Jetpack学习之---Lifecycle
2021-08-11 本文已影响0人
初夏的雪
一、介绍
Lifecycle 是Jetpack库中一个软件包，通过存储有生命周期状态的组件的生命周期信息，并允许其他的对象观察此状态，来达到能方便让自定义的组件感知到Activity ，Fragment等的生命周期，这样就可以根据生命周期来完成对应的工作了。
Lifecycle采用的是观察者模式，观察者需要实现LifecycleObserver接口， 被观察者需要实现LifecycleOwner接口，然后给被观察者与观察者通过addObserver()建立关系。这样达到观察生命周期状态的目的。
特点：
-
精简代码
-
易于维护
LifecycleOwner
他是一个只有一个方法的接口，只要实现了这个接口的类，就表示他是具有生命周期状态的提供者了。该接口提供了一个统一的获取生命周期提供者的方法getLifecycle()方法。
LifecycleObserver
这是一个空的接口，实现了该接口的类，就可以作为有生命周期状态组件的观察者进行观察了。
实现了LifecycleObserver的观察者角色的组件与实现了LifecycleOwner的被观察者组件通过addObserver()来建立观察关系，这样观察者就可以观察被观察者的生命周期了。
二、基本使用
1、添加依赖
def lifecycle_version = "2.4.0-alpha03"
// Lifecycles only (without ViewModel or LiveData)
implementation "androidx.lifecycle:lifecycle-runtime-ktx:$lifecycle_version"
// Annotation processor
annotationProcessor "androidx.lifecycle:lifecycle-compiler:$lifecycle_version"
// alternately - if using Java8, use the following instead of lifecycle-compiler
implementation "androidx.lifecycle:lifecycle-common-java8:$lifecycle_version"
2、自定义Observer,需要实现LifecycleObserver接口
通过注解@OnLifecycleEvent() 参数是监听的生命周期的事件
public class CustomLifecycleObserver implements LifecycleObserver {
private static final String TAG = CustomLifecycleObserver.class.getSimpleName();
@OnLifecycleEvent(Lifecycle.Event.ON_CREATE)
void onCreateX() {
Log.e(TAG, "CustomLifecycleObserver---onCreateX:");
}
@OnLifecycleEvent(Lifecycle.Event.ON_RESUME)
void onResumeX() {
Log.e(TAG, "CustomLifecycleObserver---onResumeX:");
}
@OnLifecycleEvent(Lifecycle.Event.ON_STOP)
void onStopX() {
Log.e(TAG, "CustomLifecycleObserver---onStopX:");
}
@OnLifecycleEvent(Lifecycle.Event.ON_DESTROY)
void onDestoryX() {
Log.e(TAG, "CustomLifecycleObserver---onDestoryX:");
}
}
3、观察者与被观察者建立关联
MainActivity--->AppCompatActivity--->FragmentActivity--->ComponentActivity 他们之间的这种父子关系，而ComponentActivity就是LifecycleOwner接口，所以MainActivity是生命周期的提供者，是被观察者角色。
public class MainActivity extends AppCompatActivity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
getLifecycle().addObserver(new CustomLifecycleObserver());
}
}
经过上面简单的三步自定义的Observer就可以感知到MainActivity的生命周期了，打印的日志如下：
2021-08-11 10:38:09.105 19493-19493/com.leon.study_jetpack_lifecycle E/CustomLifecycleObserver: CustomLifecycleObserver---onCreateX:
2021-08-11 10:38:09.134 19493-19493/com.leon.study_jetpack_lifecycle E/CustomLifecycleObserver: CustomLifecycleObserver---onResumeX:
2021-08-11 10:38:09.401 19493-19493/com.leon.study_jetpack_lifecycle E/SpannableStringBuilder: SPAN_EXCLUSIVE_EXCLUSIVE spans cannot have a zero length
2021-08-11 10:38:09.401 19493-19493/com.leon.study_jetpack_lifecycle E/SpannableStringBuilder: SPAN_EXCLUSIVE_EXCLUSIVE spans cannot have a zero length
2021-08-11 10:39:03.591 19493-19493/com.leon.study_jetpack_lifecycle E/CustomLifecycleObserver: CustomLifecycleObserver---onStopX:
2021-08-11 10:39:03.605 19493-19493/com.leon.study_jetpack_lifecycle E/CustomLifecycleObserver: CustomLifecycleObserver---onDestoryX:
三、实现原理
1、原理概述
观察者Observer是通过注解处理器将生命周期相关的事件添加到一个集合里面去
给被观察者中添加了一个没有UI界面的Fragment，Fragment与被观察者的生命周期绑定，然后在Fragment的生命周期方法中根据当前Activity的生命周期的状态，与事件的对应关系，然后开始对observer集合进行事件分发，当observer的状态发送变化时，就会通过反射来反射调用observer对应的方法，这样就掉到观察者的方法中去了。
其中核心的一点是同步观察者和被观察者的状态。
2、源码分析：
2.1) 先来分析一下建立观察关系的流程
基本思路：
1、通过反射技术将使用注解的方法的相关信息保存到一个map中去；
2、根据当前被观察者的状态，遍历(1)中map里面的方法，找到对应注解事件中的方法，然后分发事件来触发并同步观察者当前被观察者的状态。
Lifecycle的状态:
是以一个从0 开始的枚举值，DESTROYED=0 < INITIALIZED=1 < CREATED=2 < STARTED=3 < RESUMED=4
/**
* 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 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;
}
}
第一步：添加观察者
//LifecycleRegistry.java文件
@Override
public void addObserver(@NonNull LifecycleObserver observer) {
State initialState = mState == DESTROYED ? DESTROYED : INITIALIZED;
//这里将传递进来的观察者与当前的初始化状态打包一下
ObserverWithState statefulObserver = new ObserverWithState(observer, initialState);
//将打包好的观察者保存到成员变量map中去
ObserverWithState previous = mObserverMap.putIfAbsent(observer, statefulObserver);
//如果已经在map中存在，就直接返回了
if (previous != null) {
return;
}
//获取被观察者
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();//这里一个同步方法来同步状态
}
mAddingObserverCounter--;
}
第二步：包装观察者
//LifecycleRegistry.java
static class ObserverWithState {
State mState;
LifecycleEventObserver mLifecycleObserver;
ObserverWithState(LifecycleObserver observer, State initialState) {
//在此构造方法中包装，继续进入Lifecycling类中
mLifecycleObserver = Lifecycling.lifecycleEventObserver(observer);
mState = initialState;
}
void dispatchEvent(LifecycleOwner owner, Event event) {
State newState = getStateAfter(event);
mState = min(mState, newState);
mLifecycleObserver.onStateChanged(owner, event);
mState = newState;
}
}
//Lifecycling.java
static LifecycleEventObserver lifecycleEventObserver(Object object) {
//......这里省略了很多源码
//从这里return 的地方看
return new ReflectiveGenericLifecycleObserver(object);
}
//ReflectiveGenericLifecycleObserver.java
class ReflectiveGenericLifecycleObserver implements LifecycleEventObserver {//他是一个生命周期事件的观察者
private final Object mWrapped;
private final CallbackInfo mInfo;
ReflectiveGenericLifecycleObserver(Object wrapped) {
mWrapped = wrapped;
//在这里看看getInfo
mInfo = ClassesInfoCache.sInstance.getInfo(mWrapped.getClass());
}
@Override
public void onStateChanged(LifecycleOwner source, Event event) {
mInfo.invokeCallbacks(source, event, mWrapped);
}
}
//ClassesInfoCache.java
CallbackInfo getInfo(Class klass) {
CallbackInfo existing = mCallbackMap.get(klass);
if (existing != null) {
return existing;
}
existing = createInfo(klass, null);
return existing;
}
//此处才是最关键的，通过注解来回去生命周期注解对应的方法及其实例对象
private CallbackInfo createInfo(Class klass, @Nullable Method[] declaredMethods) {
//todo 省略了源码
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);
}
}
// 反射获取注解信息的地方
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);
//这里将每一个注解过的方法打包放到map中去
verifyAndPutHandler(handlerToEvent, methodReference, event, klass);
}
CallbackInfo info = new CallbackInfo(handlerToEvent);
mCallbackMap.put(klass, info);
mHasLifecycleMethods.put(klass, hasLifecycleMethods);
return info;
}
第三步：同步状态
//LifecycleRegistry.java文件
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) {
/**
例如：mState 最初的初始值是INITIALIZED，而最早添加的观察者的状态是CREATED 这时候，就需要将状态向后转变为对应的事件来通知给观察者
*/
backwardPass(lifecycleOwner);
}
Entry<LifecycleObserver, ObserverWithState> newest = mObserverMap.newest();
if (!mNewEventOccurred && newest != null
&& mState.compareTo(newest.getValue().mState) > 0) {
/**
这里和backwardPass的方法刚好是相反的
*/
forwardPass(lifecycleOwner);
}
}
mNewEventOccurred = false;
}
//向后分发事件
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()))) {
//生命状态转变成对应的事件
Event event = downEvent(observer.mState);
pushParentState(getStateAfter(event));
//开始分发事件
observer.dispatchEvent(lifecycleOwner, event);
popParentState();
}
}
}
//将当前的状态转化成对应的事件
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);
}
通过下图来理解一下上面的状态变化：
image-20210811114726327.png
2.2）生命周期变化的通知流程
第一步：无UI的空Fragment
进入MainActivity的父类ComponentActivity的onCreate()方法
//ComponentActivity.java
@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
mSavedStateRegistryController.performRestore(savedInstanceState);
//此处有有一个ReportFragment
ReportFragment.injectIfNeededIn(this);
if (mContentLayoutId != 0) {
setContentView(mContentLayoutId);
}
}
//这里可以看到其实是在Activity中添加了一个空的没有UI界面的Fragment来处理生命周期的 这一点与Glide感知生命周期的原理类似
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();
}
}
第二步：进入ReportFragment的生命周期方法（例如：onStart()）
//>>>>>>>>>>>>>>>> ReportFragment.java
@Override
public void onStart() {
super.onStart();
//这里应该是一个处理进程间生命周期的监听回调，事件分发
dispatchStart(mProcessListener);
//这里是事件分发
dispatch(Lifecycle.Event.ON_START);
}
//进入方法
private void dispatch(Lifecycle.Event event) {
Activity activity = getActivity();
//此处已经弃用
if (activity instanceof LifecycleRegistryOwner) {
((LifecycleRegistryOwner) activity).getLifecycle().handleLifecycleEvent(event);
return;
}
//我们的ComponentActivity是实现了LifecycleOwner接口
if (activity instanceof LifecycleOwner) {
Lifecycle lifecycle = ((LifecycleOwner) activity).getLifecycle();
if (lifecycle instanceof LifecycleRegistry) {
((LifecycleRegistry) lifecycle).handleLifecycleEvent(event);
}
}
}
第三步：同步状态
//>>>>>>>>>>>>>>>> LifecycleRegistry.java
public void handleLifecycleEvent(@NonNull Lifecycle.Event event) {
State next = getStateAfter(event);
moveToState(next);
}
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);
}
//这里添加观察者时一样，都会同步状态
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;
}
第四步：事件分发
//>>>>>>>>>>>>>>>> LifecycleRegistry.java
private void sync() {
//////...............
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) {
//该方法
backwardPass(lifecycleOwner);
}
////...............
}
mNewEventOccurred = false;
}
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));
//观察者分发事件
observer.dispatchEvent(lifecycleOwner, event);
popParentState();
}
}
}
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);
//下面的onStateChanged是一个借口的方法，他的实现类之一：ReflectiveGenericLifecycleObserver
mLifecycleObserver.onStateChanged(owner, event);
mState = newState;
}
}
第五步：反射回调观察者的方法
//>>>>>>>>>>>>>>> ReflectiveGenericLifecycleObserver.java
class ReflectiveGenericLifecycleObserver implements LifecycleEventObserver {
private final Object mWrapped;
private final CallbackInfo mInfo;
ReflectiveGenericLifecycleObserver(Object wrapped) {
mWrapped = wrapped;
//这个地方之前在添加观察者时进来过
mInfo = ClassesInfoCache.sInstance.getInfo(mWrapped.getClass());
}
@Override
public void onStateChanged(LifecycleOwner source, Event event) {
//进入反射回调
mInfo.invokeCallbacks(source, event, mWrapped);
}
}
//ClassesInfoCache.java
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--) {
//这里的handlers是在添加观察者时，保存在map里面的注解过的方法的信息
handlers.get(i).invokeCallback(source, event, mWrapped);
}
}
}
//这一步就真正回调到注解注释过的观察者的方法里面去了
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);
}
}
四、总结
Jetpack的Lifecycle的理解 ，核心思想是观察者模式思想；
当给一个生命收起组件添加观察者时，通过反射来获取观察者被注解的方法信息，并将这些方法保存下来方便后续发送事件通知；
对于生命周期的提供者（目前是Activity）在其内部添加了一个空Fragment, 由这个空Fragment专门负责发送生命周期事件,在事件发送中通过反射机制来回调到观察者的方法中。
技术点：注解，APT技术，反射，观察者模式，
知识点：生命周期与对应事件的互换思路（如图：<u>生命周期状态与事件关系图</u>）
