作為一個開發(fā)人員，學(xué)習(xí)過的知識是否掌握桐臊，通過實踐驗證胎撤，并加以分析，讓自己對知識點有更深刻的認(rèn)知断凶，如果下面內(nèi)容有不當(dāng)之處伤提，請多多指教。下面進(jìn)行Lifecycle主題认烁。
正文
通過下面幾點去認(rèn)識Lifecycle的真面目肿男。
- 使用Lifecycle的含義與意義
- Lifecycle的簡單使用
- Lifecycle的組成與實現(xiàn)原理
什么是Lifecycle介汹？
Lifecycle 組件是 google開發(fā)出的JetPack架構(gòu)組件之一，它具有感知生命周期變化舶沛。主要是通過觀察者模式去感知生命周期變化嘹承。
為什么要使用Lifecycle？
- 可以讓開發(fā)者構(gòu)建能感知的組件（如指Activity 如庭、Fragment）生命周期的類叹卷。
- 生命周期感知型組件可執(zhí)行操作來響應(yīng)另一個組件（如 Activity 和 Fragment）的生命周期狀態(tài)的變化。這些組件有助于您寫出更有條理且往往更精簡的代碼坪它，這樣的代碼更易于維護(hù)豪娜。
- 通過生命周期感知型組件，從而實現(xiàn)在活動狀態(tài)下允許操作哟楷，而在銷毀狀態(tài)下需要自動禁止操作瘤载，釋放資源，防止內(nèi)存泄露卖擅。
通過MVP例子去玩一下Lifecycle
實現(xiàn)這樣一個功能：監(jiān)聽某個 Activity 生命周期的變化鸣奔，在生命周期改變的時候打印日志。
- 定義一個與和生命周期相關(guān)的接口IPresenter惩阶。
/** * Author: 安仔夏天勤奮 * Date: 2020/3/28 * Desc: */ public interface IPresenter { void onCreate(); void onStart(); void onResume(); void onPause(); void onStop(); void onDestroy(); } - 定義一個IPresenter的實現(xiàn)類BasePresenter挎狸、LifecycleObserver
/** * Author: 安仔夏天勤奮 * Date: 2020/3/28 * Desc: */ public class BasePresenter implements IPresenter, LifecycleObserver { private static final String TAG = "BasePresenter"; @OnLifecycleEvent(Lifecycle.Event.ON_CREATE) @Override public void onCreate() { Log.e(TAG,"=========>>> onCreate"); } @OnLifecycleEvent(Lifecycle.Event.ON_START) @Override public void onStart() { Log.e(TAG,"=========>>> onStart"); } @OnLifecycleEvent(Lifecycle.Event.ON_RESUME) @Override public void onResume() { Log.e(TAG,"=========>>> onResume"); } @OnLifecycleEvent(Lifecycle.Event.ON_PAUSE) @Override public void onPause() { Log.e(TAG,"=========>>> onPause"); } @OnLifecycleEvent(Lifecycle.Event.ON_STOP) @Override public void onStop() { Log.e(TAG,"=========>>> onStop"); } @OnLifecycleEvent(Lifecycle.Event.ON_DESTROY) @Override public void onDestroy() { Log.e(TAG,"=========>>> onDestroy"); } }
需要去實現(xiàn)生命周期，那么就要實現(xiàn)LifecycleObserver接口断楷，同時在每一個方法實現(xiàn)上增加@OnLifecycleEvent(Lifecycle.Event.XXXX）注解锨匆，OnLifecycleEvent對應(yīng)了Activity的生命周期方法。
-
在Activity中綁定BasePresenter
public class MainActivity extends AppCompatActivity { BasePresenter presenter; @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); presenter = new BasePresenter(); //添加一個觀察者presenter,這樣就與父類的被觀察都LifecycleOwner建立了關(guān)系 getLifecycle().addObserver(presenter); } } -
Activity啟動和退出時的日志打印
BasePresenter: =========>>> onCreate BasePresenter: =========>>> onStart BasePresenter: =========>>> onResume BasePresenter: =========>>> onPause BasePresenter: =========>>> onStop BasePresenter: =========>>> onDestroy
從上面的代碼看是不是很簡單冬筒，的確挺簡單的恐锣。還可以繼承BasePresenter擴(kuò)展成跟自己業(yè)務(wù)對應(yīng)的Presenter去監(jiān)聽生命周期，然后復(fù)寫需要的生命周期方法即可舞痰。從上面代碼也可以看出土榴，監(jiān)聽感知 Activity 的生命周期，只需要在Activity中調(diào)用一句 getLifecycle().addObserver(presenter) 就可以了响牛。Lifecycle 是怎樣實現(xiàn)感知生命周期的呢玷禽，監(jiān)聽流程是怎么樣的呢，下面開始源碼解析了呀打。
Lifecycle的組成與實現(xiàn)原理
從上面例子不難看出矢赁，我們的切入點就是getLifecycle().addObserver(presenter)。其實是通過觀察者LifecycleObserver和被觀察者LifecycleOwner（在底層的Activity/Fragment都實現(xiàn)了）進(jìn)行綁定贬丛，從而實現(xiàn)監(jiān)聽生命周期變化撩银，也是一種觀察者模式啦。至于怎么觀察監(jiān)聽的瘫寝，下面會詳細(xì)說明蜒蕾。從上述例子還是很難理解 MainActivity稠炬、LifecycleOwner、Lifecycle 和 LifecycleObserver 之間的關(guān)系咪啡，我們通過輔助類圖整理他們的關(guān)系首启，當(dāng)我們在整理繪制類圖的過程中，理深刻地了解框架大概包含哪些組成部分撤摸，有哪些比較重要的成員變量和方法毅桃。
分析Lifecycle組件內(nèi)的類與接口的關(guān)系
從上面的類圖可以分析出：
MainActivity 繼承AppCompatActivity，AppCompatActivity繼承FragmentActivity 准夷，也就是MainActivity 繼承自 FragmentActivity钥飞，而 FragmentActivity 是 LifecycleOwner 的實現(xiàn)類。所以MainActivity 通過 getLifecycle()方法返回的是 Lifecycle 的子類 LifecycleRegistry實例衫嵌。
LifecycleRegistry是控制中心读宙。它負(fù)責(zé)控制state的轉(zhuǎn)換、接受分發(fā)event事件楔绞。在 LifecycleRegistry 類中的靜態(tài)內(nèi)部類 ObserverWithState 持有LifecycleEventObserver 對象的實例结闸，LifecycleEventObserver 是 LifecycleObserver 的子類。
LifecycleOwner 是一個接口酒朵，它有一個 getLifecycle()方法要子類去實現(xiàn)桦锄。LifecycleOwner是擔(dān)當(dāng)生命周期的事件分發(fā)者，在 Activity/Fragment 他們的生命周期發(fā)生變化的時發(fā)出相應(yīng)的 Event 給 LifecycleRegistry蔫耽。
LifecycleObserver是擔(dān)當(dāng)生命周期的觀察者结耀，通過注解將處理函數(shù)與希望監(jiān)聽的Event綁定,當(dāng)相應(yīng)的Event發(fā)生時,LifecycleRegistry會通知相應(yīng)的函數(shù)進(jìn)行處理。
Lifecycle 是一個抽象類匙铡，它有兩個枚舉類型的內(nèi)部類 Event和 State图甜，分別對應(yīng)著事件類型和組件的狀態(tài)；
通過類圖分析我們可以知道了Activity慰枕、LifecycleRegistry具则、LifecycleOwner 、LifecycleObserver具帮、Lifecycle 之間的對應(yīng)關(guān)系。但是我們還不知道注冊/注銷監(jiān)聽流程和生命周期分發(fā)流程低斋。
分析Lifecycle組件的監(jiān)聽流程/生命周期分發(fā)流程
通過整理類圖蜂厅，我們腦海里有了一個大概的類與接口對應(yīng)關(guān)系和重要的變量與方法調(diào)度概念。在整理類圖的過程膊畴，要記錄調(diào)度次序掘猿，看下面的時序圖。
Lifecycle組件的監(jiān)聽流程
從時序圖中我們可以看出唇跨，在FragmentActivity中創(chuàng)建了一個LifecycleRegistry對象稠通。FragmentActivity的父類ComponentActivity中實現(xiàn)了LifecycleOwner接口衬衬，這樣LifecycleRegistry就與LifecycleOwner綁定了。在父類構(gòu)造方法中調(diào)用了getLifecycle().addObserver(observer)改橘，那么 addObserver 就可以作為源碼分析的切入口滋尉。通過一步步追蹤，最終確定addObserver 實際上是調(diào)用了 LifecycleRegistry 的 addObserver (observer)方法飞主，并將 LifecycleObserver 的對象作為參數(shù)傳遞進(jìn)去狮惜，這樣就完成了一個完整的注冊監(jiān)聽過程。
MainActivity 中調(diào)用 getLifecycle()方法得到 LifecycleRegistry 對象碌识，LifecycleRegistry 對象的初始化在 FragmentActivity 中完成碾篡。
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));
}
}
當(dāng)getLifecyle().addObserver監(jiān)聽的時候，會先初始化狀態(tài)值筏餐，然后把狀態(tài)值與傳入的observer綁定并保存到mObserverMap开泽。
@Override
public void addObserver(@NonNull LifecycleObserver observer) {
//得到初始化狀態(tài)值，把狀態(tài)與傳入的observer綁定并保存到mObserverMap
State initialState = mState == DESTROYED ? DESTROYED : INITIALIZED;
ObserverWithState statefulObserver = new ObserverWithState(observer, initialState);
ObserverWithState previous = mObserverMap.putIfAbsent(observer, statefulObserver);
if (previous != null) {
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++;
//當(dāng)生命周期發(fā)生改變的時候魁瞪，就會遍歷這個ObserverWithState集合
while ((statefulObserver.mState.compareTo(targetState) < 0
&& mObserverMap.contains(observer))) {
pushParentState(statefulObserver.mState);
//生命周期變量了穆律，分發(fā)事件
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.
//同步狀態(tài)
sync();
}
mAddingObserverCounter--;
}
// 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) {
//正向推導(dǎo)狀態(tài)
backwardPass(lifecycleOwner);
}
Entry<LifecycleObserver, ObserverWithState> newest = mObserverMap.newest();
if (!mNewEventOccurred && newest != null
&& mState.compareTo(newest.getValue().mState) > 0) {
//逆向推導(dǎo)狀態(tài)
forwardPass(lifecycleOwner);
}
}
mNewEventOccurred = false;
}
private void backwardPass(LifecycleOwner lifecycleOwner) {
Iterator<Entry<LifecycleObserver, ObserverWithState>> descendingIterator =
mObserverMap.descendingIterator();
//取得下一個狀態(tài)，與當(dāng)前狀態(tài)比較
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));
//如果下一個狀態(tài)與當(dāng)前狀態(tài)不等佩番，則分發(fā)事件
observer.dispatchEvent(lifecycleOwner, event);
popParentState();
}
}
}
private void forwardPass(LifecycleOwner lifecycleOwner) {
Iterator<Entry<LifecycleObserver, ObserverWithState>> ascendingIterator =
mObserverMap.iteratorWithAdditions();
//取得下一個狀態(tài)众旗，與當(dāng)前狀態(tài)比較
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()))) {
pushParentState(observer.mState);
//如果下一個狀態(tài)與當(dāng)前狀態(tài)不等，則分發(fā)事件
observer.dispatchEvent(lifecycleOwner, upEvent(observer.mState));
popParentState();
}
}
}
生命周期時分發(fā)事件
在 Activity 生命周期變化時趟畏，在每個生命周期回調(diào)函數(shù)中調(diào)用 LifecycleRegistry的 handleLifecycleEvent()方法贡歧，在 handleLifecycleEvent()方法的層層調(diào)用中，最后通過 observer.dispatchEvent(lifecycleOwner, event) 方法中onStateChanged(owner, event)將生命周期的變化傳遞到 LifecycleObserver 的實現(xiàn)類BasePresenter 中赋秀。
需要注意的是啟動Activity時調(diào)用onCreat()方法的時候會注入ReportFragment利朵，這個Fragment沒有界面。這樣就綁定了一個Fragment去管理生命周期了猎莲。在ReportFragment中做一些分發(fā)事件绍弟。
public class ReportFragment extends Fragment {
//省略代碼...
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();
}
}
//省略代碼...
@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(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);
}
}
}
//省略代碼...
}
接下來我們看看handleLifecycleEvent()方法中做了那些事情。
public void handleLifecycleEvent(@NonNull Lifecycle.Event event) {
//去拿下一個狀態(tài)
State next = getStateAfter(event);
//移動到下一個狀態(tài)
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);
}
拿取狀態(tài)時著洼，是按照事件取對應(yīng)的狀態(tài)樟遣，如果事件是ON_CREATE、ON_STOP身笤，狀態(tài)為CREATED豹悬，如果事件是ON_START、ON_PAUSE液荸，狀態(tài)為STARTED瞻佛，如果事件是ON_RESUME，狀態(tài)為RESUMED娇钱，如果事件是ON_DESTROY伤柄，狀態(tài)為DESTROYED绊困。根據(jù)獲取的狀態(tài)移動到下一個狀態(tài)。
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;
//狀態(tài)同步
sync();
mHandlingEvent = false;
}
因為一開始Activity是有一個init初始狀態(tài)值适刀，隨著Activity的生命周期變化時秤朗，狀態(tài)都隨之改變。addObserver初始時會同步狀態(tài)蔗彤，和Activity的生命周期變化時同步狀態(tài)川梅。下面看一個狀態(tài)時序圖可以更加好理解。
當(dāng)生命周期發(fā)生變化時然遏，會調(diào)用mLifecycleObserver.onStateChanged(owner, event)進(jìn)行分發(fā)事件贫途。
static class ObserverWithState {
State mState;
LifecycleEventObserver mLifecycleObserver;
ObserverWithState(LifecycleObserver observer, State initialState) {
//LifecycleObserver有多個實現(xiàn)類
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;
}
}
然而mLifecycleObserver = Lifecycling.lifecycleEventObserver(observer)的實現(xiàn)類有很多個，通過追蹤待侵，最終都會走到ReflectiveGenericLifecycleObserver類丢早，在構(gòu)建ReflectiveGenericLifecycleObserver類時，將ObserverWithState傳入的observer(這個observer其實就是自己寫的BasePresenter)通過ClassesInfoCache類的getInfo(observer)方法生產(chǎn)出CallbackInfo信息秧倾。CallbackInfo信息是通過反射拿到外面定義的BasePresenter類中標(biāo)了OnLifecycleEvent注解的通通解析出來put進(jìn)來怨酝。
class ReflectiveGenericLifecycleObserver implements LifecycleEventObserver {
private final Object mWrapped;
private final CallbackInfo mInfo;
ReflectiveGenericLifecycleObserver(Object wrapped) {
mWrapped = wrapped;
mInfo = ClassesInfoCache.sInstance.getInfo(mWrapped.getClass());
}
@Override
public void onStateChanged(LifecycleOwner source, Event event) {
mInfo.invokeCallbacks(source, event, mWrapped);
}
}
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) {
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);
}
CallbackInfo info = new CallbackInfo(handlerToEvent);
mCallbackMap.put(klass, info);
mHasLifecycleMethods.put(klass, hasLifecycleMethods);
return info;
}
總結(jié)
- 當(dāng)自己還不太熟悉第三方架構(gòu)時，先查閱API那先，簡單地實現(xiàn)一個DEMO农猬。
- 自己多畫類圖，時序圖售淡，這樣對自己在后面看源碼時減少阻力斤葱。
- 多思考這個架構(gòu)用了那些模式。
- Lifecycle組件架構(gòu)用了觀察者模式揖闸，注解揍堕，反射等。
