Android Jetpack组件之Lifecycles库详解

Android Jetpack 组件是库的集合,这些库是为了协同工作而构建的,不过也可以单独采用,接下来会一一详细地学习这些库, 下面源码版本是com.android.support:appcompat-v7:28.0.0, 以及库android.arch.lifecycle:extensions:1.1.1

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:

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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:

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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);
}
}

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。类图如下所示:

jetpack_lifecycle_01

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

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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的各个状态以及事件分发过程如下图所示: (该图来自google官网文档)

lifecycle_states

矩形代表状态,一共有5个状态,记录在枚举State中, 依次是DESTROYED, INITIALIZED, CREATED, STARTED, RESUMED;

箭头上面代表分发的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

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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:

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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

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 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();
...
}

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:

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//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

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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。

更多细节参考:https://developer.android.google.cn/reference/android/arch/lifecycle/ProcessLifecycleOwner

下面简单说一下ProcessLifecycleOwner的工作原理:

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// 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:

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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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