◾︎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")
}
}
}
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