為什么要引入Lifecycle?
我們?cè)谔幚鞟ctivity或者Fragment組件的生命周期相關(guān)時(shí)蜒滩，會(huì)遇到一些問題：
比如Activity#onCreate()中初始化某些成員（MVP架構(gòu)中的Presenter或者M(jìn)ediaPlayer等）滨达，onStop()的時(shí)候?qū)@些成員進(jìn)行對(duì)應(yīng)的處理，在onDestroy()中進(jìn)行資源的釋放俯艰。類似下面的代碼：
public class MyPresenter {
public void onCreate(){
//初始化操作
}
public void onStop(){
//do something
}
public void onDestroy(){
//釋放資源
}
}
class MyActivity extends AppCompatActivity {
private MyPresenter myPresenter;
@Override
public void onCreate(...) {
//資源初始化
myPresenter = new MyPresenter();
myPresenter.onCreate();
}
@Override
public void onStop() {
super.onStop();
myPresenter.onStop();
// 管理響應(yīng)activity lifecycle的組件
}
@Override
public void onDestroy() {
super.onDestroy();
myPresenter.onDestroy();
//資源的釋放
}
}
此示例看起來沒問題捡遍，但在真實(shí)的應(yīng)用中，最終會(huì)有太多管理界面和其他組件的調(diào)用竹握，以響應(yīng)生命周期的當(dāng)前狀態(tài)画株。管理多個(gè)組件會(huì)在生命周期方法（如 onStart() 和 onStop()）中放置大量的代碼啦辐，這使得它們難以維護(hù)谓传。
使用Lifecycle組件來感知Activity/Fragment生命周期，兩步操作就可以了
- MyPresenter實(shí)現(xiàn)LifecycleObserver接口
public class MyPresenter implements LifecycleObserver{
@OnLifecycleEvent(Lifecycle.Event.ON_CREATE)
public void onCreate(){
//初始化操作
}
@OnLifecycleEvent(Lifecycle.Event.ON_STOP)
public void onStop(){
}
@OnLifecycleEvent(Lifecycle.Event.ON_DESTROY)
public void onDestroy(){
//釋放資源
}
}
- Activity/Fragment中添加Observer:
class MyActivity extends AppCompatActivity {
private MyPresenter myPresenter;
@Override
public void onCreate(@Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
myPresenter = new MyPresenter();
//添加觀察者
getLifecycle().addObserver(myPresenter);
}
}
Lifecycle組件中的核心類
Lifecycle抽象類芹关，屬于觀察者模式中的被觀察者续挟。內(nèi)部?jī)蓚€(gè)enum分別定義了生命周期事件（Event）以及狀態(tài)（LifecycleState），定義了觀察者(LifecycleObserver)的添加侥衬、移除以及當(dāng)前生命周期狀態(tài)的獲取的方法
LifecycleState 與 Event的關(guān)系
1594882307855.png
LifecycleRegister是Lifecycle的實(shí)現(xiàn)類诗祸，負(fù)責(zé)接管生命周期以及LifecycleObserver的注冊(cè)和通知。
LifecycleObserver是Lifecycle的觀察者轴总。
LifecycleOwner描述了一個(gè)擁有生命周期的組件直颅，被觀察者。
ObserverWithState是LifecycleObserver的封裝類肘习，構(gòu)造函數(shù)調(diào)用了Lifecycling#lifecycleEventObserver方法际乘。
Lifecycling 把傳入的LifecycleObserver對(duì)象轉(zhuǎn)化為L(zhǎng)ifecycleEventObserver對(duì)象。
ReflectiveGenericeLifecycleObserver LifecycleEventObserver的子類漂佩，內(nèi)部實(shí)現(xiàn)依賴于反射。
ClassesInfoCache 儲(chǔ)存了我們?cè)谟^察者中注解的方法信息罪塔，并在生命周期發(fā)生改變時(shí)通過反射的方式調(diào)用對(duì)應(yīng)的方法投蝉。
Activity中ReportFragment注入
API 27位于SupportActivity#onCreate()；API 28位于ComponentActivity#onCreate()中征堪。
1594864370892.png
添加觀察者瘩缆，LifecycleRegister#addObserver
public void addObserver(@NonNull LifecycleObserver observer) {
//初始狀態(tài)值
State initialState = mState == DESTROYED ? DESTROYED : INITIALIZED;
//把observer和state封裝為ObserverWithState對(duì)象
ObserverWithState statefulObserver = new ObserverWithState(observer, initialState);
//保存到mObserverMap中，如果已經(jīng)添加過了直接返回map中保存的值
ObserverWithState previous = mObserverMap.putIfAbsent(observer, statefulObserver);
if (previous != null) {//觀察者已經(jīng)被添加過了
return;
}
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) {
//不是重入佃蚜，同步
// we do sync only on the top level.
sync();
}
mAddingObserverCounter--;
}
ObserverWithState 是LifecycleObserver的封裝類
static class ObserverWithState {
State mState;
LifecycleEventObserver mLifecycleObserver;
ObserverWithState(LifecycleObserver observer, State initialState) {
//通過Lifecycling獲取LifecycleObserver的實(shí)現(xiàn)類
mLifecycleObserver = Lifecycling.lifecycleEventObserver(observer);
mState = initialState;
}
...
}
LifecycleObserver實(shí)現(xiàn)類的獲取流程：
- Lifecycling#lifecycleEventObserver 獲取對(duì)應(yīng)的LifecycleObserver的實(shí)現(xiàn)類
@NonNull
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();
//構(gòu)造函數(shù)的類型
int type = getObserverConstructorType(klass);
if (type == GENERATED_CALLBACK) {
List<Constructor<? extends GeneratedAdapter>> constructors =
sClassToAdapters.get(klass);
//一個(gè)參數(shù)的構(gòu)造函數(shù)
if (constructors.size() == 1) {
GeneratedAdapter generatedAdapter = createGeneratedAdapter(
constructors.get(0), object);
return new SingleGeneratedAdapterObserver(generatedAdapter);
}
//參數(shù)為數(shù)組的構(gòu)造函數(shù)
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);
}
//反射生成
return new ReflectiveGenericLifecycleObserver(object);
}
- ReflectiveGenericLifecycleObserver構(gòu)造函數(shù)通過ClassesInfoCache類的單例調(diào)用getInfo()獲取CallbackInfo對(duì)象的實(shí)例
ReflectiveGenericLifecycleObserver(Object wrapped) {
mWrapped = wrapped;
mInfo = ClassesInfoCache.sInstance.getInfo(mWrapped.getClass());
}
- ClassesInfoCache 內(nèi)部有兩個(gè)Map庸娱，實(shí)現(xiàn)了類信息的緩存
/**
* Reflection is expensive, so we cache information about methods
* for {@link ReflectiveGenericLifecycleObserver}, so it can call them,
* and for {@link Lifecycling} to determine which observer adapter to use.
*/
//從描述可以看出，反射的代價(jià)是昂貴的谐算，所以這里緩存了methods信息
//存儲(chǔ)當(dāng)前類的CallbackInfo的信息
private final Map<Class, CallbackInfo> mCallbackMap = new HashMap<>();
//存儲(chǔ)當(dāng)前類是否有使用了注解的生命周期方法
private final Map<Class, Boolean> mHasLifecycleMethods = new HashMap<>();
CallbackInfo getInfo(Class klass) {
CallbackInfo existing = mCallbackMap.get(klass);
//緩存中有熟尉，直接返回
if (existing != null) {
return existing;
}
//緩存中沒有，進(jìn)行創(chuàng)建
existing = createInfo(klass, null);
return existing;
}
//通過反射查找被@OnLifecycleEvent注解修飾的方法洲脂，然后保存起來
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);
}
}
Method[] methods = declaredMethods != null ? declaredMethods : getDeclaredMethods(klass);
boolean hasLifecycleMethods = false;
for (Method method : methods) {
//獲取方法上的@OnLifecycleEvent注解信息
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();
//參數(shù)校驗(yàn)
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;
}
FastSafeIterableMap#putIfAbsent
@Override
public V putIfAbsent(@NonNull K key, @NonNull V v) {
Entry<K, V> current = get(key);
//map中存在斤儿，直接返回
if (current != null) {
return current.mValue;
}
//不存在，存入map中
mHashMap.put(key, put(key, v));
return null;
}
FastSafeIterableMap是SafeIterableMap的子類， SafeIterableMap實(shí)現(xiàn)了Iterable接口往果，它是一個(gè)鏈表結(jié)構(gòu)疆液，模擬了Map的特性，支持在對(duì)元素遍歷期間做修改陕贮，但需要注意的是該類不是線程安全的堕油。 SafeIterableMap源碼分析請(qǐng)參考
Android源碼系列之從源碼的角度深入理解SafeIterableMap
下面從ReportFragment#onStart()方法，跟蹤Lifecycle是如何實(shí)現(xiàn)生命周期的感知并把狀態(tài)同步到觀察者中的注解方法上的流程
ReportFragment#onStart()
@Override
public void onStart() {
super.onStart();
dispatchStart(mProcessListener);
//分發(fā)事件
dispatch(Lifecycle.Event.ON_START);
}
ReportFragment#dispatch() 內(nèi)部最后都是調(diào)用LifecycleRegistry#handleLifecycleEvent()
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);
}
}
}
LifecycleRegistry#handleLifecycleEvent()處理生命周期事件：
public void handleLifecycleEvent(@NonNull Lifecycle.Event event) {
//根據(jù)Event得到LifecycleState的值
State next = getStateAfter(event);
moveToState(next);
}
LifecycleState 與 Event的關(guān)系圖解描述的就是這個(gè)方法
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) {
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;
//進(jìn)行生命周期狀態(tài)的同步
sync();
mHandlingEvent = false;
}
// 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 = 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) {
//遍歷mObserverMap中元素肮之，從鏈表的尾部開始同步狀態(tài)
backwardPass(lifecycleOwner);
}
Entry<LifecycleObserver, ObserverWithState> newest = mObserverMap.newest();
if (!mNewEventOccurred && newest != null
&& mState.compareTo(newest.getValue().mState) > 0) {
//遍歷mObserverMap中元素掉缺，從鏈表的頭部開始同步狀態(tài)
forwardPass(lifecycleOwner);
}
}
mNewEventOccurred = false;
}
//遍歷mObserverMap中元素，從鏈表的尾部開始同步狀態(tài)
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));
//分發(fā)事件
observer.dispatchEvent(lifecycleOwner, event);
popParentState();
}
}
}
LifecycleRegistry$ObserverWithState#dispatchEvent
void dispatchEvent(LifecycleOwner owner, Event event) {
State newState = getStateAfter(event);
mState = min(mState, newState);
//通知觀察者生命周期狀態(tài)變更局骤，調(diào)到LifecycleEventObserver的子類的onStateChanged()
mLifecycleObserver.onStateChanged(owner, event);
mState = newState;
}
文中LifecycleEventObserver的子類為ReflectiveGenericLifecycleObserver攀圈，具體為什么是它，參考上面提到的LifecycleObserver實(shí)現(xiàn)類的獲取流程
@Override
public void onStateChanged(LifecycleOwner source, Event event) {
mInfo.invokeCallbacks(source, event, mWrapped);
}
ClassesInfoCache$CallbackInfo#invokeCallbacks
@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);
}
}
}
ClassesInfoCache$MethodReference#invokeCallback 反射調(diào)用方法
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);
}
}
至此峦甩，生命周期組件(Activity/Fragment)和觀察者生命周期的同步流程結(jié)束了赘来。
總結(jié)
Lifecycle組件通過在Activity/Fragmen中注入空的Fragment（ReportFragment），實(shí)現(xiàn)了對(duì)生命周期的感知凯傲。使用反射把使用了@OnLifecycleEvent注解的方法信息保存下來犬辰，基于觀察者模式，在Activity/Fragment生命周期改變的時(shí)候同步狀態(tài)到觀察者冰单。
