的 onActivityStarted 中计数加 1，onActivityStopped 中计数减 1，从而判断前后台状态。

有了 Lifecycle，并通过 ProcessLifecycleOwner 可以更简单的实现。

添加依赖

implementation 'androidx.lifecycle:lifecycle-process:2.3.1'

像使用 Livecycle 一样使用 ProcessLifecycleOwner。

import android.app.Application
import android.util.Log
import androidx.lifecycle.Lifecycle
import androidx.lifecycle.LifecycleObserver
import androidx.lifecycle.OnLifecycleEvent
import androidx.lifecycle.ProcessLifecycleOwner

class App : Application() 

    override fun onCreate() 
        super.onCreate()

        ProcessLifecycleOwner.get().lifecycle.addObserver(ApplicationLifecycleObserver())
    

    inner class ApplicationLifecycleObserver : LifecycleObserver 

        @OnLifecycleEvent(Lifecycle.Event.ON_START)
        fun onAppForeground() 
            Log.i("tianjf", "App foreground")
        

        @OnLifecycleEvent(Lifecycle.Event.ON_STOP)
        fun onAppBackground() 
            Log.i("tianjf", "App background")
        
    

◾︎Sample

Kotlin 版
Java 版