解耦是软件开发亘古不变的追求,而Lifecycle正是这一名言的体现。
Android开发过程中,有些功能不可避免与页面的生命周期关联,LifeCycle作为Jetpack中具有生命周期感知姓的组件,通过感知activity/fragment/service/application的生命周期状态而做不同操作,功能与页面完成解耦,避免发生内存泄露。
本文我们将记录Lifecycle在Activity中的使用，以及源码分析。
Lifecycle在Activity上的使用
1,我们首先创建一个对象,并实现LifecycleObserver,这个对象就具有感知生命周期的能力。
package com.sun.lifecycle;
import androidx.lifecycle.Lifecycle;
import androidx.lifecycle.LifecycleObserver;
import androidx.lifecycle.OnLifecycleEvent;
import com.sun.common_library.util.LogUtil;
/**
* 具有感知Activity/fragment/service/application生命周期的能力
* 说白了就是创建了一个观察者,只要它看向哪个对象,就能知道那个对象的生命周期到那了
* 这样就能选择性的在不同的生命周期做不同的操作,从而和activity/fragment/service/application实现解耦
*/
public class CustomObserver implements LifecycleObserver {
private final String TAG = "CustomObserver";
@OnLifecycleEvent(value = Lifecycle.Event.ON_RESUME)
public void onResume() {
LogUtil.e(TAG, "Observer ON_RESUME");
//TODO 做些操作
}
@OnLifecycleEvent(value = Lifecycle.Event.ON_START)
public void onStart() {
LogUtil.e(TAG, "Observer ON_START");
//TODO 做些操作
}
@OnLifecycleEvent(value = Lifecycle.Event.ON_CREATE)
public void onCreate() {
LogUtil.e(TAG, "Observer ON_CREATE");
//TODO 做些操作
}
@OnLifecycleEvent(Lifecycle.Event.ON_PAUSE)
public void onPause() {
LogUtil.e(TAG, "Observer ON_PAUSE");
//TODO 做些操作
}
@OnLifecycleEvent(Lifecycle.Event.ON_DESTROY)
public void onDestory() {
LogUtil.e(TAG, "Observer ON_DESTROY");
//TODO 做些操作
}
@OnLifecycleEvent(Lifecycle.Event.ON_STOP)
public void onStop() {
LogUtil.e(TAG, "Observer ON_STOP");
//TODO 做些操作
}
// @OnLifecycleEvent(Lifecycle.Event.ON_ANY)
// public void onAny() {
// LogUtil.e(TAG, "Observer ON_ANY 只要上面有调用,此方法也会调用");
// }
}
2,我们在activity的生命周期方法打日志留痕,同时添加观察者
getLifecycle().addObserver(new CustomObserver());
3,执行页面进去及出退操作,得到日志
进入activity
MainActivity: onCreate
CustomObserver: Observer ON_CREATE
MainActivity: onStart
CustomObserver: Observer ON_START
MainActivity: onResume
CustomObserver: Observer ON_RESUME
退出activity
CustomObserver: Observer ON_PAUSE
MainActivity: onPause
CustomObserver: Observer ON_STOP
MainActivity: onStop
CustomObserver: Observer ON_DESTROY
MainActivity: onDestroy
源码分析
一言以蔽之,就是activity和操作类组成观察者模式,activity生命周期变化会通知到操作类,下面具体源码分析。
1,androidx对原始activity进行了二次封装,在androidx.activity.ComponentActivity已经实现了LifecycleOwner接口,从而使activity变成了被观察者
2,androidx.activity.ComponentActivity
@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
mSavedStateRegistryController.performRestore(savedInstanceState);
ReportFragment.injectIfNeededIn(this);//通过创建一个空白fragment并交给它处理生命周期变化的任务
if (mContentLayoutId != 0) {
setContentView(mContentLayoutId);
}
}
3,androidx.lifecycle.ReportFragment
public class ReportFragment extends Fragment {
//创建fragment的方法
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();
manager.executePendingTransactions();
}
}
//调遣事件ON_CREATE
@Override
public void onActivityCreated(Bundle savedInstanceState) {
super.onActivityCreated(savedInstanceState);
dispatch(Lifecycle.Event.ON_CREATE);
}
//调遣事件ON_START
@Override
public void onStart() {
super.onStart();
dispatch(Lifecycle.Event.ON_START);
}
//调遣事件ON_RESUME
@Override
public void onResume() {
super.onResume();
dispatchResume(mProcessListener);
dispatch(Lifecycle.Event.ON_RESUME);
}
//调遣事件ON_PAUSE
@Override
public void onPause() {
super.onPause();
dispatch(Lifecycle.Event.ON_PAUSE);
}
//调遣事件ON_STOP
@Override
public void onStop() {
super.onStop();
dispatch(Lifecycle.Event.ON_STOP);
}
//调遣事件ON_DESTROY
@Override
public void onDestroy() {
super.onDestroy();
dispatch(Lifecycle.Event.ON_DESTROY);
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);//再把具体处理放在被观察者的具体实现上
}
}
}
}
分析LifecycleRegistry之前,我们需要先看下LifeCycle源码,它是生命周期被观察者的基类,定义了 生命周期被观察者 的行为规范。
package androidx.lifecycle;
import androidx.annotation.MainThread;
import androidx.annotation.NonNull;
import androidx.annotation.RestrictTo;
import java.util.concurrent.atomic.AtomicReference;
public abstract class Lifecycle {
@RestrictTo(RestrictTo.Scope.LIBRARY_GROUP)
@NonNull
AtomicReference
@MainThread
public abstract void addObserver(@NonNull LifecycleObserver observer);
@MainThread
public abstract void removeObserver(@NonNull LifecycleObserver observer);
@MainThread
@NonNull
public abstract State getCurrentState();
@SuppressWarnings("WeakerAccess")
public enum Event {
ON_CREATE,
ON_START,
ON_RESUME,
ON_PAUSE,
ON_STOP,
ON_DESTROY,
ON_ANY
}
/**
* Lifecycle states. You can consider the states as the nodes in a graph and
* {@link Event}s as the edges between these nodes.
*/
@SuppressWarnings("WeakerAccess")
public enum State {
DESTROYED,
INITIALIZED,
CREATED,
STARTED,
RESUMED;
public boolean isAtLeast(@NonNull State state) {
return compareTo(state) >= 0;
}
}
}
Lifecycle是一个抽象类,定义了添加/移除观察者的抽象方法,通知定义了事件枚举类和状态枚举类。
而LifecycleRegistry是Lifecycle的具体继承者,LifecycleRegistry在类中实现增加/删除观察者,关键还有对观察者发送状态变化的接收方法,具体如下。
package androidx.lifecycle;
public class LifecycleRegistry extends Lifecycle {
private FastSafeIterableMap
new FastSafeIterableMap<>();
private State mState;
private final WeakReference
public LifecycleRegistry(@NonNull LifecycleOwner provider) {
mLifecycleOwner = new WeakReference<>(provider);
mState = INITIALIZED;
}
//添加观察者
@Override
public void addObserver(@NonNull LifecycleObserver observer) {
State initialState = mState == DESTROYED ? DESTROYED : INITIALIZED;
ObserverWithState statefulObserver = new ObserverWithState(observer, initialState);//把观察者封装成ObserverWithState
ObserverWithState previous = mObserverMap.putIfAbsent(observer, statefulObserver);
...
}
//删除观察者
@Override
public void removeObserver(@NonNull LifecycleObserver observer) {
mObserverMap.remove(observer);
}
//这里对接上了ReportFragment的代码-dispatch(Lifecycle.Event)-->((LifecycleRegistry) lifecycle).handleLifecycleEvent(event);
public void handleLifecycleEvent(@NonNull Lifecycle.Event event) {
State next = getStateAfter(event);
moveToState(next);
}
//字面意思 移动状态
private void moveToState(State next) {
...
mHandlingEvent = true;
sync();//状态同步 准备把状态同步给观察者
mHandlingEvent = false;
}
//通知状态 到这里了
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);//根据状态选择执行
}
Entry
if (!mNewEventOccurred && newest != null
&& mState.compareTo(newest.getValue().mState) > 0) {
forwardPass(lifecycleOwner);//根据状态选择执行
}
}
mNewEventOccurred = false;
}
//对保存在map中的观察者进行逐一通知
private void forwardPass(LifecycleOwner lifecycleOwner) {
Iterator
mObserverMap.iteratorWithAdditions();
while (ascendingIterator.hasNext() && !mNewEventOccurred) {
Entry
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();
}
}
}
//对保存在map中的观察者进行逐一通知
private void backwardPass(LifecycleOwner lifecycleOwner) {
Iterator
mObserverMap.descendingIterator();
while (descendingIterator.hasNext() && !mNewEventOccurred) {
Entry
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);//执行通知事件的方法
popParentState();
}
}
}
static class ObserverWithState {
State mState;
LifecycleEventObserver mLifecycleObserver;
ObserverWithState(LifecycleObserver observer, State initialState) {
mLifecycleObserver = Lifecycling.lifecycleEventObserver(observer);//具体的实现类在这里面
mState = initialState;
}
void dispatchEvent(LifecycleOwner owner, Event event) {//这里是执行通知具体操作的方法
State newState = getStateAfter(event);
mState = min(mState, newState);
mLifecycleObserver.onStateChanged(owner, event);//事件操作,下面是实际的操作流程,由上面的Lifecycling.lifecycleEventObserver
mState = newState;
}
}
}
分析Lifecycling中的lifecycleEventObserver()
@NonNull
static LifecycleEventObserver lifecycleEventObserver(Object object) {//我们的观察者实现的是 LifecycleObserver
boolean isLifecycleEventObserver = object instanceof LifecycleEventObserver;//false
boolean isFullLifecycleObserver = object instanceof FullLifecycleObserver; //false
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
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);//最终是这句话执行了
}
ReflectiveGenericLifecycleObserver类分析
package androidx.lifecycle;
class ReflectiveGenericLifecycleObserver implements LifecycleEventObserver {
private final Object mWrapped;
private final CallbackInfo mInfo;
ReflectiveGenericLifecycleObserver(Object wrapped) {
mWrapped = wrapped;
mInfo = ClassesInfoCache.sInstance.getInfo(mWrapped.getClass());//转到ClassesInfoCache中获取CallbackInfo类,里面包含所有带Lifecycle.Event注解的方法
}
@Override
public void onStateChanged(LifecycleOwner source, Event event) {//LifecycleRegistry类中类ObserverWithState中的dispatchEvent()
mInfo.invokeCallbacks(source, event, mWrapped);//最终执行的方法
}
}
ClassesInfoCache部分关键方法的梳理
//androidx.lifecycle.ClassesInfoCache
CallbackInfo getInfo(Class klass) {
CallbackInfo existing = mCallbackMap.get(klass);
if (existing != null) {
return existing;
}
existing = createInfo(klass, null);//关键代码
return existing;
}
private CallbackInfo createInfo(Class klass, @Nullable Method[] declaredMethods) {
Class superclass = klass.getSuperclass();
Map
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
intrfc).mHandlerToEvent.entrySet()) {
verifyAndPutHandler(handlerToEvent, entry.getKey(), entry.getValue(), klass);
}
}
Method[] methods = declaredMethods != null ? declaredMethods : getDeclaredMethods(klass);
boolean hasLifecycleMethods = false;
for (Method method : methods) {//遍历所有方法,并把带OnLifecycleEvent的方法搞出来
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);//又把观察者中的注解方法封装一下
verifyAndPutHandler(handlerToEvent, methodReference, event, klass);//然后又放到一个map中
}
CallbackInfo info = new CallbackInfo(handlerToEvent);
mCallbackMap.put(klass, info);
mHasLifecycleMethods.put(klass, hasLifecycleMethods);
return info;
}
//ReflectiveGenericLifecycleObserver类的onStateChanged具体执行方法
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
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);//终于快到尾巴上了
}
}
}
//通过invoke执行观察者中被生命周期关联的方法
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);
}
}
Android-Jetpack代码位置:github
Lifecycle在activity的整体流程梳理，大致如下：
总结：Lifecycle在activity中使用可以说是相当简单，注册-->空页面感知-->通知被观察者-->反射观察者包含OnLifecycleEvent的方法->invoke触发,
完美实现业务逻辑和activity的生命周期方法解耦。
下一篇文章将分析Lifecycle在Service和Application中的使用。
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