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jetpack 最新功能来了(Jetpack组件库)

jetpack 最新功能来了(Jetpack组件库)Support Library 26.1.0 版本以及之后的版本,AppCompatActivity和Fragment实现了LifecycleOwner。class MyLocationListener implements LifecycleObserver{ public MyLocationListener(Context context Callback callback) { // ... } @OnLifecycleEvent(Lifecycle.Event.ON_START) void start() { // 开始连接位置服务 } @OnLifecycleEvent(Lifecycle.Event.ON_STOP) void stop() { // 停止连接位置服务

引言

Jetpack 是一个由多个库组成的套件,可帮助开发者遵循最佳做法,减少样板代码并编写可在各种 Android 版本和设备中一致运行的代码,让开发者精力集中编写重要的代码

jetpack 最新功能来了(Jetpack组件库)(1)

Jetpack 是 Google 为解决 Android 开发碎片化,打造成熟健康生态圈提出的战略规划,是 Google 对 Android 未来提出的发展方向,同时它也是众多优秀 Android 组件的集合

Lifecycles 库是拿来干什么的

这个库从系统框架层去管理具有生命周期的组件,例如Activity fragment。让开发更方便地去管理自己应用里需要和activity或者fragment绑定的组件,让代码更容易维护。

也许有点抽象,举个例子说明一下,比如有个需求,需要在一个界面比较频繁更新地理位置信息。当Activity走了onstop之后,你应该也要暂停更新地理位置,或者当Activity走destroy后,你要释放一些资源

下面用一些代码实例解析一下,你的代码也许是这样的:

class MyLocationListener { public MyLocationListener(Context context callback callback) { // ... } void start() { // 开始连接位置服务 } void stop() { // 停止连接位置服务 } void destroy(){ //释放资源 } } class MyActivity extends AppCompatActivity { private MyLocationListener myLocationListener; @Override public void onCreate(...) { myLocationListener = new MyLocationListener(this new Callback(){ //回调更新UI }); } @Override public void onStart() { super.onStart(); myLocationListener.start(); //绑定actiivty的onStart周期函数 } @Override public void onStop() { super.onStop(); myLocationListener.stop(); //绑定actiivty的onStop周期函数 } @Override public void onDestroy() { super.onDestroy(); myLocationListener.destroy(); //绑定actiivty的onDestroy周期函数 } }

上面的代码在简单app看起来也许还好,但是当你activity业务逻辑比较多,可能包含很多和生命周期绑定的自定义组件 代码长期积累就很难维护啦。

下面在看看使用Lifecycles库的代码做对比:

class MyLocationListener implements LifecycleObserver{ public MyLocationListener(Context context Callback callback) { // ... } @OnLifecycleEvent(Lifecycle.Event.ON_START) void start() { // 开始连接位置服务 } @OnLifecycleEvent(Lifecycle.Event.ON_STOP) void stop() { // 停止连接位置服务 } @OnLifecycleEvent(Lifecycle.Event.ON_DESTROY) void destroy(){ //释放资源 } } class MyActivity extends AppCompatActivity { private MyLocationListener myLocationListener; @Override public void onCreate(...) { myLocationListener = new MyLocationListener(this new Callback(){ //回调更新UI }); getLifecycle().addObserver(myLocationListener); } } 复制代码

MyLocationListener实现LifecycleObserver 在相应的方法添加OnLifecycleEvent注解就可以收到相应的回调,在Activity的onCreate方法里调用 getLifecycle().addObserver(myLocationListener)即可。下面结合源码分析Lifecycles库 去更好地学习这个库。

Lifecycles 库核心类与结构

Support Library 26.1.0 版本以及之后的版本,AppCompatActivity和Fragment实现了LifecycleOwner。

Lifecycles库核心就是订阅者模式。

  • LifecycleOberver类:只是个空接口 安卓生命周期观察者
  • Lifecycle类: 是个抽象类,定义了安卓生命周期对象,有3个方法,添加观察者,移除观察者,获取当前状态
  • LifecycleOwner类: 是个接口 安卓生命周期的拥有者
  • LifecycleRegistry: Lifecycle的实现类,实现了添加、移除观察者,分派观察者状态等

自定义类实现LifecycleOberver接口,在方法中添加OnLifecycleEvent注解就可以收到相应生命周期的状态

public @interface OnLifecycleEvent { Lifecycle.Event value(); } public enum Event { ON_CREATE ON_START ON_RESUME ON_PAUSE ON_STOP ON_DESTROY ON_ANY } //Event是Lifecycle内部类一个枚举类 分别定义了onCreate onStart onResume onPause onStop onDestroy onAny这几个Event public enum state { DESTROYED INITIALIZED CREATED STARTED RESUMED; public boolean isAtLeast(@NonNull State state) { return compareTo(state) >= 0; } }//State也是Lifecycle内部类一个枚举类 定义了INITIALIZED CREATED STARTED RESUMED DESTROYED几种状态

Lifecycle的各个状态一共有5个,记录在枚举State中, 依次是DESTROYED INITIALIZED CREATED STARTED RESUMED;

代表分发的Event:

  • 当分发ON_CREATE事件时,State由INITIALIZED -> CREATED;
  • 当分发ON_START事件时 State由CREATED -> STARTED
  • 当分发ON_RESUME事件时 State由STARTED -> RESUMED
  • 当分发ON_PAUSE事件时 State由RESUMED -> STARTED
  • 当分发ON_STOP事件时 State由STARTED -> CREATED
  • 当分发ON_DESTROY事件时 State由CREATED -> DESTROYED

你会发现State没STOPED和PAUSED的状态 当State=CREATED时 Activity大概是在onCreate调用后或者onStop调用后;当State=STARTED时 Activity大概是在onStart调用后或者onPause调用后

ComponentActivity 分发 Event 的过程
  • 下面截取部分ComponentActivity的关键代码:

public class ComponentActivity extends Activity implements LifecycleOwner{ private LifecycleRegistry mLifecycleRegistry = new LifecycleRegistry(this); protected void onCreate(@Nullable Bundle savedInstanceState) { super.onCreate(savedInstanceState); ReportFragment.injectIfNeededIn(this); //利用Fragment来分发 } @CallSuper @Override protected void onSaveInstanceState(Bundle outState) { mLifecycleRegistry.markState(Lifecycle.State.CREATED); //onSaveInstanceState是用来恢复Activity状态的 这里记录的状态是CREATED super.onSaveInstanceState(outState); } @Override public Lifecycle getLifecycle() { return mLifecycleRegistry; //返回LifecycleRegistry } }

  • 下面再看ReportFragment类关键代码:

public static void injectIfNeededIn(Activity activity) { // 为当前activity add 一个ReportFragment 用于分发event android.app.FragmentManager manager = activity.getFragmentManager(); if (manager.findFragmentByTag(REPORT_FRAGMENT_TAG) == null) { manager.beginTransaction().add(new ReportFragment() REPORT_FRAGMENT_TAG).commit(); manager.executePendingTransactions(); } } @Override public void onActivityCreated(Bundle savedInstanceState) { super.onActivityCreated(savedInstanceState); dispatch(Lifecycle.Event.ON_CREATE); //分发ON_CREATE Event } @Override public void onStart() { super.onStart(); dispatch(Lifecycle.Event.ON_START);//分发ON_START Event } @Override public void onResume() { super.onResume(); dispatch(Lifecycle.Event.ON_RESUME);//分发ON_RESUME Event } @Override public void onPause() { super.onPause(); dispatch(Lifecycle.Event.ON_PAUSE);//分发ON_PAUSE Event } @Override public void onStop() { super.onStop(); dispatch(Lifecycle.Event.ON_STOP);//分发ON_STOP Event } @Override public void onDestroy() { super.onDestroy(); dispatch(Lifecycle.Event.ON_DESTROY);//分发ON_DESTROY Event } private void dispatch(Lifecycle.Event event) { Activity activity = getActivity(); // Activity是实现LifecycleOwner接口,这里可以跳过 if (activity instanceof LifecycleRegistryOwner) { ((LifecycleRegistryOwner) activity).getLifecycle().handleLifecycleEvent(event); return; } if (activity instanceof LifecycleOwner) { Lifecycle lifecycle = ((LifecycleOwner) activity).getLifecycle(); if (lifecycle instanceof LifecycleRegistry) { ((LifecycleRegistry) lifecycle).handleLifecycleEvent(event); //最终调用LifecycleRegistry.handleLifecycleEvent(event)处理event分发 } } }

ComponentActivity的是Event分发是通过添加一个ReportFragment 通过重写ReportFragment的onActivityCreated onStart onResume onStop onPause onDestroy方法,最终交给LifecycleRegistry.handleLifecycleEvent(event)处理。

Fragment 分发 Event 的过程
  • 下面也是截去相关v4里Fragment的相关源码

LifecycleRegistry mLifecycleRegistry = new LifecycleRegistry(this); @Override public Lifecycle getLifecycle() { return mLifecycleRegistry; } void performCreate(Bundle savedInstanceState) { ... onCreate(savedInstanceState); ... mLifecycleRegistry.handleLifecycleEvent(Lifecycle.Event.ON_CREATE); } void performStart() { ... onStart(); ... mLifecycleRegistry.handleLifecycleEvent(Lifecycle.Event.ON_START); } void performResume() { ... onResume(); ... mLifecycleRegistry.handleLifecycleEvent(Lifecycle.Event.ON_RESUME); } void performPause() { mLifecycleRegistry.handleLifecycleEvent(Lifecycle.Event.ON_PAUSE); ... onPause(); ... } void performStop() { mLifecycleRegistry.handleLifecycleEvent(Lifecycle.Event.ON_STOP); ... onStop(); ... } void performDestroy() { mLifecycleRegistry.handleLifecycleEvent(Lifecycle.Event.ON_DESTROY); ... onDestroy(); ... } 复制代码

LifecycleRegistry.handleLifecycleEvent(event)处理。 至于LifecycleRegistry这个类更多的细节就不展开啦

自定义 LifecycleOwner

前面提到Support Library 26.1.0 版本以及之后的版本,AppCompatActivity和Fragment实现了LifecycleOwner 如果你还用旧的版本或者继承Activity 你可以通过自定义Activity或者Fragment实现

  • 自定义实现代码如下:

//可以单独引入androidx.lifecycle:lifecycle-runtime:$lifecycle_version库 public class MyActivity extends Activity implements LifecycleOwner { private LifecycleRegistry mLifecycleRegistry; @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); mLifecycleRegistry = new LifecycleRegistry(this); mLifecycleRegistry.markState(Lifecycle.State.CREATED); } @Override public void onStart() { super.onStart(); mLifecycleRegistry.markState(Lifecycle.State.STARTED); } .... @NonNull @Override public Lifecycle getLifecycle() { return mLifecycleRegistry; } }使用 ProcessLifecycleOwner 监听整个 App 进程的前后台

要注意ON_CREATE的Event之后分发一次,ON_DESTROY不会分发

public class App extends Application { @Override public void onCreate() { super.onCreate(); ProcessLifecycleOwner.get().getLifecycle().addObserver(new LifecycleObserver() { private static final String TAG = "ProcessLifecycleOwner"; @OnLifecycleEvent(Lifecycle.Event.ON_CREATE) public void onCreate(){ Log.d(TAG "onCreate: "); //应用启动只被调用一次 } @OnLifecycleEvent(Lifecycle.Event.ON_START) public void onStart(){ Log.d(TAG "onStart: "); //应用启动会调用一次 从后台回来也会调用 } @OnLifecycleEvent(Lifecycle.Event.ON_RESUME) public void onResume(){ Log.d(TAG "onResume: "); //应用启动会调用一次 从后台回来也会调用 } @OnLifecycleEvent(Lifecycle.Event.ON_PAUSE) public void onPause(){ Log.d(TAG "onPause: "); //按home键或者切换应用会调用 } @OnLifecycleEvent(Lifecycle.Event.ON_STOP) public void onStop(){ Log.d(TAG "onStop: "); //按home键或者切换应用会调用 } }); }

要注意ON_PAUSE ON_STOP的回调会有700毫秒的延迟 官方的解析是保证不要由于配置更改而销毁和重新Activity时不会分发任何事件。还有一点,如果你的app是多进程应用,ProcessLifecycleOwner只能用来监听主进程。

  • 下面简单说一下ProcessLifecycleOwner的工作原理:

// ProcessLifecycleOwner 关键代码 public class ProcessLifecycleOwner implements LifecycleOwner{ private int mStartedCounter = 0; // 计数器 private int mResumedCounter = 0; // 计数器 void activityResumed() { mResumedCounter ; if (mResumedCounter == 1) { if (mPauseSent) { mRegistry.handleLifecycleEvent(Lifecycle.Event.ON_RESUME); mPauseSent = false; } else { mHandler.removeCallbacks(mDelayedPauseRunnable); } } } void activityPaused() { mResumedCounter--; if (mResumedCounter == 0) { mHandler.postDelayed(mDelayedPauseRunnable TIMEOUT_MS); } } void attach(Context context) { mHandler = new Handler(); //mRegistry是LifecycleRegistry对象,依靠LifecycleRegistry分发Event,不多说 mRegistry.handleLifecycleEvent(Lifecycle.Event.ON_CREATE); Application app = (Application) context.getApplicationContext(); //利用registerActivityLifecycleCallbacks注册callback监听activity生命周期 //细看activityResumed和activityPaused方法,通过Activity计数法来实现应用前后台的监听 app.registerActivityLifecycleCallbacks(new EmptyActivityLifecycleCallbacks() { @Override public void onActivityCreated(Activity activity Bundle savedInstanceState) { ReportFragment.get(activity).setProcessListener(mInitializationListener); } @Override public void onActivityPaused(Activity activity) { activityPaused(); } @Override public void onActivityStopped(Activity activity) { activityStopped(); } }); } static void init(Context context) { // 初始化 sInstance.attach(context); } } public class ProcessLifecycleOwnerInitializer extends ContentProvider { @Override public boolean onCreate() { LifecycleDispatcher.init(getContext()); //这是个ContentProvider,在onCreate方法初始化ProcessLifecycleOwner //主进程的第一个ContentProvider.onCreate是比Application.onCreate先调用的 //这个ContentProvider会注册在Androidmenifest中,从而不用再Application中进行ProcessLifecycleOwner初始化 ProcessLifecycleOwner.init(getContext()); return true; } } LifecycleService 的使用

LifecycleService 继承Service 并实现LifecycleOwner 可以自定义一个服务继承LifecycleService来使用

  • 下面是代码实例:

public class MyService extends LifecycleService { public MyService() { } @Override public void onCreate() { super.onCreate(); getLifecycle().addObserver(new LifecycleObserver() { @OnLifecycleEvent(Lifecycle.Event.ON_CREATE) public void onCreate(){ Log.d(TAG "onCreate: "); } @OnLifecycleEvent(Lifecycle.Event.ON_START) public void onStart(){ Log.d(TAG "onStart: "); } @OnLifecycleEvent(Lifecycle.Event.ON_STOP) public void onStop(){ Log.d(TAG "onStop: "); } @OnLifecycleEvent(Lifecycle.Event.ON_DESTROY) public void onDestroy(){ Log.d(TAG "onDestroy: "); } }); } }总结以及其他的 Tips

Lifecycles 库为 Jetpack 其他组件打下了基础,通过LifecycleObserver观察者减少对Activity Fragment 和Service这些具有生命周期类的依赖。

  • 是使用LifecycleService和ProcessLifecycleOwner需要引入android.arch.lifecycle:extensions:1.1.1库,它并没有包含在com.android.support:appcompat-v7:version中
  • 在使用到Lifecycles库时最好在gradle引入apt编译器库annotationProcessor "android.arch.lifecycle:compiler:1.1.1" 没引入这库 对应注解@OnLifecycleEvent的方法就是使用反射来实现的,当引入这库后,会在编译时期自动生成YourObserverName_LifecycleAdapter类实现0反射提高性能。

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jetpack 最新功能来了(Jetpack组件库)(2)

jetpack 最新功能来了(Jetpack组件库)(3)

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