一、添加依赖
- 1、非androidX项目引入:
dependencies {
//运行时
implementation "android.arch.lifecycle:runtime:1.1.1"
// 编译期
annotationProcessor "android.arch.lifecycle:compiler:1.1.1"
}
- 2、androidX项目引入:
implementation "androidx.lifecycle:lifecycle-runtime:2.0.0"
implementation "androidx.lifecycle:lifecycle-extensions:2.0.0"
implementation "androidx.lifecycle:lifecycle-common-java8:2.0.0"
annotationProcessor "androidx.lifecycle:lifecycle-compiler:2.0.0"
类讲解
-
LifecycleOwner： 被观察者：实现该接口的类持有生命周期(Lifecycle对象)，该接口的生命周期(Lifecycle对象)的改变会被其注册的观察者LifecycleObserver观察到并触发其对应的事件。 -
LifecycleObserver： 观察者：实现该接口的类，通过注解的方式，可以通过被LifecycleOwner类的addObserver(LifecycleObserver o)方法注册,被注册后，LifecycleObserver便可以观察到LifecycleOwner的生命周期事件。 -
LifecycleRegistry：Lifecycle的子类， 起着添加观察者，响应生命周期事件，分发生命周期事件的作用。 -
ReportFragment：在ComponentActivity 里面注入，用来绑定生命周期。 -
State：当前生命周期所处状态。 -
Event：当前生命周期改变对应的事件，当Lifecycle发生改变，如进入onCreate,会自动发出ON_CREATE事件。
二、源码讲解
2.1、向Activity中添加ReportFragment 类
在ComponentActivity 中的onCretae方法
@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
mSavedStateRegistryController.performRestore(savedInstanceState);
ReportFragment.injectIfNeededIn(this);
if (mContentLayoutId != 0) {
setContentView(mContentLayoutId);
}
}
ReportFragment 类是一个Fragment，它负责分派生命周期的时间，injectIfNeededIn()就是在当前的Activity里添加一个ReportFragment。
public static void injectIfNeededIn(Activity activity) {
if (Build.VERSION.SDK_INT >= 29) {
// On API 29+, we can register for the correct Lifecycle callbacks directly
activity.registerActivityLifecycleCallbacks(
new LifecycleCallbacks());
}
//添加Fragment
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();
}
}
2.2、添加Observer
public ComponentActivity() {
Lifecycle lifecycle = getLifecycle();
//noinspection ConstantConditions
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.");
}
if (Build.VERSION.SDK_INT >= 19) {
getLifecycle().addObserver(new LifecycleEventObserver() {
@Override
public void onStateChanged(@NonNull LifecycleOwner source,
@NonNull Lifecycle.Event event) {
if (event == Lifecycle.Event.ON_STOP) {
Window window = getWindow();
final View decor = window != null ? window.peekDecorView() : null;
if (decor != null) {
decor.cancelPendingInputEvents();
}
}
}
});
}
getLifecycle().addObserver(new LifecycleEventObserver() {
@Override
public void onStateChanged(@NonNull LifecycleOwner source,
@NonNull Lifecycle.Event event) {
if (event == Lifecycle.Event.ON_DESTROY) {
if (!isChangingConfigurations()) {
getViewModelStore().clear();
}
}
}
});
if (19 <= SDK_INT && SDK_INT <= 23) {
getLifecycle().addObserver(new ImmLeaksCleaner(this));
}
}
LifecycleRegister 类中的addObserver方法
@Override
public void addObserver(@NonNull LifecycleObserver observer) {
//这个状态值是一个枚举类型的，生命周期越靠后，值越小 DESTROYED < INITIALIZED < CREATED < STARTED < RESUMED
State initialState = mState == DESTROYED ? DESTROYED : INITIALIZED;
ObserverWithState statefulObserver = new ObserverWithState(observer, initialState);
ObserverWithState previous = mObserverMap.putIfAbsent(observer, statefulObserver);
if (previous != null) {
return;
}
//由上面的使用可知，LifecycleOwner相当于是观察者模式中的被观察者，它来向各LifecycleObserver发送生命周期消息
LifecycleOwner lifecycleOwner = mLifecycleOwner.get();
if (lifecycleOwner == null) {
// it is null we should be destroyed. Fallback quickly
return;
}
//mAddingObserverCounter 表示正在添加的观察者的数量，
//开始添加新的observe之前，对该变量进行 ++ 操作，添加完成后，进行 -- 操作。
//如果mAddingObserverCounter 不为0 ，说明在添加observe A 时，在while循环中更新observe A 的生命周期时，又添加了新的observe B。
//这就是嵌套事件。而由于B 是后添加的，所以同一生命周期的回调顺序，要保证B 在A后 被回调。
//mHandlingEvent 表示是否正在处理事件中
boolean isReentrance = mAddingObserverCounter != 0 || mHandlingEvent;
State targetState = calculateTargetState(observer);
mAddingObserverCounter++;
//这里是while 循环，新添加的LifecycleObserver ，会不断更新自己的生命周期到指定的targetState
while ((statefulObserver.mState.compareTo(targetState) < 0
&& mObserverMap.contains(observer))) {
//为了 嵌套添加observer，保证各observe的回调顺序，这个状态是保存在变量mParentStates中的
pushParentState(statefulObserver.mState);
//注意这里的upEvent，获取statefulObserver.mState下一状态的值,
//也就是说mParentStates中的状态 比 即将执行的生命周期 要晚一个阶段
statefulObserver.dispatchEvent(lifecycleOwner, upEvent(statefulObserver.mState));
popParentState();
// mState / subling may have been changed recalculate
targetState = calculateTargetState(observer);
}
// 没有事件正在处理，或者是添加了observe,就需要重新同步一次状态
// 嵌套添加observer，只有最先添加的observer,isReentrance 才是false，才会执行sync()
// 也就是说嵌套添加observer，sync 只会被执行一次。
if (!isReentrance) {
// we do sync only on the top level.
//防止在添加过程中，生命周期发生了变化，状态不一致的问题
sync();
}
mAddingObserverCounter--;
}
我们看LifecycleRegistry中的内部类ObserverWithState的创建
static class ObserverWithState {
//保存 Observer 的state
State mState;
LifecycleEventObserver mLifecycleObserver;
ObserverWithState(LifecycleObserver observer, State initialState) {
//打包我们定义的观察者
mLifecycleObserver = Lifecycling.lifecycleEventObserver(observer);
mState = initialState;
}
void dispatchEvent(LifecycleOwner owner, Event event) {
State newState = getStateAfter(event);
mState = min(mState, newState);
//分发事件
mLifecycleObserver.onStateChanged(owner, event);
//更新 Observer 的 state
mState = newState;
}
}
执行Lifecycling.lifecycleEventObserver
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);
}
创建ReflectiveGenericLifecycleObserver观察者
class ReflectiveGenericLifecycleObserver implements LifecycleEventObserver {
private final Object mWrapped;
private final CallbackInfo mInfo;
ReflectiveGenericLifecycleObserver(Object wrapped) {
mWrapped = wrapped;
//获取观察这的class并保存
mInfo = ClassesInfoCache.sInstance.getInfo(mWrapped.getClass());
}
@Override
public void onStateChanged(LifecycleOwner source, Event event) {
mInfo.invokeCallbacks(source, event, mWrapped);
}
}
接下来看 ClassesInfoCache类的方法
CallbackInfo getInfo(Class klass) {
//从map缓存中获取class
CallbackInfo existing = mCallbackMap.get(klass);
if (existing != null) {
return existing;
}
//map没有创建
existing = createInfo(klass, null);
return existing;
}
private CallbackInfo createInfo(Class klass, @Nullable Method[] declaredMethods) {
//获取class
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);
}
}
//反射获取方法
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);
}
//初始化回调信息 CallbackInfo，在下面分发消息用的到
CallbackInfo info = new CallbackInfo(handlerToEvent);
//把class信息保存map中，方便上面方法下次直接获取 class
mCallbackMap.put(klass, info);
mHasLifecycleMethods.put(klass, hasLifecycleMethods);
return info;
}
然后Event发生变化的时候会从mCallbackMap中拿去对应的class文件，然后通过反射执行对应生命周期方法。源码分析如下：
2.3、ReportFragment的生命周期函数
@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(@NonNull Lifecycle.Event event) {
if (Build.VERSION.SDK_INT < 29) {
dispatch(getActivity(), event);
}
}
@SuppressWarnings("deprecation")
static void dispatch(@NonNull Activity activity, @NonNull Lifecycle.Event event) {
if (activity instanceof LifecycleRegistryOwner) {
((LifecycleRegistryOwner) activity).getLifecycle().handleLifecycleEvent(event);
return;
}
//ComponentActivity 实现了 LifecycleOwner 接口，所以执行此处
if (activity instanceof LifecycleOwner) {
Lifecycle lifecycle = ((LifecycleOwner) activity).getLifecycle();
if (lifecycle instanceof LifecycleRegistry) {
((LifecycleRegistry) lifecycle).handleLifecycleEvent(event);
}
}
}
你会发现都调用了dispatch()方法，而dispatch()方法则会判断Activity是否实现了LifecycleOwner接口，如果实现了该接口就调用LifecycleRegister#handleLifecycleEvent()，这样生命周期的状态就会借由LifecycleRegistry通知给各个LifecycleObserver从而调用其中对应Lifecycle.Event的方法。这种通过Fragment来感知Activity生命周期的方法其实在Glide的中也是有体现的。
public void handleLifecycleEvent(@NonNull Lifecycle.Event event) {
//获取事件对应的状态
State next = getStateAfter(event);
moveToState(next);
}
//通过事件Event获取状态State
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 void moveToState(State next) {
//判断状态是否对齐
if (mState == next) {
return;
}
//设置最新状态
mState = next;
if (mHandlingEvent || mAddingObserverCounter != 0) {
mNewEventOccurred = true;
// we will figure out what to do on upper level.
return;
}
mHandlingEvent = true;
//同步状态
sync();
mHandlingEvent = false;
}
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.");
}
//状态是否同步
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); //向后对齐
}
Entry<LifecycleObserver, ObserverWithState> newest = mObserverMap.newest();
//第一个 mSate 是被观察者（ReportFragment）的state
//第二个 mState 是观察者的状态，保存在 ObserverWithState 中
//只要比较两者的状态进行对齐
if (!mNewEventOccurred && newest != null
&& mState.compareTo(newest.getValue().mState) > 0) {
forwardPass(lifecycleOwner); //向前对齐
}
}
mNewEventOccurred = 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;
}
我们这边只看前进流程，后退流程同理
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 的当前的state
pushParentState(observer.mState);
//根据 state 获取 observer 的下一个 event，并分发
observer.dispatchEvent(lifecycleOwner, upEvent(observer.mState));
//移出上一次保存的 state
popParentState();
}
}
}
//通过状态获取事件
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 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();
}
}
}
ObserverWithState是LifecycleRegistry的实现类
static class ObserverWithState {
State mState;
LifecycleEventObserver mLifecycleObserver;
ObserverWithState(LifecycleObserver observer, State initialState) {
mLifecycleObserver = Lifecycling.lifecycleEventObserver(observer);
mState = initialState;
}
void dispatchEvent(LifecycleOwner owner, Event event) {
State newState = getStateAfter(event);
mState = min(mState, newState);
//改变事件
mLifecycleObserver.onStateChanged(owner, event);
mState = newState;
}
}
根据前面添加观察者分析，我们的到会进入ReflectiveGenericLifecycleObserver中执行onStateChanged
@Override
public void onStateChanged(LifecycleOwner source, Event event) {
mInfo.invokeCallbacks(source, event, mWrapped);
}
执行ClassesInfoCache内部类CallbackInfo#invokeCallbacks
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);
}
}
}
}
在ClassesInfoCache 内部类 MethodReference#invokeCallback我们可以看到通过反射执行生命周期方法
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();
}
}
三、使用
实现LifecycleObserver的实现
public class MyLifeCycleObserver implements LifecycleObserver {
public MyLifeCycleObserver() {
}
@OnLifecycleEvent(value = Lifecycle.Event.ON_CREATE)
private void onCreate(){
Log.i("--->", "onCreate");
}
@OnLifecycleEvent(value = Lifecycle.Event.ON_START)
private void onStart(){
Log.i("--->", "onStart");
}
@OnLifecycleEvent(value = Lifecycle.Event.ON_RESUME)
public void onResume(){
Log.i("--->", "onResume");
}
@OnLifecycleEvent(value = Lifecycle.Event.ON_PAUSE)
public void onPause(){
Log.i("--->", "onPause");
}
@OnLifecycleEvent(value = Lifecycle.Event.ON_STOP)
public void onStop(){
Log.i("--->", "onStop");
}
@OnLifecycleEvent(value = Lifecycle.Event.ON_DESTROY)
public void onDestroy(){
Log.i("--->", "onDestroy");
}
}
使用
public class LifecycleActivity extends AppCompatActivity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_lifecycle);
//添加自定义的观察者
getLifecycle().addObserver(new MyLifeCycleObserver());
}
}
四、关系图
事件分发图
Event驱动State图
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