什么是生命周期观察器以及如何正确使用它？

Question

我在 Android 中阅读了有关新架构组件的信息。所以，我想问一下什么是生命周期观察者，我们为什么需要它们？在什么情况下有用？感谢您的回答！

Answer 1

您可以使用 ProcessLifecycleOwner 获取应用程序的生命周期并添加一个 class 作为这些事件的观察者。您可以在您的应用程序中实现 LifecycleObserver Class:

public class MyApplication extends MultiDexApplication implements LifecycleObserver

@Override
public void onCreate() {
    super.onCreate();

    ProcessLifecycleOwner.get().getLifecycle().addObserver(this);

}

// 添加这些生命周期方法以观察您的应用何时进入后台或前台：

@OnLifecycleEvent(Lifecycle.Event.ON_RESUME)
public void appInResumeState() {
    Toast.makeText(this,"In Foreground",Toast.LENGTH_LONG).show();
}

@OnLifecycleEvent(Lifecycle.Event.ON_PAUSE)
public void appInPauseState() {
    Toast.makeText(this,"In Background",Toast.LENGTH_LONG).show();
}

// 在您的 build.gradle 文件中添加以下内容

implementation 'android.arch.lifecycle:extensions:1.1.1'

//也在活动或片段中

您还可以使用它们来降低代码的复杂性，方法是创建实现 LifecycleObserver 的不同组件，然后将 activity 的生命周期传递给这些组件。这样您就可以将巨大的复杂性分解为不同的组件。

class MainActivity : AppCompatActivity(), LifecycleObserver {

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        ReduceComplexComponent().registerLifecycle(lifecycle)

    }

}

class ReduceComplexComponent : LifecycleObserver{

    registerLifecycle(lifecycle : Lifecycle){
       lifecycle.addObserver(this)
    }

    @OnLifecycleEvent(Lifecycle.Event.ON_RESUME)
    fun resume() {
       Log.d("OnResume","ON_RESUME")
    }

    @OnLifecycleEvent(Lifecycle.Event.ON_PAUSE)
    fun pause() {
       Log.d("onPause","ON_PAUSE")
    }
}

这样您就可以在单独的组件中监听 activity 或片段生命周期事件。

我们还可以在 Activity 中手动获取生命周期实例的当前状态，我们可以使用它的 getCurrentState()

State 也有一个 isAtLeast() 方法，我们可以使用它来与当前生命周期状态进行比较

Answer 2

LifeCycleObserver is part of Google released Android Jetpack LifeCycle Architecture components, and it is an interface that allows you to observe a LifeCycle-aware observable component, typically a LifeCycleOwner (Activity/Fragment), in order to interact with the LifeCycle events and states 关联到此组件；这样您就可以监控前台和后台生命周期事件。

以下是一些具有典型用法的有用链接

Answer 3

派对可能有点晚了，但另一个很好的生命周期用例（除了明显的 ViewModel 东西）是让应用程序的许多组件在相关 activity 正在被摧毁，或者只是在屏幕之外。

例如，我有一个创建对话框的静态工厂，并且使用生命周期我可以关闭对话框而不用像 activity 这样的旧东西弄乱主机=12=] 和 void onPause(){ ... if (mDialog !null && mDialog.isShowing()) mDialog.cancel() }

一些代码：

DialogUtils.java:

public static void showConfirmDialog(Lifecycle lifecycle, String title, String msg, Runnable okRunnable) {
    AlertDialog dialog = AlertDialog.Builder(mAppContext)
    /* configuration stuff ... */
        .build();

    dialog.show();

    lifecycle.addObserver(new LifecycleObserver() {
          @OnLifecycleEvent(Lifecycle.Event.ON_PAUSE)
          public void cancelDialog() {
            if (dialog.isShowing()) { // if not already dismissed by main-button tap 
              dialog.cancel();
            }
          }
    });
}

MyActivity.java:

public class MyActivity extends AppCompatActivity {

    /* stuff... onCreate()... other stuff... */

    private void confirmDeleteUser(User user){
        DialogUtils.showConfirmDialog(
            MyActivity.this.getLifecycle(), // all activities & fragment have lifecycles
            "Confirm Delete",
            "Action cannot be undone. Sure to continue?",
            new Runnable() { /* stuff... */ }
        );
        // Voilà! 
        // activity no needs to store reference to the dialog and cancel manually on pause
        // it's a fire-and-forget action
    }
}

Answer 4

您使用它们是为了减少由于您的应用 Lifecycle events 而引起的组件回调和清理。例如，你有一个 Handler/Runnable 是运行来自某处的 Thread 并且你需要一些 callbacks/interfaces 以便在 onStop() 被调用；因此使用新的 "Jetpack LifeCycle Architecture components" 你可以使你的 component/class "lifecycle-aware" 从而从 Activity/Fragment 使用它轻松控制它！

更多信息如上所述：https://developer.android.com/topic/libraries/architecture/lifecycle

Answer 5

lifecycle-extensions API 已弃用。

lifecycle-extensions Artifact Deprecation: With the above deprecation of ViewModelProviders.of(), this release marks the deprecation of the last API in lifecycle-extensions and this artifact should now be considered deprecated in its entirety. We strongly recommend depending on the specific Lifecycle artifacts you need (such as lifecycle-service if you’re using LifecycleService and lifecycle-process if you’re using ProcessLifecycleOwner) rather than lifecycle-extensions as there will not be a future 2.3.0 release of lifecycle-extensions.

如果您想继续使用ProcessLifecycleOwner，建议添加这个依赖：

implementation "androidx.lifecycle:lifecycle-process:2.2.0"

参考：
https://developer.android.com/jetpack/androidx/releases/lifecycle#version_220_3

https://androidx.tech/artifacts/lifecycle/lifecycle-extensions/

https://developer.android.com/reference/androidx/lifecycle/ProcessLifecycleOwner

Answer 6

@OnLifecycleEvent 已弃用因此许多曾经在这里有用的答案不再有用。如果您目前尚未使用 Java 8，则需要先更新 build.gradle。我们现在可以使用 DefaultLifecycleObserver 或 LifecycleEventObserver。请参阅下面的示例。

首先，您可以使用 ProcessLifecycleOwner 获取应用程序的生命周期：

ProcessLifecycleOwner.get().getLifecycle()

DefaultLifecycleObserver:

lifecycle.addObserver(object: DefaultLifecycleObserver {
    override fun onResume(owner: LifecycleOwner) {
        super.onResume(owner)
        TODO()
    }
    override fun onPause(owner: LifecycleOwner) {
        TODO()
        super.onPause(owner)
    }
})

LifecycleEventObserver:

lifecycle.addObserver(object: LifecycleEventObserver {
    override fun onStateChanged(source: LifecycleOwner, event: Lifecycle.Event) {
        when (event) {
            Lifecycle.Event.ON_RESUME -> TODO()
            Lifecycle.Event.ON_PAUSE -> TODO()
            else -> { }
        }
    }
})

如何更新到 Java 8 以及为什么 OnLifecycleEvent 被弃用的解释：

什么是生命周期观察器以及如何正确使用它？

What is lifecycle observer and how to use it correctly?

java

android

mvvm

android-lifecycle

android-livedata

DialogUtils.java:

MyActivity.java: