1.使用
Acitivty或者Fragment里面调用getLifecycle().addObserver(MyObserver())
//这些注解已经过时
class MyObserver : LifecycleObserver {
@OnLifecycleEvent(Lifecycle.Event.ON_CREATE)
fun onCreate() {}
@OnLifecycleEvent(Lifecycle.Event.ON_START)
fun onStart() {}
@OnLifecycleEvent(Lifecycle.Event.ON_RESUME)
fun onResume() {}
@OnLifecycleEvent(Lifecycle.Event.ON_PAUSE)
fun onPause() {}
@OnLifecycleEvent(Lifecycle.Event.ON_STOP)
fun onStop() {}
@OnLifecycleEvent(Lifecycle.Event.ON_DESTROY)
fun onDestroy() {}
}
最新的使用
public interface DefaultLifecycleObserver extends FullLifecycleObserver {
//在onCreate之后调用
@Override
default void onCreate(@NonNull LifecycleOwner owner) {
}
//在onStart之后调用
@Override
default void onStart(@NonNull LifecycleOwner owner) {
}
//在onResume之后调用
@Override
default void onResume(@NonNull LifecycleOwner owner) {
}
//在onPause之前调用
@Override
default void onPause(@NonNull LifecycleOwner owner) {
}
//onStop
@Override
default void onStop(@NonNull LifecycleOwner owner) {
}
//onDestroy
@Override
default void onDestroy(@NonNull LifecycleOwner owner) {
}
}
或者
public interface LifecycleEventObserver extends LifecycleObserver {
void onStateChanged(@NonNull LifecycleOwner source, @NonNull Lifecycle.Event event);
}
2.源码解析
activity和fragment内通过
androidx.fragment.app.Fragment
androidx.activity.ComponentActivity
这两个类中的
public Lifecycle getLifecycle() {
return mLifecycleRegistry;
}
获取mLifecycleRegistry。
//activity内
private LifecycleRegistry mLifecycleRegistry = new LifecycleRegistry(this);
//fragment里面
private void initLifecycle() {
mLifecycleRegistry = new LifecycleRegistry(this);
//继承关系
自己的MainActivity
androidx.appcompat.app.AppCompatActivity
androidx.fragment.app.FragmentActivity
androidx.activity.ComponentActivity
androidx.core.app.ComponentActivity
android.app.Activity
ComponentActivity的onCreate里面 调用
ReportFragment.injectIfNeededIn(this);
可以看到往activity里面添加了一个无界面的ReportFragment
public static void injectIfNeededIn(Activity activity) {
...
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();
}
}
ReportFragment 的生命周期
@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;
}
@SuppressWarnings("deprecation")
static void dispatch(@NonNull Activity activity, @NonNull Lifecycle.Event event) {
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);
}
}
}
上面调用了LifecycleRegistry的 handleLifecycleEvent
public void handleLifecycleEvent(@NonNull Lifecycle.Event event) {
enforceMainThreadIfNeeded("handleLifecycleEvent");
moveToState(event.getTargetState());
}
可以看到最后调用了moveToState，就跟FragmentActivity 的 onCreate方法里面调用handleLifecycleEvent后也调用了moveToState一样
@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
mFragmentLifecycleRegistry.handleLifecycleEvent(Lifecycle.Event.ON_CREATE);
mFragments.dispatchCreate();
}
public void handleLifecycleEvent(@NonNull Lifecycle.Event event) {
enforceMainThreadIfNeeded("handleLifecycleEvent");
moveToState(event.getTargetState());
}
mState就是一个状态机
private void moveToState(State next) {
if (mState == next) {
return;
}
if (mState == INITIALIZED && next == DESTROYED) {
throw new IllegalStateException("no event down from " + mState);
}
mState = next;
if (mHandlingEvent || mAddingObserverCounter != 0) {
mNewEventOccurred = true;
// we will figure out what to do on upper level.
return;
}
mHandlingEvent = true;
sync();
mHandlingEvent = false;
if (mState == DESTROYED) {
mObserverMap = new FastSafeIterableMap<>();
}
}
private void sync() {
LifecycleOwner lifecycleOwner = mLifecycleOwner.get();
if (lifecycleOwner == null) {
throw new IllegalStateException("LifecycleOwner of this LifecycleRegistry is already"
+ "garbage collected. It is too late to change lifecycle state.");
}
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);
}
Map.Entry<LifecycleObserver, ObserverWithState> newest = mObserverMap.newest();
if (!mNewEventOccurred && newest != null
&& mState.compareTo(newest.getValue().mState) > 0) {
forwardPass(lifecycleOwner);
}
}
mNewEventOccurred = false;
}
不管是backwardPass或者forwardPass都调用
private void backwardPass(LifecycleOwner lifecycleOwner) {
Iterator<Map.Entry<LifecycleObserver, ObserverWithState>> descendingIterator =
mObserverMap.descendingIterator();
while (descendingIterator.hasNext() && !mNewEventOccurred) {
Map.Entry<LifecycleObserver, ObserverWithState> entry = descendingIterator.next();
ObserverWithState observer = entry.getValue();
while ((observer.mState.compareTo(mState) > 0 && !mNewEventOccurred
&& mObserverMap.contains(entry.getKey()))) {
Event event = Event.downFrom(observer.mState);
if (event == null) {
throw new IllegalStateException("no event down from " + observer.mState);
}
pushParentState(event.getTargetState());
observer.dispatchEvent(lifecycleOwner, event);
popParentState();
}
}
}
observer是 LifecycleRegistry
void dispatchEvent(LifecycleOwner owner, Event event) {
State newState = event.getTargetState();
mState = min(mState, newState);
mLifecycleObserver.onStateChanged(owner, event);
mState = newState;
}
onstateChanged有很多实现
看ComponentActivity 的构造函数
public ComponentActivity() {
......
Lifecycle lifecycle = this.getLifecycle();
if (lifecycle == null) {
throw new IllegalStateException("getLifecycle() returned null in ComponentActivity's constructor. Please make sure you are lazily constructing your Lifecycle in the first call to getLifecycle() rather than relying on field initialization.");
} else {
if (VERSION.SDK_INT >= 19) {
this.getLifecycle().addObserver(new LifecycleEventObserver() {
public void onStateChanged(@NonNull LifecycleOwner source, @NonNull Lifecycle.Event event) {
if (event == Event.ON_STOP) {
Window window = ComponentActivity.this.getWindow();
View decor = window != null ? window.peekDecorView() : null;
if (decor != null) {
ComponentActivity.Api19Impl.cancelPendingInputEvents(decor);
}
}
}
});
}
this.getLifecycle().addObserver(new LifecycleEventObserver() {
public void onStateChanged(@NonNull LifecycleOwner source, @NonNull Lifecycle.Event event) {
if (event == Event.ON_DESTROY) {
ComponentActivity.this.mContextAwareHelper.clearAvailableContext();
if (!ComponentActivity.this.isChangingConfigurations()) {
ComponentActivity.this.getViewModelStore().clear();
}
}
}
});
this.getLifecycle().addObserver(new LifecycleEventObserver() {
public void onStateChanged(@NonNull LifecycleOwner source, @NonNull Lifecycle.Event event) {
ComponentActivity.this.ensureViewModelStore();
ComponentActivity.this.getLifecycle().removeObserver(this);
}
});
....
}
}
addObserver方法
ObserverWithState statefulObserver = new ObserverWithState(observer, initialState);
ObserverWithState(LifecycleObserver observer, State initialState) {
mLifecycleObserver = Lifecycling.lifecycleEventObserver(observer);
mState = initialState;
}
@SuppressWarnings("deprecation")
static LifecycleEventObserver lifecycleEventObserver(Object object) {
boolean isLifecycleEventObserver = object instanceof LifecycleEventObserver;
boolean isFullLifecycleObserver = object instanceof FullLifecycleObserver;
if (isLifecycleEventObserver && isFullLifecycleObserver) {
return new FullLifecycleObserverAdapter((FullLifecycleObserver) object,
(LifecycleEventObserver) object);
}
if (isFullLifecycleObserver) {
return new FullLifecycleObserverAdapter((FullLifecycleObserver) object, null);
}
if (isLifecycleEventObserver) {
return (LifecycleEventObserver) 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);
}
这些return的就是保存了Observer的方法，比如我们看 ReflectiveGenericLifecycleObserver(object);
ReflectiveGenericLifecycleObserver(Object wrapped) {
mWrapped = wrapped;
mInfo = ClassesInfoCache.sInstance.getInfo(mWrapped.getClass());
}
实际上就是把有注解的方法都加进来
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);
}
}
//把观察者有注解的方法都加进来
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;
}
我们回到状态机 Lifecycle.java里面定义的
public enum State {
/**
* Destroyed state for a LifecycleOwner. After this event, this Lifecycle will not dispatch
* any more events. For instance, for an {@link android.app.Activity}, this state is reached
* <b>right before</b> Activity's {@link android.app.Activity#onDestroy() onDestroy} call.
*/
DESTROYED,
/**
* Initialized state for a LifecycleOwner. For an {@link android.app.Activity}, this is
* the state when it is constructed but has not received
* {@link android.app.Activity#onCreate(android.os.Bundle) onCreate} yet.
*/
INITIALIZED,
/**
* Created state for a LifecycleOwner. For an {@link android.app.Activity}, this state
* is reached in two cases:
* <ul>
* <li>after {@link android.app.Activity#onCreate(android.os.Bundle) onCreate} call;
* <li><b>right before</b> {@link android.app.Activity#onStop() onStop} call.
* </ul>
*/
CREATED,
/**
* Started state for a LifecycleOwner. For an {@link android.app.Activity}, this state
* is reached in two cases:
* <ul>
* <li>after {@link android.app.Activity#onStart() onStart} call;
* <li><b>right before</b> {@link android.app.Activity#onPause() onPause} call.
* </ul>
*/
STARTED,
/**
* Resumed state for a LifecycleOwner. For an {@link android.app.Activity}, this state
* is reached after {@link android.app.Activity#onResume() onResume} is called.
*/
RESUMED;
/**
* Compares if this State is greater or equal to the given {@code state}.
*
* @param state State to compare with
* @return true if this State is greater or equal to the given {@code state}
*/
public boolean isAtLeast(@NonNull State state) {
return compareTo(state) >= 0;
}
}
public abstract State getCurrentState();
@SuppressWarnings("WeakerAccess")
public enum Event {
/**
* Constant for onCreate event of the {@link LifecycleOwner}.
*/
ON_CREATE,
/**
* Constant for onStart event of the {@link LifecycleOwner}.
*/
ON_START,
/**
* Constant for onResume event of the {@link LifecycleOwner}.
*/
ON_RESUME,
/**
* Constant for onPause event of the {@link LifecycleOwner}.
*/
ON_PAUSE,
/**
* Constant for onStop event of the {@link LifecycleOwner}.
*/
ON_STOP,
/**
* Constant for onDestroy event of the {@link LifecycleOwner}.
*/
ON_DESTROY,
/**
* An {@link Event Event} constant that can be used to match all events.
*/
ON_ANY;
/**
* Returns the {@link Lifecycle.Event} that will be reported by a {@link Lifecycle}
* leaving the specified {@link Lifecycle.State} to a lower state, or {@code null}
* if there is no valid event that can move down from the given state.
*
* @param state the higher state that the returned event will transition down from
* @return the event moving down the lifecycle phases from state
*/
@Nullable
public static Event downFrom(@NonNull State state) {
switch (state) {
case CREATED:
return ON_DESTROY;
case STARTED:
return ON_STOP;
case RESUMED:
return ON_PAUSE;
default:
return null;
}
}
/**
* Returns the {@link Lifecycle.Event} that will be reported by a {@link Lifecycle}
* entering the specified {@link Lifecycle.State} from a higher state, or {@code null}
* if there is no valid event that can move down to the given state.
*
* @param state the lower state that the returned event will transition down to
* @return the event moving down the lifecycle phases to state
*/
@Nullable
public static Event downTo(@NonNull State state) {
switch (state) {
case DESTROYED:
return ON_DESTROY;
case CREATED:
return ON_STOP;
case STARTED:
return ON_PAUSE;
default:
return null;
}
}
/**
* Returns the {@link Lifecycle.Event} that will be reported by a {@link Lifecycle}
* leaving the specified {@link Lifecycle.State} to a higher state, or {@code null}
* if there is no valid event that can move up from the given state.
*
* @param state the lower state that the returned event will transition up from
* @return the event moving up the lifecycle phases from state
*/
@Nullable
public static Event upFrom(@NonNull State state) {
switch (state) {
case INITIALIZED:
return ON_CREATE;
case CREATED:
return ON_START;
case STARTED:
return ON_RESUME;
default:
return null;
}
}
/**
* Returns the {@link Lifecycle.Event} that will be reported by a {@link Lifecycle}
* entering the specified {@link Lifecycle.State} from a lower state, or {@code null}
* if there is no valid event that can move up to the given state.
*
* @param state the higher state that the returned event will transition up to
* @return the event moving up the lifecycle phases to state
*/
@Nullable
public static Event upTo(@NonNull State state) {
switch (state) {
case CREATED:
return ON_CREATE;
case STARTED:
return ON_START;
case RESUMED:
return ON_RESUME;
default:
return null;
}
}
/**
* Returns the new {@link Lifecycle.State} of a {@link Lifecycle} that just reported
* this {@link Lifecycle.Event}.
*
* Throws {@link IllegalArgumentException} if called on {@link #ON_ANY}, as it is a special
* value used by {@link OnLifecycleEvent} and not a real lifecycle event.
*
* @return the state that will result from this event
*/
@NonNull
public State getTargetState() {
switch (this) {
case ON_CREATE:
case ON_STOP:
return State.CREATED;
case ON_START:
case ON_PAUSE:
return State.STARTED;
case ON_RESUME:
return State.RESUMED;
case ON_DESTROY:
return State.DESTROYED;
case ON_ANY:
break;
}
throw new IllegalArgumentException(this + " has no target state");
}
}
根据顺序和状态机，找到state对应的event然后分发给Observer
3.全部流程
activity在创建的时候会被add进一个空白的fragment，而activity的生命周期发生变化的时候，fragment的生命周期也会发生变化，在fragment的相关生命周期方法中会dispatch对应的event，event与state又有着某种关系。而event中对应的state与mState不一致时，会根据event参数通过反射去执行lifecyclerobserver里面的方法。
