jetpack之Lifecycle的源码解读
源码版本：Android 10
一、Lifecycle基本使用方法
class MainActivity : AppCompatActivity() {
override fun onCreate(savedInstanceState: Bundle?) {
//源码分析入口：1
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main)
//源码分析入口：2 给当前Activity添加观察者
lifecycle.addObserver(MainActivityLifeObserver())
tv_exit.setOnClickListener {
finish()
}
}
}
class MainActivityLifeObserver : LifecycleObserver {
@OnLifecycleEvent(Lifecycle.Event.ON_CREATE)
fun onCreate() {
Log.e("Observer==", "OnCreate方法调用了")
}
@OnLifecycleEvent(Lifecycle.Event.ON_DESTROY)
fun onDestroy() {
Log.e("Observer==", "OnDestroy被调用了")
}
}
二、Lifecycle源码分析
如下是谷歌官方网站绘制的Lifecycle对象状态转换图
2.1 源码入口1
点击super.onCreate方法，往上一直找到ComponentActivity。CompontActvity的onCreate方法源码如下： @Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
//实现Activity生命周期监听的核心类，通过在Activity上面添加一个空白的没有任何UI的Fragment来监控
//Activity的生命周期
ReportFragment.injectIfNeededIn(this);
}
接下来看 ReportFragment.injectIfNeededIn(this)主要干了什么，其源码如下所示：
public static void injectIfNeededIn(Activity activity) {
android.app.FragmentManager manager = activity.getFragmentManager();
if (manager.findFragmentByTag(REPORT_FRAGMENT_TAG) == null) {
//给当前activity添加一个ReportFragment
manager.beginTransaction().add(new ReportFragment(), REPORT_FRAGMENT_TAG).commit();
manager.executePendingTransactions();
}
}
2.2 源码入口2
点击addObserver方法找到其实现类LifecycleRegistry，LifecycleRegistry中addObserver源码如下所示： @Override
public void addObserver(@NonNull LifecycleObserver observer) {
//初始化observer的初始状态
State initialState = mState == DESTROYED ? DESTROYED : INITIALIZED;
//对传入的observer和它的状态进行封装
ObserverWithState statefulObserver = new ObserverWithState(observer, initialState);
//将oberver和其封装类装入mObserverMap中，其底层是借助HashMap实现的
ObserverWithState previous = mObserverMap.putIfAbsent(observer, statefulObserver);
//最开始添加时pervious为null
if (previous != null) {
return;
}
LifecycleOwner lifecycleOwner = mLifecycleOwner.get();
//最开始添加时lifecycleOwner不为null
if (lifecycleOwner == null) {
return;
}
boolean isReentrance = mAddingObserverCounter != 0 || mHandlingEvent;
State targetState = calculateTargetState(observer);
mAddingObserverCounter++;
//最开始添加时statefulObserver.mState.compareTo(targetState）等于0，所以不走while里面的内容
while ((statefulObserver.mState.compareTo(targetState) < 0
&& mObserverMap.contains(observer))) {
pushParentState(statefulObserver.mState);
statefulObserver.dispatchEvent(lifecycleOwner, upEvent(statefulObserver.mState));
popParentState();
// mState / subling may have been changed recalculate
targetState = calculateTargetState(observer);
}
//最开始添加isReentrance为false
if (!isReentrance) {
//同步状态
sync();
}
mAddingObserverCounter--;
}
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;
}
//最开始添加isSysnced()方法返回true，所以不执行while里面的内容
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;
}
private boolean isSynced() {
if (mObserverMap.size() == 0) {
return true;
}
State eldestObserverState = mObserverMap.eldest().getValue().mState;
State newestObserverState = mObserverMap.newest().getValue().mState;
//最开始添加时eldestObserverState与newestObserverState指向同一对象，所以返回true
return eldestObserverState == newestObserverState && mState == newestObserverState;
}
2.3 ObserverWidthState源码
ObserverWidthState是实现生命周期监听回调的核心类，该类源码如下： static class ObserverWithState {
State mState;
GenericLifecycleObserver mLifecycleObserver;
ObserverWithState(LifecycleObserver observer, State initialState) {
//通过反射解析自定义Observer中的方法，将其中的方法和监听的事件进行绑定
mLifecycleObserver = Lifecycling.getCallback(observer);
mState = initialState;
}
void dispatchEvent(LifecycleOwner owner, Event event) {
State newState = getStateAfter(event);
mState = min(mState, newState);
//当Activity生命周期发生变化时通知自定义Observer
mLifecycleObserver.onStateChanged(owner, event);
mState = newState;
}
}
Lifecycling.getCallback(observer)源码如下：
@NonNull
static GenericLifecycleObserver getCallback(Object object) {
if (object instanceof FullLifecycleObserver) {
return new FullLifecycleObserverAdapter((FullLifecycleObserver) object);
}
//LiveData的观察者走这条分支
if (object instanceof GenericLifecycleObserver) {
return (GenericLifecycleObserver) object;
}
final Class<?> klass = object.getClass();
//通过该方法将自定义Oberver中的事件和方法进行绑定
int type = getObserverConstructorType(klass);
//此Demo中调用时type不等于GENERATED_CALLBACK，所以不进入if分支
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);
}
private static int getObserverConstructorType(Class<?> klass) {
if (sCallbackCache.containsKey(klass)) {
return sCallbackCache.get(klass);
}
int type = resolveObserverCallbackType(klass);
sCallbackCache.put(klass, type);
return type;
}
private static int resolveObserverCallbackType(Class<?> klass) {
//....
//是否拥有监听Activity生命周期的方法，有的话将会返回true
boolean hasLifecycleMethods = ClassesInfoCache.sInstance.hasLifecycleMethods(klass);
if (hasLifecycleMethods) {
return REFLECTIVE_CALLBACK;
}
//....
return REFLECTIVE_CALLBACK;
}
ClassesInfoCache的部分源码分析如下所示：
class ClassesInfoCache {
static ClassesInfoCache sInstance = new ClassesInfoCache();
private static final int CALL_TYPE_NO_ARG = 0;
private static final int CALL_TYPE_PROVIDER = 1;
private static final int CALL_TYPE_PROVIDER_WITH_EVENT = 2;
private final Map<Class, CallbackInfo> mCallbackMap = new HashMap<>();
private final Map<Class, Boolean> mHasLifecycleMethods = new HashMap<>();
boolean hasLifecycleMethods(Class klass) {
if (mHasLifecycleMethods.containsKey(klass)) {
return mHasLifecycleMethods.get(klass);
}
//获取自定义Observer中的所有方法
Method[] methods = getDeclaredMethods(klass);
//遍历方法，获取方法上面的注解
for (Method method : methods) {
OnLifecycleEvent annotation = method.getAnnotation(OnLifecycleEvent.class);
if (annotation != null) {
createInfo(klass, methods);
return true;
}
}
mHasLifecycleMethods.put(klass, false);
return false;
}
private Method[] getDeclaredMethods(Class klass) {
try {
return klass.getDeclaredMethods();
} catch (NoClassDefFoundError e) {
throw new IllegalArgumentException("The observer class has some methods that use "
+ "newer APIs which are not available in the current OS version. Lifecycles "
+ "cannot access even other methods so you should make sure that your "
+ "observer classes only access framework classes that are available "
+ "in your min API level OR use lifecycle:compiler annotation processor.", e);
}
}
//将封装的方法与事件保存起来
private void verifyAndPutHandler(Map<MethodReference, Lifecycle.Event> handlers,
MethodReference newHandler, Lifecycle.Event newEvent, Class klass) {
Lifecycle.Event event = handlers.get(newHandler);
if (event != null && newEvent != event) {
Method method = newHandler.mMethod;
throw new IllegalArgumentException(
"Method " + method.getName() + " in " + klass.getName()
+ " already declared with different @OnLifecycleEvent value: previous"
+ " value " + event + ", new value " + newEvent);
}
if (event == null) {
handlers.put(newHandler, newEvent);
}
}
//将监听事件与监听回调方法进行绑定
private CallbackInfo createInfo(Class klass, @Nullable Method[] declaredMethods) {
//....
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;
//当前方法参数的个数
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");
}
//将方法参数的个数与该方法进行封装
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;
}
static class CallbackInfo {
//...
@SuppressWarnings("ConstantConditions")
void invokeCallbacks(LifecycleOwner source, Lifecycle.Event event, Object target) {
invokeMethodsForEvent(mEventToHandlers.get(event), source, event, target);
invokeMethodsForEvent(mEventToHandlers.get(Lifecycle.Event.ON_ANY), source, event,
target);
}
//根据事件获取对应的MethodReference对象，从而执行监听事件的方法
private static void invokeMethodsForEvent(List<MethodReference> handlers,
LifecycleOwner source, Lifecycle.Event event, Object mWrapped) {
if (handlers != null) {
for (int i = handlers.size() - 1; i >= 0; i--) {
handlers.get(i).invokeCallback(source, event, mWrapped);
}
}
}
}
static class MethodReference {
//....
//通过反射执行自定义Observer中监听Activity生命周期的方法
void invokeCallback(LifecycleOwner source, Lifecycle.Event event, Object target) {
//noinspection TryWithIdenticalCatches
try {
switch (mCallType) {
case CALL_TYPE_NO_ARG:
mMethod.invoke(target);
break;
case CALL_TYPE_PROVIDER:
mMethod.invoke(target, source);
break;
case CALL_TYPE_PROVIDER_WITH_EVENT:
mMethod.invoke(target, source, event);
break;
}
} catch (InvocationTargetException e) {
throw new RuntimeException("Failed to call observer method", e.getCause());
} catch (IllegalAccessException e) {
throw new RuntimeException(e);
}
}
//....
}
}
2.4 ReportFragment如何分发Activity生命周期事件
调用ReportFragment中的dispatch方法进行事件的传递，实际上调用的是LifecycleRegistry的handleLifecycleEvent方法，该方法源码如下：public void handleLifecycleEvent(@NonNull Lifecycle.Event event) {
//获取当前事件对应的自定义Observer的状态，也就是当前Observer的下一个状态
State next = getStateAfter(event);
//将Observer的状态移动到下一个状态
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();//同步状态,因为最开始Observer的状态是INITIALIZED，现在是CREATED,所以调用的是forwardPass方法
mHandlingEvent = false;
}
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()))) {
//保存observer.mState之前的状态
pushParentState(observer.mState);
//到了给自定义Observer回调事件监听的时候了
observer.dispatchEvent(lifecycleOwner, upEvent(observer.mState));
//事件分发完毕之后弹出之前保存的状态
popParentState();
}
}
}
observer.dispatchEvent最终调用的是ReflectiveGenericLifecycleObserver中的onStateChanged方法，ReflectiveGenericLifecycleObserver源码如下：
class ReflectiveGenericLifecycleObserver implements GenericLifecycleObserver {
private final Object mWrapped;
private final CallbackInfo mInfo;
ReflectiveGenericLifecycleObserver(Object wrapped) {
mWrapped = wrapped;
//在MainActivity添加自定义Observer的时候已经将监听事件和方法进行了绑定
mInfo = ClassesInfoCache.sInstance.getInfo(mWrapped.getClass());
}
@Override
public void onStateChanged(LifecycleOwner source, Event event) {
//执行自定义Observer方法的回调
mInfo.invokeCallbacks(source, event, mWrapped);
}
}