Android 架构组件 LifeCycle 源码解析
随着 Android系统的发展,碎片化也越来越严重,APP开发考虑的东西越来越多,如后台任务,用户操作,电量消耗,性能等等,给APP 的开发带来沉重的负担。不同的公司开发 APP 的技术水平不一样,导致整个 APP 应用市场上的产品质量参差不齐。为了保障 APP 的质量,Google推出了 Jetpack 组件,该组件从基础库、架构、行为、UI 四个大的方面入手,旨在统一 APP 开发框架,提升稳定性。整个组件库比较庞大,本文首先对 Jetpack 整体进行简介,然后着重分析架构模块中 LifeCycle 的实现原理,感兴趣的读者可以用类似的方法去分析其他组件,借以抛砖引玉。
1. Android Jetpack 简介
Jetpack是一系列库、工具、架构组成的,帮助开发人员快速方便的构建高效、稳定的Anroid App,使用该组件库可以:
\1. 轻松管理应用程序的生命周期
\2. 构建可观察的数据对象,以便在基础数据库更改时通知视图
\3. 存储在应用程序轮换中未销毁的UI相关数据,在界面重建后恢复数据
\4. 轻松的实现SQLite数据库
\5. 系统自动调度后台任务的执行,优化使用性能
其构成如下:
![/C:/0885f05e82cae96eaa692a8069d3d162](
1.1 基础库Foundation
基础库提供向下兼容、测试、Multidex 分包以及 Kotlin 扩展支持
- AppCompact : 提供各个版本的兼容 API,以及基础库,包含 v4,v7,v13等,随着版本的不断提升还会有更多的兼容库。
- Android KTX: 提供对 Kotlin 语言相关的支持库,方便 Kotlin 开发
- Multidex: 通过 Dex 拆分,解决 App 包过大的问题
- Test: Android 单元测试和 UI 测试的库
1.2 架构组件 Architecture
架构组件的目的是让 APP 更加健壮、可测试、易维护,主要包含:
- Data Binding: 声明式数据绑定,即 MVVM 在 Android 平台的实现
- Lifecycles:管理应用生命周期
- LiveData: 动态观察数据变化,感知界面生命周期,自动更新数据
- Navigation: App 内部导航框架
- Paging: 优雅地按需加载数据
- Room: SQLite ORM 对象关系映射
- ViewModel: 在可感知界面状态下,管理 UI 数据
- WorkManager: 统一管理后台任务
1.3 行为 Behavior
- Download Manager: 统一管理(定时、周期)下载
- Media & playback :提供兼容 API 处理多媒体播放、投射
- Notifications: 提供兼容 API 处理通知,同时能够兼容手表和车载设备
- Permissions: 权限检查和申请
- Sharing: 提供标题栏通用分享
- Slices: 创建灵活的 UI 组件模块,在 APP 外展示数据(如 Google 搜索)
- Preferences: 提供统一的偏好设置
1.4 界面 UI
- Animation & transitions: 在不同场景切换时,播放组件动画
- Auto: 开发车载工具箱
- Emoji: 为老系统提供最新的 Emoji 颜文字
- Fragment: 界面组件单元
- Layout: 使用不同的算法进行界面布局
- Palette: 从调色板中提取有用的信息
- TV : TV开发工具箱
- Wear OS by Google: 手表开发工具箱
Android Jetpack 所包含的内容及其丰富,其源码是没办法在一篇文章中解释清除。因此在接下来的章节以 Lifecycles 组件为突破口,基于2.0.0版本进行分析,其他的组件可以顺藤摸瓜,依次研究。
2. 架构组件LifeCycles 原理解析
2.1 架构组件概览
第一部分对 Jetpack 进行整体介绍时,在1.2节中提到架构组件的构成,其使用方式如下图。
Lifecycle 已经默认包含在 support-compact / fragment.2.0.0版本就具备了 Lifecycle 的能力库中的 AppCompactActivity 、Fragment中,也就是下图中顶部的 Activity/Fragment。
![/C:/74c6f50adc7de0b3776f58d091a7925a](
2.2 Lifecycles 使用方式
Lifecycle是一个生命周期感知组件,它通过类似@OnLifecycleEvent(Lifecycle.Event.ON_RESUME)的注解标注方法,使之可以监听UI组件生命周期的变化。
以往在生命周期中执行相应的方法需要设置接口,然后在声明周期中回调接口,造成了代码的耦合。
假如我们要在界面可见时监听 GPS 获取定位,在界面不可见时停止定位。
在没有使用LifeCycle 时,我们监听Lifecycle的方式是这样的:
class MyLocationListener {
public MyLocationListener(Context context, Callback callback) {
// ...
}
void start() {
// connect to system location service
}
void stop() {
// disconnect from system location service
}
}
class MyActivity extends AppCompatActivity {
private MyLocationListener myLocationListener;
@Override
public void onCreate(...) {
myLocationListener = new MyLocationListener(this, (location) -> {
// update UI
});
}
@Override
public void onStart() {
super.onStart();
myLocationListener.start();
// manage other components that need to respond
// to the activity lifecycle
}
@Override
public void onStop() {
super.onStop();
myLocationListener.stop();
// manage other components that need to respond
// to the activity lifecycle
}
}
这种方式有明显的缺陷,我们要在 Activity 的各种生命周期做不同的处理。如果我们还有其他的类似需求,这里将不断膨胀,难以维护,牵一发而动全身。
Lifecycle框架的使用方式
public class MyObserver implements LifecycleObserver {
@OnLifecycleEvent(Lifecycle.Event.ON_RESUME)
public void connectListener() {
...
}
@OnLifecycleEvent(Lifecycle.Event.ON_PAUSE)
public void disconnectListener() {
...
}
}
myLifecycleOwner.getLifecycle().addObserver(new MyObserver());
2.3 Lifecycle 原理
Lifecycle 统一监听 UI 组件声明周期,并且允许其他组件监听自己,来实现生命周期感知。
其定义了 UI 组件的状态和对应的事件,类似状态机,通过UI 组件不同生命周期,触发Lifecycle的事件并更新状态,其状态变化如下:
![/C:/4a4ea796bc8071d76a5377bc601b47bc](
其本质仍然是观察者模式,观察UI界面生命周期变化。
2.4 源码解析
我们先看一下 Lifecycle 的使用步骤:
- 实现LifecycleObserver观察者
- 编写执行方法
- 在方法加上对应的事件注解
- 获取 LifeOwner并添加观察者
按照使用步骤来分析实现原理,讲真个源码分为:
- 状态和事件定义
- 事件注解
- 生命周期感知
- 通知观察者
2.4.1 事件和状态定义
如下是对2.3节状态的定义,不同的状态和事件,对应着界面的变化,比较简单,直接看如下代码中状态的定义:
public abstract class Lifecycle {
@MainThread
public abstract void addObserver(@NonNull LifecycleObserver observer);
@MainThread
public abstract void removeObserver(@NonNull LifecycleObserver observer);
@MainThread
@NonNull
public abstract State getCurrentState();
@SuppressWarnings("WeakerAccess")
public enum Event {
ON_CREATE,
ON_START,
ON_RESUME,
ON_PAUSE,
ON_STOP,
ON_DESTROY,
ON_ANY
}
@SuppressWarnings("WeakerAccess")
public enum State {
DESTROYED,
INITIALIZED,
CREATED,
STARTED,
RESUMED;
public boolean isAtLeast(@NonNull State state) {
return compareTo(state) >= 0;
}
}
}
2.4.2 注解的实现
@SuppressWarnings("unused")
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.METHOD)
public @interface OnLifecycleEvent {
Lifecycle.Event value();
}
从代码@Retention(RetentionPolicy.RUNTIME)可以看出使用的是运行时注解(运行时注解反射是很耗时的,所以在CallbackInfo 中有对反射进行缓存),注解取值为事件枚举Lifecycle.Event。
注解是如何调用的?我们需要看一下观察者的继承结构
![/C:/7de455a52467a048929bdc1450fc5b57](
基于注解的观察值使用了 CallbackInfo 来封装注解事件和对应的处理方法,其代码如下次:
private CallbackInfo createInfo(Class klass, @Nullable Method[] declaredMethods) {
Class superclass = klass.getSuperclass();
Map<MethodReference, Lifecycle.Event> handlerToEvent = new HashMap<>();
if (superclass != null) {
CallbackInfo superInfo = getInfo(superclass);
if (superInfo != null) {
handlerToEvent.putAll(superInfo.mHandlerToEvent);
}
}
Class[] interfaces = klass.getInterfaces();
for (Class intrfc : interfaces) {
for (Map.Entry<MethodReference, Lifecycle.Event> entry : getInfo(
intrfc).mHandlerToEvent.entrySet()) {
verifyAndPutHandler(handlerToEvent, entry.getKey(), entry.getValue(), klass);
}
}
Method[] methods = declaredMethods != null ? declaredMethods : getDeclaredMethods(klass);
boolean hasLifecycleMethods = false;
for (Method method : methods) {
OnLifecycleEvent annotation = method.getAnnotation(OnLifecycleEvent.class);
if (annotation == null) {
continue;
}
hasLifecycleMethods = true;
Class<?>[] params = method.getParameterTypes();
int callType = CALL_TYPE_NO_ARG;
if (params.length > 0) {
callType = CALL_TYPE_PROVIDER;
if (!params[0].isAssignableFrom(LifecycleOwner.class)) {
throw new IllegalArgumentException(
"invalid parameter type. Must be one and instanceof LifecycleOwner");
}
}
Lifecycle.Event event = annotation.value();
if (params.length > 1) {
callType = CALL_TYPE_PROVIDER_WITH_EVENT;
if (!params[1].isAssignableFrom(Lifecycle.Event.class)) {
throw new IllegalArgumentException(
"invalid parameter type. second arg must be an event");
}
if (event != Lifecycle.Event.ON_ANY) {
throw new IllegalArgumentException(
"Second arg is supported only for ON_ANY value");
}
}
if (params.length > 2) {
throw new IllegalArgumentException("cannot have more than 2 params");
}
MethodReference methodReference = new MethodReference(callType, method);
verifyAndPutHandler(handlerToEvent, methodReference, event, klass);
}
CallbackInfo info = new CallbackInfo(handlerToEvent);
mCallbackMap.put(klass, info);
mHasLifecycleMethods.put(klass, hasLifecycleMethods);
return info;
}
createInfo 会依次解析 Class 并且将解析结果放入缓存:
private final Map<Class, CallbackInfo> mCallbackMap 缓存了所有观察者的信息 private final Map<Class, Boolean> mHasLifecycleMethods 缓存了观察者是否有生命周期方法
final Map<Lifecycle.Event, List
在注册观察者时,基于注解的会走到最后的分支,保存ReflectiveGenericLifecycleObserver,并通过 CallbackInfo记录上述注解信息,如下代码所示:
@NonNull
static GenericLifecycleObserver getCallback(Object object) {
if (object instanceof FullLifecycleObserver) {
return new FullLifecycleObserverAdapter((FullLifecycleObserver) object);
}
if (object instanceof GenericLifecycleObserver) {
return (GenericLifecycleObserver) object;
}
final Class<?> klass = object.getClass();
int type = getObserverConstructorType(klass);
if (type == GENERATED_CALLBACK) {
List<Constructor<? extends GeneratedAdapter>> constructors =
sClassToAdapters.get(klass);
if (constructors.size() == 1) {
GeneratedAdapter generatedAdapter = createGeneratedAdapter(
constructors.get(0), object);
return new SingleGeneratedAdapterObserver(generatedAdapter);
}
GeneratedAdapter[] adapters = new GeneratedAdapter[constructors.size()];
for (int i = 0; i < constructors.size(); i++) {
adapters[i] = createGeneratedAdapter(constructors.get(i), object);
}
return new CompositeGeneratedAdaptersObserver(adapters);
}
return new ReflectiveGenericLifecycleObserver(object);
}
2.4.3 生命周期感知
我们示例采用的是基于注解反射的观察者,从这里展开分析,首先看是如何注册的。注册需要获取 LifeOwner,然后获取 LifeCycle(只有唯一实现者LifecycleRegistry),而这一切都应该在 AppCompactActivity 中,更具体一点是在父类ComponentActivity,代码如下:
@RestrictTo(LIBRARY_GROUP)
public class ComponentActivity extends Activity
implements LifecycleOwner, KeyEventDispatcher.Component {
private LifecycleRegistry mLifecycleRegistry = new LifecycleRegistry(this);
@Override
@SuppressWarnings("RestrictedApi")
protected void onCreate(@Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
ReportFragment.injectIfNeededIn(this);
}
@Override
public Lifecycle getLifecycle() {
return mLifecycleRegistry;
}
注意在 onCreate 中有一个ReportFragment.injectIfNeededIn(this),注入了一个 Fragment
public class ReportFragment extends Fragment {
private static final String REPORT_FRAGMENT_TAG = "androidx.lifecycle"
+ ".LifecycleDispatcher.report_fragment_tag";
public static void injectIfNeededIn(Activity activity) {
// ProcessLifecycleOwner should always correctly work and some activities may not extend
// FragmentActivity from support lib, so we use framework fragments for activities
android.app.FragmentManager manager = activity.getFragmentManager();
if (manager.findFragmentByTag(REPORT_FRAGMENT_TAG) == null) {
manager.beginTransaction().add(new ReportFragment(), REPORT_FRAGMENT_TAG).commit();
// Hopefully, we are the first to make a transaction.
manager.executePendingTransactions();
}
}
static ReportFragment get(Activity activity) {
return (ReportFragment) activity.getFragmentManager().findFragmentByTag(
REPORT_FRAGMENT_TAG);
}
private ActivityInitializationListener mProcessListener;
private void dispatchCreate(ActivityInitializationListener listener) {
if (listener != null) {
listener.onCreate();
}
}
private void dispatchStart(ActivityInitializationListener listener) {
if (listener != null) {
listener.onStart();
}
}
private void dispatchResume(ActivityInitializationListener listener) {
if (listener != null) {
listener.onResume();
}
}
@Override
public void onActivityCreated(Bundle savedInstanceState) {
super.onActivityCreated(savedInstanceState);
dispatchCreate(mProcessListener);
dispatch(Lifecycle.Event.ON_CREATE);
}
@Override
public void onStart() {
super.onStart();
dispatchStart(mProcessListener);
dispatch(Lifecycle.Event.ON_START);
}
@Override
public void onResume() {
super.onResume();
dispatchResume(mProcessListener);
dispatch(Lifecycle.Event.ON_RESUME);
}
@Override
public void onPause() {
super.onPause();
dispatch(Lifecycle.Event.ON_PAUSE);
}
@Override
public void onStop() {
super.onStop();
dispatch(Lifecycle.Event.ON_STOP);
}
@Override
public void onDestroy() {
super.onDestroy();
dispatch(Lifecycle.Event.ON_DESTROY);
// just want to be sure that we won't leak reference to an activity
mProcessListener = null;
}
private void dispatch(Lifecycle.Event event) {
Activity activity = getActivity();
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);
}
}
}
}
原来是通过插入一个 Fragment,在各个生命周期回调中转发到 注册中心 LifecycleRegistry。
2.4.4 通知观察者
上一节提到 ReportFragment 会将各个生命周期转发给 LifecycleRegistry.handleLifecycleEvent,我们继续分析代码
public void handleLifecycleEvent(@NonNull Lifecycle.Event event) {
State next = getStateAfter(event);
moveToState(next);
}
private void moveToState(State next) {
if (mState == next) {
return;
}
mState = next;
if (mHandlingEvent || mAddingObserverCounter != 0) {
mNewEventOccurred = true;
// we will figure out what to do on upper level.
return;
}
mHandlingEvent = true;
sync();
mHandlingEvent = false;
}
@NonNull
@Override
public State getCurrentState() {
return mState;
}
static State getStateAfter(Event event) {
switch (event) {
case ON_CREATE:
case ON_STOP:
return CREATED;
case ON_START:
case ON_PAUSE:
return STARTED;
case ON_RESUME:
return RESUMED;
case ON_DESTROY:
return DESTROYED;
case ON_ANY:
break;
}
throw new IllegalArgumentException("Unexpected event value " + event);
}
private static Event downEvent(State state) {
switch (state) {
case INITIALIZED:
throw new IllegalArgumentException();
case CREATED:
return ON_DESTROY;
case STARTED:
return ON_STOP;
case RESUMED:
return ON_PAUSE;
case DESTROYED:
throw new IllegalArgumentException();
}
throw new IllegalArgumentException("Unexpected state value " + state);
}
private static Event upEvent(State state) {
switch (state) {
case INITIALIZED:
case DESTROYED:
return ON_CREATE;
case CREATED:
return ON_START;
case STARTED:
return ON_RESUME;
case RESUMED:
throw new IllegalArgumentException();
}
throw new IllegalArgumentException("Unexpected state value " + state);
}
private void forwardPass(LifecycleOwner lifecycleOwner) {
Iterator<Entry<LifecycleObserver, ObserverWithState>> ascendingIterator =
mObserverMap.iteratorWithAdditions();
while (ascendingIterator.hasNext() && !mNewEventOccurred) {
Entry<LifecycleObserver, ObserverWithState> entry = ascendingIterator.next();
ObserverWithState observer = entry.getValue();
while ((observer.mState.compareTo(mState) < 0 && !mNewEventOccurred
&& mObserverMap.contains(entry.getKey()))) {
pushParentState(observer.mState);
observer.dispatchEvent(lifecycleOwner, upEvent(observer.mState));
popParentState();
}
}
}
private void backwardPass(LifecycleOwner lifecycleOwner) {
Iterator<Entry<LifecycleObserver, ObserverWithState>> descendingIterator =
mObserverMap.descendingIterator();
while (descendingIterator.hasNext() && !mNewEventOccurred) {
Entry<LifecycleObserver, ObserverWithState> entry = descendingIterator.next();
ObserverWithState observer = entry.getValue();
while ((observer.mState.compareTo(mState) > 0 && !mNewEventOccurred
&& mObserverMap.contains(entry.getKey()))) {
Event event = downEvent(observer.mState);
pushParentState(getStateAfter(event));
observer.dispatchEvent(lifecycleOwner, event);
popParentState();
}
}
}
// happens only on the top of stack (never in reentrance),
// so it doesn't have to take in account parents
private void sync() {
LifecycleOwner lifecycleOwner = mLifecycleOwner.get();
if (lifecycleOwner == null) {
Log.w(LOG_TAG, "LifecycleOwner is garbage collected, you shouldn't try dispatch "
+ "new events from it.");
return;
}
while (!isSynced()) {
mNewEventOccurred = false;
// no need to check eldest for nullability, because isSynced does it for us.
if (mState.compareTo(mObserverMap.eldest().getValue().mState) < 0) {
backwardPass(lifecycleOwner);
}
Entry<LifecycleObserver, ObserverWithState> newest = mObserverMap.newest();
if (!mNewEventOccurred && newest != null
&& mState.compareTo(newest.getValue().mState) > 0) {
forwardPass(lifecycleOwner);
}
}
mNewEventOccurred = false;
}
我们梳理一下顺序:
1.->handleLifecycleEvent()
2.->moveToState()
3.->sync()
4.->backwardPass()/forwardPass()
5.->observer.dispatchEvent(lifecycleOwner, event);
总结一下,就是按照状态切换流程,如下图所示,切换状态并通知生命周期。
![/C:/5c3d20ff2429c4edadb198fa6d194d69](
2.5 结尾
APP 开发最佳实践并不是一蹴而就的,而是随着平台、业务的不断发展,一同向前演进。没有最好的架构,只有最适合的架构。Google 推出的 Jetpack 开发套件 内容及其丰富,本文只是以其中的 Lifecycle 组件为核心,分析介绍其实现原理,管中窥豹。对其他模块感兴趣的读者也可以参照本文的分析方式进行庖丁解牛。这些组件现在正在融入我们当前的项目中,哪些部分适合,那些水土不服,仍需要实践的检验。