当使用MVVM框架时，ViewModel会实现LifecycleObserver接口类，使用OnLifecycleEvent注解声明当生命周期发生改变时会回调的方法,从而在ViewModel层也可以监听到生命周期。那么它是如何实现的？
首先我们要了解一些基础相关的知识点
:是一个接口，没有声明任何抽象方法，就是为了标识其子类实现是LifecycleObserver这一类型;也是一个观察者，观察Activity/Fragment的生命周期状态改变
public interface LifecycleObserver {
}
Lifecycle:是一个抽象类，可用于添加/移除LifecycleObserver;使用Event枚举来定义生命周期标识符；使用State枚举来定义Activity/Fragment的状态
public abstract class Lifecycle {
@RestrictTo(.LIBRARY_GROUP)
@NonNull
AtomicReference<Object> mInternalScopeRef = new AtomicReference<>();
/**
* 添加一个LifecycleObserver
* @param observer
*/
@MainThread//指定该方法要在主线程 若在异步线程则抛异常
public abstract void addObserver(@NonNull LifecycleObserver observer);
/**
* 移除一个LifecycleObserver
* @param observer
*/
@MainThread
public abstract void removeObserver(@NonNull LifecycleObserver observer);
/**
* 获取当前状态
* @return
*/
@MainThread
@NonNull
public abstract State getCurrentState();
/**
* 生命周期事件
*/
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} constant that can be used to match all events.
*/
ON_ANY
}
public enum State {
/**
* Destroyed state for a LifecycleOwner. After this event, this Lifecycle will not dispatch
* any more events. For instance, for an {@link }, this state is reached
* <b>right before</b> Activity's {@link #onDestroy() onDestroy} call.
*/
DESTROYED,
/**
* Initialized state for a LifecycleOwner. For an {@link }, this is
* the state when it is constructed but has not received
* {@link #onCreate() onCreate} yet.
*/
INITIALIZED,
/**
* Created state for a LifecycleOwner. For an {@link }, this state
* is reached in two cases:
* <ul>
* <li>after {@link #onCreate() onCreate} call;
* <li><b>right before</b> {@link #onStop() onStop} call.
* </ul>
*/
CREATED,
/**
* Started state for a LifecycleOwner. For an {@link }, this state
* is reached in two cases:
* <ul>
* <li>after {@link #onStart() onStart} call;
* <li><b>right before</b> {@link #onPause() onPause} call.
* </ul>
*/
STARTED,
/**
* Resumed state for a LifecycleOwner. For an {@link }, this state
* is reached after {@link #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;
}
}
}
OnLifecycleEvent：是一个注解类，用于注解一个方法,其返回值是
@Retention()
@Target()
public @interface OnLifecycleEvent {
value();
}
IBaseViewModel：实现LifecycleObserver接口，定义了当Activity/Fragment的生命周期发生改变时会被调用的方法。(注意：需要使用OnLifecycleEvent注解声明)
interface IBaseViewModel implement LifecycleObserver {
@OnLifecycleEvent(.ON_CREATE)
void onCreate()
@OnLifecycleEvent(.ON_START)
void onStart()
@OnLifecycleEvent(.ON_RESUME)
void onResume()
@OnLifecycleEvent(.ON_PAUSE)
void onPause()
@OnLifecycleEvent(.ON_STOP)
void onStop()
@OnLifecycleEvent(.ON_DESTROY)
void onDestroy()
@OnLifecycleEvent(.ON_ANY)
void onAny(owner: LifecycleOwner, event: )
}
LifecycleOwner：是一个接口类，用于获取Lifecycle对象，在ComponentActivity/Fragment中实现类该接口类
interface LifecycleOwner {
@NonNull
Lifecycle getLifecycle();
}
接下来我们看一下调用流程
1.首先让BaseViewModel实现IBaseViewModel接口，那么BaseViewModel也是LifecycleObserver的子类
abstract class BaseViewModel<M : BaseModel> exdends AndroidViewModel implement IBaseViewModel {
}
2.在Activity/Fragment中调用该方法将BaseViewModel作为LifecycleObserver被添加到观察者列表中
getLifecycle().addObserver(LifecycleObserver)
3.我们可以看到在Fragment/ComponentActivity中会创建LifecycleRegistry对象，通过getLifecycle()获取的就是LifecycleRegistry，而LifecycleRegistry将会将LifecycleObserver添加到观察者列表中
public class Fragment implements ComponentCallbacks, OnCreateContextMenuListener, LifecycleOwner,
ViewModelStoreOwner {
LifecycleRegistry mLifecycleRegistry = new LifecycleRegistry(this);
@Override
public Lifecycle getLifecycle() {
return mLifecycleRegistry;
}
.......
}
public class ComponentActivity extends Activity
implements LifecycleOwner, {
LifecycleRegistry mLifecycleRegistry = new LifecycleRegistry(this);
@Override
public Lifecycle getLifecycle() {
return mLifecycleRegistry;
}
@Override
@SuppressWarnings("RestrictedApi")
protected void onCreate(@Nullable Bundle savedInstanceState) {
(savedInstanceState);
(this);
}
.......
}
4.在ComponentActivity的onCreate()中调用 (this)，即为Activity增加了一个ReportFragment，这个fragment没有其它操作，就是用来同步Activity生命周期的，在各个生命周期方法中通过调用dispatch()将当前生命周期状态通知给 LifecycleRegistry的观察者列表，而观察者就是LifecycleObserver，它里面有我们注解的生命周期方法，而LifecycleRegistry会通过反射获取注解的方法，之后根据对应的生命周期状态调用对应的方法，从而使我们的ViewModel(LifecycleObserver的子类)也能够同步Activity的生命周期状态，那么LifecycleRegistry到底如何操作的?
public class ComponentActivity extends Activity
implements LifecycleOwner, {
protected void onCreate(@Nullable Bundle savedInstanceState) {
(savedInstanceState);
(this);
}
}
public class ReportFragment extends Fragment {
private static final String REPORT_FRAGMENT_TAG = ""
+ ".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
manager = ();
if ((REPORT_FRAGMENT_TAG) == null) {
().add(new ReportFragment(), REPORT_FRAGMENT_TAG).commit();
// Hopefully, we are the first to make a transaction.
();
}
}
static ReportFragment get(Activity activity) {
return (ReportFragment) ().findFragmentByTag(
REPORT_FRAGMENT_TAG);
}
private ActivityInitializationListener mProcessListener;
private void dispatchCreate(ActivityInitializationListener listener) {
if (listener != null) {
();
}
}
private void dispatchStart(ActivityInitializationListener listener) {
if (listener != null) {
();
}
}
private void dispatchResume(ActivityInitializationListener listener) {
if (listener != null) {
();
}
}
@Override
public void onActivityCreated(Bundle savedInstanceState) {
(savedInstanceState);
dispatchCreate(mProcessListener);
dispatch(.ON_CREATE);
}
@Override
public void onStart() {
();
dispatchStart(mProcessListener);
dispatch(.ON_START);
}
@Override
public void onResume() {
();
dispatchResume(mProcessListener);
dispatch(.ON_RESUME);
}
@Override
public void onPause() {
();
dispatch(.ON_PAUSE);
}
@Override
public void onStop() {
();
dispatch(.ON_STOP);
}
@Override
public void onDestroy() {
();
dispatch(.ON_DESTROY);
// just want to be sure that we won't leak reference to an activity
mProcessListener = null;
}
private void dispatch( 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);
}
}
}
void setProcessListener(ActivityInitializationListener processListener) {
mProcessListener = processListener;
}
interface ActivityInitializationListener {
void onCreate();
void onStart();
void onResume();
}
}
继承于Lifecycler，重写了addObserver()/removeObserver(),添加和移除观察者；内部有一个内部类ObserverWithState,存储LifecycleObserver以及LifecycleObserver的状态，它会通过LifecycleObserver的子类类型来判断是那一种GenericLifecycleObserver，默认返回ReflectiveGenericLifecycleObserver，而ReflectiveGenericLifecycleObserver会通过反射获取所有的注解OnLifecycleEvent的方法并将其存储起来;FastSafeIterableMap是以LifecycleObserver(我们传进来的ViewModel，实现了LifecycleObserver)为key，以存储LifecycleObserver和状态的ObserverWithState为value，其优势是能在遍历的过程中也能进行添加和移除操作;mLifecycleOwner其实是Activity/Fragment的弱引用，主要判断Activity/Fragment是否被销毁，若被销毁将不再进行后续操作;mState存储当前Activity/Fragment的状态 其顺序为DESTROYED < INITIALIZED < CREATED < STARTED < RESUMED；mHandlingEvent用来判断是否正在同步状态，true：正在同步所有观察者的状态 false：没有同步所有观察者的状态；mAddingObserverCounter:当添加一个观察者的时候加1 添加完成后减1;mNewEventOccurred：当正在同步或者添加观察者的时候为true 默认为false，当为true时会中断当前的同步操作，之后根据最后的状态同步所欲观察者的状态
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() {
// (this);
// (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 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 (() == 0) {
return true;
}
State eldestObserverState = ().getValue().mState;
State newestObserverState = ().getValue().mState;
return eldestObserverState == newestObserverState && mState == newestObserverState;
}
private State calculateTargetState(LifecycleObserver observer) {
Entry<LifecycleObserver, ObserverWithState> previous = (observer);
State siblingState = previous != null ? ().mState : null;
State parentState = !() ? (() - 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 = (observer, statefulObserver);
if (previous != null) {
return;
}
LifecycleOwner lifecycleOwner = ();
if (lifecycleOwner == null) {
// it is null we should be destroyed. Fallback quickly
return;
}
boolean isReentrance = mAddingObserverCounter != 0 || mHandlingEvent;
State targetState = calculateTargetState(observer);
mAddingObserverCounter++;
while (((targetState) < 0
&& (observer))) {
pushParentState();
(lifecycleOwner, upEvent());
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() {
(() - 1);
}
private void pushParentState(State state) {
(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.
(observer);
}
/**
* The number of observers.
*
* @return The number of observers.
*/
@SuppressWarnings("WeakerAccess")
public int getObserverCount() {
return ();
}
@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 =
();
while (() && !mNewEventOccurred) {
Entry<LifecycleObserver, ObserverWithState> entry = ();
ObserverWithState observer = ();
while (((mState) < 0 && !mNewEventOccurred
&& (()))) {
pushParentState();
(lifecycleOwner, upEvent());
popParentState();
}
}
}
private void backwardPass(LifecycleOwner lifecycleOwner) {
Iterator<Entry<LifecycleObserver, ObserverWithState>> descendingIterator =
();
while (() && !mNewEventOccurred) {
Entry<LifecycleObserver, ObserverWithState> entry = ();
ObserverWithState observer = ();
while (((mState) > 0 && !mNewEventOccurred
&& (()))) {
Event event = downEvent();
pushParentState(getStateAfter(event));
(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 = ();
if (lifecycleOwner == null) {
(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 ((().getValue().mState) < 0) {
backwardPass(lifecycleOwner);
}
Entry<LifecycleObserver, ObserverWithState> newest = ();
if (!mNewEventOccurred && newest != null
&& (().mState) > 0) {
forwardPass(lifecycleOwner);
}
}
mNewEventOccurred = false;
}
static State min(@NonNull State state1, @Nullable State state2) {
return state2 != null && (state1) < 0 ? state2 : state1;
}
static class ObserverWithState {
State mState;
GenericLifecycleObserver mLifecycleObserver;
ObserverWithState(LifecycleObserver observer, State initialState) {
mLifecycleObserver = (observer);
mState = initialState;
}
void dispatchEvent(LifecycleOwner owner, Event event) {
State newState = getStateAfter(event);
mState = min(mState, newState);
(owner, event);
mState = newState;
}
}
}
说明
- 1.在addObserver()中会将LifecycleObserver以及状态封装到ObserverWithState中，然后存到FastSafeIterableMap，在添加之后若当前界面的状态和 LifecycleObserver(ViewModel)的状态不一样，将会更新当前LifecycleObserver(ViewModel)的状态。最后若当前没有同步状态操作，那么将调用sync()进行所有观察者的同步操作
- 2.在removeObserver()中直接从FastSafeIterableMap移除对应的LifecycleObserver
- 3.在getCurrentState()中获取当前Activity/Fragment的生命周期状态
- ()在ReportFragment中被调用，通知所有观察者生命周期改变
- () while循环查看所有观察者的状态是否和当前的状态相同，若相同，表示所有的观察者的状态都是最新的；若不相同，则需要同步所有的观察者的的状态，遍历观察者，若观察者的状态小于当前状态，则状态需要向前移动，会调用forwardPass();若观察者的状态大于当前状态，则状态需要向后移动，会调用backwardPass()
- ():当有观察者的状态小于当前状态时会被调用，将观察者的状态变成下一个状态，之后调用ObserverWithState的dispatchEvent()触发对应LifecycleObserver的注解事件方法
- ():当有观察者的状态大于当前状态时会被调用，将观察者的状态变成上一个状态，之后调用ObserverWithState的dispatchEvent()触发对应LifecycleObserver的注解事件方法
- ():用来修改LifecycleObserver的状态，会调用ReflectiveGenericLifecycleObserver的onStateChanged()，因ReflectiveGenericLifecycleObserver通过ClassesInfoCache存储了所有的注解方法，当调用该方法时会获取所有对应的注解事件的方法集合，然后通过反射调用对应的方法，那么ViewModel中的使用OnLifecycleEvent注解的方法被调用
是GenericLifecycleObserver的子类，当LifecycleObserver被封装到ObserverWithState时会通过(observer)获取一个GenericLifecycleObserver，若observer不是FullLifecycleObserver/GenericLifecycleObserver/SingleGeneratedAdapterObserver/CompositeGeneratedAdaptersObserver,那么将使用ReflectiveGenericLifecycleObserver。 当Activity/Fragment的状态发生改变通知ViewModel时，最终会通过它来通过反射调用方法来实现通知
class ReflectiveGenericLifecycleObserver implements GenericLifecycleObserver {
//LifecycleObserver
private final Object mWrapped;
private final mInfo;
ReflectiveGenericLifecycleObserver(Object wrapped) {
mWrapped = wrapped;
mInfo = (());
}
@Override
public void onStateChanged(LifecycleOwner source, event) {
(source, event, mWrapped);
}
}
说明
- 1.实现GenericLifecycleObserver接口类，重写了onStateChanged()方法
- 2.在构造函数中持有LifecycleObserver(ViewModel)的引用，同时通过()获取LifecycleObserver所有的使用了OnLifecycleEvent注解声明的方法信息
- 3.当Activity/Fragment的生命周期发生了改变，将会调用ObserverWithState的dispatchEvent()，而在dispatchEvent()中会调用onStateChanged()，那么最终ClassesInfoCache会根据事件获取对应事件的所有方法，最后通过反射调用触发这些方法
主要用来收集Class中所有的使用OnLifecycleEvent的方法，并根据事件类型，来触发对应事件类型的方法
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 ((klass)) {
return (klass);
}
Method[] methods = getDeclaredMethods(klass);
for (Method method : methods) {
OnLifecycleEvent annotation = ();
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;
}
}
(klass, false);
return false;
}
private Method[] getDeclaredMethods(Class klass) {
try {
return ();
} 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 = (klass);
if (existing != null) {
return existing;
}
existing = createInfo(klass, null);
return existing;
}
private void verifyAndPutHandler(Map<MethodReference, > handlers,
MethodReference newHandler, newEvent, Class klass) {
event = (newHandler);
if (event != null && newEvent != event) {
Method method = ;
throw new IllegalArgumentException(
"Method " + () + " in " + ()
+ " already declared with different @OnLifecycleEvent value: previous"
+ " value " + event + ", new value " + newEvent);
}
if (event == null) {
(newHandler, newEvent);
}
}
private CallbackInfo createInfo(Class klass, @Nullable Method[] declaredMethods) {
Class superclass = ();
Map<MethodReference, > handlerToEvent = new HashMap<>();
if (superclass != null) {
CallbackInfo superInfo = getInfo(superclass);
if (superInfo != null) {
();
}
}
Class[] interfaces = ();
for (Class intrfc : interfaces) {
for (<MethodReference, > entry : getInfo(
intrfc).()) {
verifyAndPutHandler(handlerToEvent, (), (), klass);
}
}
Method[] methods = declaredMethods != null ? declaredMethods : getDeclaredMethods(klass);
boolean hasLifecycleMethods = false;
for (Method method : methods) {
OnLifecycleEvent annotation = ();
if (annotation == null) {
continue;
}
hasLifecycleMethods = true;
Class<?>[] params = ();
int callType = CALL_TYPE_NO_ARG;
if ( > 0) {
callType = CALL_TYPE_PROVIDER;
if (!params[0].isAssignableFrom()) {
throw new IllegalArgumentException(
"invalid parameter type. Must be one and instanceof LifecycleOwner");
}
}
event = ();
if ( > 1) {
callType = CALL_TYPE_PROVIDER_WITH_EVENT;
if (!params[1].isAssignableFrom()) {
throw new IllegalArgumentException(
"invalid parameter type. second arg must be an event");
}
if (event != .ON_ANY) {
throw new IllegalArgumentException(
"Second arg is supported only for ON_ANY value");
}
}
if ( > 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);
(klass, info);
(klass, hasLifecycleMethods);
return info;
}
@SuppressWarnings("WeakerAccess")
static class CallbackInfo {
final Map<, List<MethodReference>> mEventToHandlers;
final Map<MethodReference, > mHandlerToEvent;
CallbackInfo(Map<MethodReference, > handlerToEvent) {
mHandlerToEvent = handlerToEvent;
mEventToHandlers = new HashMap<>();
for (<MethodReference, > entry : ()) {
event = ();
List<MethodReference> methodReferences = (event);
if (methodReferences == null) {
methodReferences = new ArrayList<>();
(event, methodReferences);
}
(());
}
}
@SuppressWarnings("ConstantConditions")
void invokeCallbacks(LifecycleOwner source, event, Object target) {
invokeMethodsForEvent((event), source, event, target);
invokeMethodsForEvent((.ON_ANY), source, event,
target);
}
private static void invokeMethodsForEvent(List<MethodReference> handlers,
LifecycleOwner source, event, Object mWrapped) {
if (handlers != null) {
for (int i = () - 1; i >= 0; i--) {
(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;
(true);
}
void invokeCallback(LifecycleOwner source, event, Object target) {
//noinspection TryWithIdenticalCatches
try {
switch (mCallType) {
case CALL_TYPE_NO_ARG:
(target);
break;
case CALL_TYPE_PROVIDER:
(target, source);
break;
case CALL_TYPE_PROVIDER_WITH_EVENT:
(target, source, event);
break;
}
} catch (InvocationTargetException e) {
throw new RuntimeException("Failed to call observer method", ());
} catch (IllegalAccessException e) {
throw new RuntimeException(e);
}
}
@Override
public boolean equals(Object o) {
if (this == o) {
return true;
}
if (o == null || getClass() != ()) {
return false;
}
MethodReference that = (MethodReference) o;
return mCallType == && ().equals(());
}
@Override
public int hashCode() {
return 31 * mCallType + ().hashCode();
}
}
}
说明
- 1.CALL_TYPE_NO_ARG： 标识符，表示通过反射调用方法时不传递参数
- 2.CALL_TYPE_PROVIDER：标识符，表示通过反射调用方法时传递一个参数
- 3.CALL_TYPE_PROVIDER_WITH_EVENT：标识符，表示通过反射调用方法时传递两个参数
- ：map集合，以class为key，以boolean为value，标识这个class中是否含有OnLifecycleEvent注解的方法
- ：map集合，以class为key，以存储了class相关信息的CallbackInfo为value，它包含了这个class所有的OnLifecycleEvent注解的方法
- 是一个静态内部类，它存储OnLifecycleEvent注解的方法以及调用时的传参类型，其中invokeCallback()方法实现了Method的反射调用
- 是一个静态内部类，当我们把所有OnLifecycleEvent注解的方法获取到后进行整理，将同一个事件的方法，放入一个集合中，并以事件为key，以对应方法集合为value存储到map中
- ()：调用invokeMethodsForEvent()触发对应的事件，但同时我们可以看到，调用任何一个事件都会触发ON_ANY事件
- ():根据事件找到对应事件的方法集合，然后遍历集合所有的方法进行调用
总结
- 是一个接口类，没有定义任何方法，它表示是Activity/Fragment生命周期变化的观察者
- 2.自定义的ViewModel实现了IBaseViewModel接口类，而IBaseViewModel实现了LifecycleObserver，并定义了使用OnLifecycleEvent注解声明的方法,那么ViewModel就是一个LifecycleObserver。那么当Activity/Fragment的生命周期发生改变时，如何被触发对应的方法呢?那么就要借助LifecycleRegistry
- 是Lifecycle的子类，它在Activity/Fragment中被创建，它用来添加/移除生命周期变化观察者，在生命周期发生改变后同步生命周期状态
- 会添加一个ReportFragment，这个framgent没有界面，不做其他操作 ，只是用来同步生命周期的，当fragment在不同的生命周期状态时，将会调用LifecycleRegistry的handleLifecycleEvent()来通知观察者
- 会有一个FragmentManager的子类FragmentManagerImpl，用于对Fragment进行管理，而在Fragment的生命周期状态发生改变时会调用FragmentManagerImpl的moveToState()，在方法中会根据当前Fragment的State触发不同的操作，但最终都会调用 (Event)来通知观察者
- 的addObserver()用来添加观察者，并将其对应的状态一起封装到ObserverWithState中，之后同步添加的观察者状态，最后调用sync()将当前最新状态同步到所有的观察者中
- 的removeObserver()用来移除观察者
- 是LifecycleRegistry的一个静态内部类，它包含一个LifecycleObserver以及对应的状态信心，同时通过(observer)得到一个GenericLifecycleObserver，最终是ReflectiveGenericLifecycleObserver
- 是GenericLifecycleObserver的子类，它会通过反射获取LifecycleObserver中所有使用OnLifecycleEvent注解声明的方法，之后将其归类，将所有相同注解事件放入集合，以Event为key，以集合为value，存入到map中
- 10.当Activity/Fragment的进入不同生命周期时会调用LifecycleRegistry的handleLifecycleEvent()，之后调用sync()遍历所有的观察者列表，将当前状态传给观察者的包装对象ObserverWithState，ObserverWithState会触发dispatchEvent()，在dispatchEvent()中调用ReflectiveGenericLifecycleObserver的onStateChanged()，而在onStateChanged()中会调用的invokeCallbacks()，最后会根据事件Event获取到对应的方法集合，通过反射触发方法，这样就完成当Acitvity/Fragment的生命周期发生改变时，ViewModel(LifecycleObserver)中使用OnLifecycleEvent注解的方法会被触发