Lifecycle是一个持有组件生命周期状态信息的类,并且允许其他对象观察该状态.
本文基于 Lifecycle 2.0.0版本，Android API 28. 首先我们先看一下如何使用Lifecycle实现生命周期的监听.
基础用法
public class HippoX implements LifecycleObserver {
private static final String TAG_LOG = "HippoX";
@OnLifecycleEvent(Lifecycle.Event.ON_RESUME)
public void init() {
Log.e(TAG_LOG, "init exec");
}
}
复制代码public class MainActivity extends AppCompatActivity {
private static final String TAG_LOG = "HippoX";
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
getLifecycle().addObserver(new HippoX());
}
@Override
protected void onResume() {
super.onResume();
Log.e(TAG_LOG, "onResumes execute");
}
}
复制代码Logcat日志:
E/HippoX: onResumes execute
E/HippoX: init exec
复制代码日志打印结果表明上述代码确实实现了观察生命周期的功能. 别问我为啥这样就监听成功了,往下看.
生命周期事件及状态
Lifecycle使用两个主要枚举来跟踪其关联组件的生命周期状态.
- Event 描述从框架和Lifecycle类中派发的生命周期事件.
- State 描述Lifecycle对象跟踪的组件的当前状态.
Lifecycle.Event
public enum Event {
ON_CREATE, //用于匹配生命周期所有者的onCreate事件.
ON_START, //用于匹配生命周期所有者的onStart事件.
ON_RESUME, //用于匹配生命周期所有者的onResume事件.
ON_PAUSE, //用于匹配生命周期所有者的onCreate事件.
ON_STOP, //用于匹配生命周期所有者的onStop事件.
ON_DESTROY, //用于匹配生命周期所有者的onDestroy事件.
ON_ANY //用于匹配生命周期所有者的所有事件.
}
复制代码Lifecycle.State
public enum State {
DESTROYED, //表示生命周期所有者创建的状态.对于Activity来说,在onCreate执行之后,onStop执行之前.
INITIALIZED, //表示生命周期所有者销毁的状态.
CREATED, //表示生命周期所有者初始化的状态.
STARTED, //表示生命周期所有者恢复的状态.
RESUMED; //表示生命周期所有者启动的状态.
public boolean isAtLeast(@NonNull State state) { //比较此状态是否大于或等于给定状态
return compareTo(state) >= 0;
}
}
复制代码Event与State的关系
生命周期所有者和观察者
LifecycleOwner 生命周期所有者
public interface LifecycleOwner {
@NonNull
Lifecycle getLifecycle(); //返回lifecycle
}
复制代码官方文档的描述是持有Android生命周期的类,通过实现该类可以在非Activity和Fragment中来处理生命周期事件. 注:Activity和Fragment都实现了该接口,并提供
LifecycleObserver 生命周期观察者
public interface LifecycleObserver {
}
复制代码官方文档的描述是将实现该接口的类标记为生命周期观察者,且依赖于通过OnLifecycleEvent注解实现的方法. 先了解到这里,我们开始正式的分析lifecycle是如何实现生命周期的监听的. Activity和Fragment的实现过程基本相同,我们在这里就分析Activity. 首先我们由MainActivity的getLifecycle()方法往里跟,最终找到他的父父父类ComponentActivity实现了LifecycleOwner接口.
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;
}
}
复制代码其中比较重要的是LifecycleRegistry类和ReportFragment.我们再来继续分析.
LifecycleRegistry
LifecycleRegistry是Lifecycle的实现类,能够管理多个生命周期观察者. 提供的主要方法如下:
- void addObserver(LifecycleObserver observer) 添加一个生命周期观察者(下文简称观察者),当生命周期所有者状态改变时将会进行通知.
- Lifecycle.State getCurrentState() 获取生命周期的当前状态.
- int getObserverCount() 返回观察者的个数.
- void handleLifecycleEvent(Lifecycle.Event event) 设置当前状态并通知观察者.
- void removeObserver(LifecycleObserver observer) 从观察者列表中移除指定的观察者.
- void setCurrentState(Lifecycle.State state) 设置生命周期为指定的状态,并将事件分派给观察者.
ReportFragment
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);
}
...
@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);
}
}
}
...
}
复制代码查看源码可以知道,lifecycle是通过ReportFragment来实现生命周期的监听的,重写了生命周期的回调方法,在生命周期回调方法的内部调用dispatch的方法来派发生命周期事件.并且ComponentActivity的onCreate方法中通过injectIfNeededIn方法进行了注入. 接下来我们分析一下我们实现了LifecycleObserver接口的类是如何得知生命周期变化的.
LifecycleRegistry
public class LifecycleRegistry extends Lifecycle {
private static final String LOG_TAG = "LifecycleRegistry";
/**
* Custom list that keeps observers and can handle removals / additions during traversal.
*
* Invariant: at any moment of time for observer1 & observer2:
* if addition_order(observer1) < addition_order(observer2), then
* state(observer1) >= state(observer2),
*/
private FastSafeIterableMap<LifecycleObserver, ObserverWithState> mObserverMap =
new FastSafeIterableMap<>();
/**
* Current state
*/
private State mState;
/**
* The provider that owns this Lifecycle.
* Only WeakReference on LifecycleOwner is kept, so if somebody leaks Lifecycle, they won't leak
* the whole Fragment / Activity. However, to leak Lifecycle object isn't great idea neither,
* because it keeps strong references on all other listeners, so you'll leak all of them as
* well.
*/
private final WeakReference<LifecycleOwner> mLifecycleOwner;
private int mAddingObserverCounter = 0;
private boolean mHandlingEvent = false;
private boolean mNewEventOccurred = false;
// we have to keep it for cases:
// void onStart() {
// mRegistry.removeObserver(this);
// mRegistry.add(newObserver);
// }
// newObserver should be brought only to CREATED state during the execution of
// this onStart method. our invariant with mObserverMap doesn't help, because parent observer
// is no longer in the map.
private ArrayList<State> mParentStates = new ArrayList<>();
/**
* Creates a new LifecycleRegistry for the given provider.
* <p>
* You should usually create this inside your LifecycleOwner class's constructor and hold
* onto the same instance.
*
* @param provider The owner LifecycleOwner
*/
public LifecycleRegistry(@NonNull LifecycleOwner provider) {
mLifecycleOwner = new WeakReference<>(provider);
mState = INITIALIZED;
}
/**
* Moves the Lifecycle to the given state and dispatches necessary events to the observers.
*
* @param state new state
*/
@SuppressWarnings("WeakerAccess")
@MainThread
public void markState(@NonNull State state) {
moveToState(state);
}
/**
* Sets the current state and notifies the observers.
* <p>
* Note that if the {@code currentState} is the same state as the last call to this method,
* calling this method has no effect.
*
* @param event The event that was received
*/
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;
}
private boolean isSynced() {
if (mObserverMap.size() == 0) {
return true;
}
State eldestObserverState = mObserverMap.eldest().getValue().mState;
State newestObserverState = mObserverMap.newest().getValue().mState;
return eldestObserverState == newestObserverState && mState == newestObserverState;
}
private State calculateTargetState(LifecycleObserver observer) {
Entry<LifecycleObserver, ObserverWithState> previous = mObserverMap.ceil(observer);
State siblingState = previous != null ? previous.getValue().mState : null;
State parentState = !mParentStates.isEmpty() ? mParentStates.get(mParentStates.size() - 1)
: null;
return min(min(mState, siblingState), parentState);
}
@Override
public void addObserver(@NonNull LifecycleObserver observer) {
State initialState = mState == DESTROYED ? DESTROYED : INITIALIZED;
ObserverWithState statefulObserver = new ObserverWithState(observer, initialState);
ObserverWithState previous = mObserverMap.putIfAbsent(observer, statefulObserver);
if (previous != null) {
return;
}
LifecycleOwner lifecycleOwner = mLifecycleOwner.get();
if (lifecycleOwner == null) {
// it is null we should be destroyed. Fallback quickly
return;
}
boolean isReentrance = mAddingObserverCounter != 0 || mHandlingEvent;
State targetState = calculateTargetState(observer);
mAddingObserverCounter++;
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);
}
if (!isReentrance) {
// we do sync only on the top level.
sync();
}
mAddingObserverCounter--;
}
private void popParentState() {
mParentStates.remove(mParentStates.size() - 1);
}
private void pushParentState(State state) {
mParentStates.add(state);
}
@Override
public void removeObserver(@NonNull LifecycleObserver observer) {
// we consciously decided not to send destruction events here in opposition to addObserver.
// Our reasons for that:
// 1. These events haven't yet happened at all. In contrast to events in addObservers, that
// actually occurred but earlier.
// 2. There are cases when removeObserver happens as a consequence of some kind of fatal
// event. If removeObserver method sends destruction events, then a clean up routine becomes
// more cumbersome. More specific example of that is: your LifecycleObserver listens for
// a web connection, in the usual routine in OnStop method you report to a server that a
// session has just ended and you close the connection. Now let's assume now that you
// lost an internet and as a result you removed this observer. If you get destruction
// events in removeObserver, you should have a special case in your onStop method that
// checks if your web connection died and you shouldn't try to report anything to a server.
mObserverMap.remove(observer);
}
/**
* The number of observers.
*
* @return The number of observers.
*/
@SuppressWarnings("WeakerAccess")
public int getObserverCount() {
return mObserverMap.size();
}
@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;
}
static State min(@NonNull State state1, @Nullable State state2) {
return state2 != null && state2.compareTo(state1) < 0 ? state2 : state1;
}
static class ObserverWithState {
State mState;
GenericLifecycleObserver mLifecycleObserver;
ObserverWithState(LifecycleObserver observer, State initialState) {
mLifecycleObserver = Lifecycling.getCallback(observer);
mState = initialState;
}
void dispatchEvent(LifecycleOwner owner, Event event) {
State newState = getStateAfter(event);
mState = min(mState, newState);
mLifecycleObserver.onStateChanged(owner, event);
mState = newState;
}
}
}
复制代码从上面的分析中我们得知,ReportFragment的dispatch方法中,调用了LifecycleRegistry类的handleLifecycleEvent方法.该方法接收了传递的事件后调用getStateAfter获取了下一状态并调用moveToState更新,然后调用sync方法通知了生命周期观察者,也就是我们实现了LifecycleObserver接口的类.
在sync方法中,通过对比当前状态和上一状态来完成当前State的状态更新,在forwardPass和backwardPass方法中我们就可以看到事件派发方法了,ObserverWithState内部类的dispatchEvent方法.
static class ObserverWithState {
State mState;
GenericLifecycleObserver mLifecycleObserver;
ObserverWithState(LifecycleObserver observer, State initialState) {
mLifecycleObserver = Lifecycling.getCallback(observer);
mState = initialState;
}
void dispatchEvent(LifecycleOwner owner, Event event) {
State newState = getStateAfter(event);
mState = min(mState, newState);
mLifecycleObserver.onStateChanged(owner, event);
mState = newState;
}
}
}
复制代码@RestrictTo(RestrictTo.Scope.LIBRARY)
@SuppressWarnings({"WeakerAccess", "unused"})
public interface GenericLifecycleObserver extends LifecycleObserver {
/**
* Called when a state transition event happens.
*
* @param source The source of the event
* @param event The event
*/
void onStateChanged(LifecycleOwner source, Lifecycle.Event event);
}
复制代码ObserverWithState类的dispatchEvent方法调用了GenericLifecycleObserver接口的onStateChanged方法,那么这个mLifecycleObserver是从何而来的呢,在ObserverWithState方法中通过Lifecycling.getCallback(observer)方法获得,继续往下跟.
@RestrictTo(RestrictTo.Scope.LIBRARY_GROUP)
public class Lifecycling {
@RestrictTo(RestrictTo.Scope.LIBRARY_GROUP)
public class Lifecycling {
private static final int REFLECTIVE_CALLBACK = 1;
private static final int GENERATED_CALLBACK = 2;
private static Map<Class, Integer> sCallbackCache = new HashMap<>();
private static Map<Class, List<Constructor<? extends GeneratedAdapter>>> sClassToAdapters =
new HashMap<>();
@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);
}
private static GeneratedAdapter createGeneratedAdapter(
Constructor<? extends GeneratedAdapter> constructor, Object object) {
//noinspection TryWithIdenticalCatches
try {
return constructor.newInstance(object);
} catch (IllegalAccessException e) {
throw new RuntimeException(e);
} catch (InstantiationException e) {
throw new RuntimeException(e);
} catch (InvocationTargetException e) {
throw new RuntimeException(e);
}
}
@Nullable
private static Constructor<? extends GeneratedAdapter> generatedConstructor(Class<?> klass) {
try {
Package aPackage = klass.getPackage();
String name = klass.getCanonicalName();
final String fullPackage = aPackage != null ? aPackage.getName() : "";
final String adapterName = getAdapterName(fullPackage.isEmpty() ? name :
name.substring(fullPackage.length() + 1));
@SuppressWarnings("unchecked") final Class<? extends GeneratedAdapter> aClass =
(Class<? extends GeneratedAdapter>) Class.forName(
fullPackage.isEmpty() ? adapterName : fullPackage + "." + adapterName);
Constructor<? extends GeneratedAdapter> constructor =
aClass.getDeclaredConstructor(klass);
if (!constructor.isAccessible()) {
constructor.setAccessible(true);
}
return constructor;
} catch (ClassNotFoundException e) {
return null;
} catch (NoSuchMethodException e) {
// this should not happen
throw new RuntimeException(e);
}
}
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) {
// anonymous class bug:35073837
if (klass.getCanonicalName() == null) {
return REFLECTIVE_CALLBACK;
}
Constructor<? extends GeneratedAdapter> constructor = generatedConstructor(klass);
if (constructor != null) {
sClassToAdapters.put(klass, Collections
.<Constructor<? extends GeneratedAdapter>>singletonList(constructor));
return GENERATED_CALLBACK;
}
boolean hasLifecycleMethods = ClassesInfoCache.sInstance.hasLifecycleMethods(klass);
if (hasLifecycleMethods) {
return REFLECTIVE_CALLBACK;
}
Class<?> superclass = klass.getSuperclass();
List<Constructor<? extends GeneratedAdapter>> adapterConstructors = null;
if (isLifecycleParent(superclass)) {
if (getObserverConstructorType(superclass) == REFLECTIVE_CALLBACK) {
return REFLECTIVE_CALLBACK;
}
adapterConstructors = new ArrayList<>(sClassToAdapters.get(superclass));
}
for (Class<?> intrface : klass.getInterfaces()) {
if (!isLifecycleParent(intrface)) {
continue;
}
if (getObserverConstructorType(intrface) == REFLECTIVE_CALLBACK) {
return REFLECTIVE_CALLBACK;
}
if (adapterConstructors == null) {
adapterConstructors = new ArrayList<>();
}
adapterConstructors.addAll(sClassToAdapters.get(intrface));
}
if (adapterConstructors != null) {
sClassToAdapters.put(klass, adapterConstructors);
return GENERATED_CALLBACK;
}
return REFLECTIVE_CALLBACK;
}
private static boolean isLifecycleParent(Class<?> klass) {
return klass != null && LifecycleObserver.class.isAssignableFrom(klass);
}
/**
* Create a name for an adapter class.
*/
public static String getAdapterName(String className) {
return className.replace(".", "_") + "_LifecycleAdapter";
}
private Lifecycling() {
}
}
}
复制代码我们可以看见,在getCallback方法中,是通过反射来实现的,因为我们是实现的LifecycleObserver接口,所以最后我们得到的是ReflectiveGenericLifecycleObserver,接着往下跟.
class ReflectiveGenericLifecycleObserver implements GenericLifecycleObserver {
private final Object mWrapped;
private final CallbackInfo mInfo;
ReflectiveGenericLifecycleObserver(Object wrapped) {
mWrapped = wrapped;
mInfo = ClassesInfoCache.sInstance.getInfo(mWrapped.getClass());
}
@Override
public void onStateChanged(LifecycleOwner source, Event event) {
mInfo.invokeCallbacks(source, event, mWrapped);
}
}
复制代码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);
}
Method[] methods = getDeclaredMethods(klass);
for (Method method : methods) {
OnLifecycleEvent annotation = method.getAnnotation(OnLifecycleEvent.class);
if (annotation != null) {
// Optimization for reflection, we know that this method is called
// when there is no generated adapter. But there are methods with @OnLifecycleEvent
// so we know that will use ReflectiveGenericLifecycleObserver,
// so we createInfo in advance.
// CreateInfo always initialize mHasLifecycleMethods for a class, so we don't do it
// here.
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);
}
}
CallbackInfo getInfo(Class klass) {
CallbackInfo existing = mCallbackMap.get(klass);
if (existing != null) {
return existing;
}
existing = createInfo(klass, null);
return existing;
}
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);
}
}
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;
}
@SuppressWarnings("WeakerAccess")
static class CallbackInfo {
final Map<Lifecycle.Event, List<MethodReference>> mEventToHandlers;
final Map<MethodReference, Lifecycle.Event> mHandlerToEvent;
CallbackInfo(Map<MethodReference, Lifecycle.Event> handlerToEvent) {
mHandlerToEvent = handlerToEvent;
mEventToHandlers = new HashMap<>();
for (Map.Entry<MethodReference, Lifecycle.Event> entry : handlerToEvent.entrySet()) {
Lifecycle.Event event = entry.getValue();
List<MethodReference> methodReferences = mEventToHandlers.get(event);
if (methodReferences == null) {
methodReferences = new ArrayList<>();
mEventToHandlers.put(event, methodReferences);
}
methodReferences.add(entry.getKey());
}
}
@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);
}
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);
}
}
}
}
@SuppressWarnings("WeakerAccess")
static class MethodReference {
final int mCallType;
final Method mMethod;
MethodReference(int callType, Method method) {
mCallType = callType;
mMethod = method;
mMethod.setAccessible(true);
}
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);
}
}
@Override
public boolean equals(Object o) {
if (this == o) {
return true;
}
if (o == null || getClass() != o.getClass()) {
return false;
}
MethodReference that = (MethodReference) o;
return mCallType == that.mCallType && mMethod.getName().equals(that.mMethod.getName());
}
@Override
public int hashCode() {
return 31 * mCallType + mMethod.getName().hashCode();
}
}
}
复制代码最终执行了invokeCallbacks方法,追根溯源可以发现,在Lifecycling的getCallback方法中同时执行了getObserverConstructorType方法,一步步往下跟,最后执行到ClassesInfoCache的hasLifecycleMethods方法中,随后调用createInfo(Class klass, @Nullable Method[] declaredMethods)方法,在这个方法中,通过反射获取到我们通过OnLifecycleEvent注解修饰的方法,并且按照Event的类型存储到CallbackInfo中.
那么总结一下,我们在生命周期观察者(实现了LifecycleObserver接口的类)中用注解修饰的方法会通过反射被获取并保存,在生命周期发生改变的时候再找到对应的方法,通过反射来调用.
那么到此,整个Lifecycle监听生命周期的实现原理就分析完毕了.
如果本文能够帮助到你,麻烦您动动小手给我点一个喜欢,如有不足请指正. 下一篇文章为Lifecycle的进阶使用.
转载于:https://juejin.im/post/5d05f11251882548ec7c933f