介紹
Lifecycle 它是 Jetpack 組件庫的的核心基礎(chǔ) 甸各，具備宿主生命周期感知能力的組件团甲。它能持有組件（如 Activity 或 Fragment）生命周期狀態(tài)的信息溉贿，并且允許其他觀察者監(jiān)聽宿主的狀態(tài)仑嗅，大大減少了業(yè)務(wù)代碼發(fā)生內(nèi)存泄漏和 NPE 的風(fēng)險，LiveData 痹束、 ViewModel 組件等也都是基于它來實現(xiàn)的（觀察者模式）
工程引入
implementation 'androidx.appcompat:appcompat:1.1.0'
//或者
implementation "androidx.lifecycle:lifecycle-common:2.2.0"
implementation "androidx.lifecycle:lifecycle-common-java8:2.2.0"
implementation "androidx.lifecycle:lifecycle-runtime:2.2.0"
Lifecycle觀察宿主
每當(dāng) Activity/Fragment 的生命周期函數(shù)被觸發(fā)時检疫，該接口的相應(yīng)同名函數(shù)就會在之前或者之后被調(diào)用，以此來獲得相應(yīng)生命周期事件變化的通知
//使用案例
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main)
lifecycle.addObserver(object : DefaultLifecycleObserver {
override fun onCreate(owner: LifecycleOwner) {}
override fun onResume(owner: LifecycleOwner) {}
override fun onDestroy(owner: LifecycleOwner) {}
})
}
實現(xiàn)原理源碼分析
一祷嘶、Activity 實現(xiàn) Lifecycle
-
LifecycleOwner接口用于標(biāo)記其實現(xiàn)類具備 Lifecycle 對象
ComponentActivity和Fragment都實現(xiàn)了該接口
public interface LifecycleOwner {
@NonNull
Lifecycle getLifecycle();
}
class ComponentActivity implements LifecycleOwner{
//LifecycleRegistry觀察者添加和事件分發(fā)
private final LifecycleRegistry mLifecycleRegistry = new LifecycleRegistry(this);
@NonNull
@Override
public Lifecycle getLifecycle() {
return mLifecycleRegistry;
}
@SuppressLint("RestrictedApi")
@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
//往Activity上添加一個fragment,用以監(jiān)聽生命周期的變化
//其目的是為了兼顧不是繼承自AppCompactActivity的場景.
ReportFragment.injectIfNeededIn(this);
}
public ComponentActivity() {
//添加觀察者
getLifecycle().addObserver(new LifecycleEventObserver() {
@Override
public void onStateChanged(@NonNull LifecycleOwner source,
@NonNull Lifecycle.Event event) {
}
});
}
}
- Lifecycle接口Lifecycle 聲明了 添加觀察者 移除觀察者等相關(guān)函數(shù)（抽象）
public abstract class Lifecycle {
@RestrictTo(RestrictTo.Scope.LIBRARY_GROUP)
@NonNull
AtomicReference<Object> mInternalScopeRef = new AtomicReference<>();
@MainThread
public abstract void addObserver(@NonNull LifecycleObserver observer);
@MainThread
public abstract void removeObserver(@NonNull LifecycleObserver observer);
@MainThread
@NonNull
public abstract State getCurrentState();
}
- Event類(Lifecycle內(nèi)部靜態(tài)類)用于抽象 Activity/Fragment 的生命周期事件發(fā)生變化時所觸發(fā)的事件屎媳。例如，當(dāng) Activity 的每個生命周期事件回調(diào)函數(shù)（onCreate论巍、onStart 等）被觸發(fā)時都會被抽象為相應(yīng)的 ON_CREATE烛谊、ON_START 兩個 Event
public enum Event {
ON_CREATE,
ON_START,
ON_RESUME,
ON_PAUSE,
ON_STOP,
ON_DESTROY,
ON_ANY
}
- State(Lifecycle內(nèi)部靜態(tài)類) 類用于標(biāo)記 Lifecycle 的當(dāng)前生命周期狀態(tài)
public enum State {
//當(dāng)處于 DESTROYED 狀態(tài)時，Lifecycle 將不會發(fā)布其它 Event 值
//當(dāng) Activity 即將回調(diào) onDestory 時則處于此狀態(tài)
DESTROYED,
//已初始化的狀態(tài)嘉汰。例如丹禀，當(dāng) Activity 的構(gòu)造函數(shù)已完成，
//但還未回調(diào) onCreate 時則處于此狀態(tài)
INITIALIZED,
CREATED,
STARTED,
RESUMED;
//如果當(dāng)前狀態(tài)大于入?yún)⒅?state 時鞋怀，則返回 true
public boolean isAtLeast(@NonNull State state) {
return compareTo(state) >= 0;
}
}
- LifecycleRegistry 觀察者添加和事件分發(fā)
public class LifecycleRegistry extends Lifecycle {
private FastSafeIterableMap<LifecycleObserver, ObserverWithState>
mObserverMap =new FastSafeIterableMap<>();
//一般一個 LifecycleRegistry 對應(yīng)一個 LifecycleOwner 對象（Activity/Fragment等）
//mState 就用來標(biāo)記 LifecycleOwner 對象所處的當(dāng)前生命周期狀態(tài)
private State mState;
// 持有對 LifecycleOwner 的弱引用双泪，避免內(nèi)存泄露
private final WeakReference<LifecycleOwner> mLifecycleOwner;
public LifecycleRegistry(@NonNull LifecycleOwner provider) {
mLifecycleOwner = new WeakReference<>(provider);
mState = INITIALIZED;
}
}
-
LifecycleRegistry addObserver 添加觀察者 將觀察者包裝成
ObserverWithState對象 存儲到mObserverMap中
@Override
public void addObserver(@NonNull LifecycleObserver observer) {
//添加新的Observer時，會首先根據(jù)宿主的狀態(tài)計算出它的初始狀態(tài)接箫，只要不是在onDestroy中注冊
// 的攒读，它的初始狀態(tài)都是INITIALIZED
State initialState = mState == DESTROYED ? DESTROYED : INITIALIZED;
//接著會把observer包裝成ObserverWithState，這個類主要是包含了觀察者及其狀態(tài)辛友。每個事
//件都會經(jīng)由這個對象類轉(zhuǎn)發(fā),這個類后面會來分析
ObserverWithState statefulObserver = new ObserverWithState(observer, initialState);
ObserverWithState previous = mObserverMap.putIfAbsent(observer, statefulObserver);
//添加到集合，如果之前已經(jīng)添加過了，則return
if (previous != null) {
return;
}
LifecycleOwner lifecycleOwner = mLifecycleOwner.get();
if (lifecycleOwner == null) {
// it is null we should be destroyed. Fallback quickly
//如果 LifecycleOwner 對象已經(jīng)被回收了废累，則直接返回
return;
}
//如果 isReentrance 為 true邓梅，則說明此時以下兩種情況至少有一個成立：
//1. mAddingObserverCounter != 0。會出現(xiàn)這種情況邑滨，是由于開發(fā)者先添加了一個
// LifecycleObserver 日缨，當(dāng)還在向其回調(diào)事件的過程中，
//在回調(diào)函數(shù)里又再次調(diào)用了 addObserver 方法添加了一個新的 LifecycleObserver
//2.mHandlingEvent 為 true掖看。即此時正處于向外回調(diào) Lifecycle.Event 的狀態(tài)
boolean isReentrance = mAddingObserverCounter != 0 || mHandlingEvent;
State targetState = calculateTargetState(observer);
//遞增加一匣距，標(biāo)記當(dāng)前正處于向新添加的 LifecycleObserver 回調(diào) Event 值的過程
mAddingObserverCounter++;
//拿觀察者的狀態(tài)和宿主當(dāng)前狀態(tài)做比較，如果小于0哎壳，說明兩者狀態(tài)還沒有對齊
while ((statefulObserver.mState.compareTo(targetState) < 0
&& mObserverMap.contains(observer))) {
//將 observer 已經(jīng)遍歷到的當(dāng)前的狀態(tài)值 mState 保存下來
pushParentState(statefulObserver.mState);
//向 observer 回調(diào)進(jìn)入“statefulObserver.mState”前需要收到的 Event 值
statefulObserver.dispatchEvent(lifecycleOwner, upEvent(statefulObserver.mState));
//移除 mState
popParentState();
// mState / subling may have been changed recalculate
targetState = calculateTargetState(observer);
}
if (!isReentrance) {
// we do sync only on the top level.
sync();
}
mAddingObserverCounter--;
}
- ObserverWithState 將外界傳入的 LifecycleObserver 對象傳給 Lifecycling 進(jìn)行類型包裝毅待，將反射邏輯和接口回調(diào)邏輯都給匯總綜合成一個新的 LifecycleEventObserver 對象，從而使得 Event 分發(fā)過程都統(tǒng)一為一個入口
static class ObserverWithState {
State mState;
LifecycleEventObserver mLifecycleObserver;
ObserverWithState(LifecycleObserver observer, State initialState) {
mLifecycleObserver = Lifecycling.lifecycleEventObserver(observer);
mState = initialState;
}
void dispatchEvent(LifecycleOwner owner, Event event) {
//再一次根據(jù)需要分發(fā)的事件類型反推出該觀察者的狀態(tài)
State newState = getStateAfter(event);
mState = min(mState, newState);
//把事件分發(fā)給被包裝的對象归榕，完成本次流程尸红。
mLifecycleObserver.onStateChanged(owner, event);
mState = newState;
}
}
//再一次根據(jù)需要分發(fā)的事件類型反推出該觀察者的狀態(tài),這樣的好處是事件與狀態(tài)對應(yīng)，不會出現(xiàn)跳躍
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);
}
- Lifecycling.lifecycleEventObserver 獲取觀察者對象類型 根據(jù)傳入的不同觀察者對象類型返回不同的觀察者對象
@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;
}
......一些反射邏輯....
}
-
LifecycleEventObserver和FullLifecycleObserver 都實現(xiàn)了
LifecycleObserver（Java多態(tài)）
public interface LifecycleObserver {
}
public interface LifecycleEventObserver extends LifecycleObserver {
void onStateChanged(@NonNull LifecycleOwner source, @NonNull Lifecycle.Event event);
}
interface FullLifecycleObserver extends LifecycleObserver {
void onCreate(LifecycleOwner owner);
void onStart(LifecycleOwner owner);
void onResume(LifecycleOwner owner);
void onPause(LifecycleOwner owner);
void onStop(LifecycleOwner owner);
void onDestroy(LifecycleOwner owner);
}
-
FullLifecycleObserverAdapter 實現(xiàn)了
LifecycleEventObserver接口刹泄，用于在收到 Lifecycle 生命周期事件狀態(tài)變化時外里，對其兩個構(gòu)造函數(shù)參數(shù)（FullLifecycleObserver、LifecycleEventObserver）進(jìn)行生命周期事件做轉(zhuǎn)發(fā)
class FullLifecycleObserverAdapter implements LifecycleEventObserver {
private final FullLifecycleObserver mFullLifecycleObserver;
private final LifecycleEventObserver mLifecycleEventObserver;
FullLifecycleObserverAdapter(FullLifecycleObserver fullLifecycleObserver,
LifecycleEventObserver lifecycleEventObserver) {
mFullLifecycleObserver = fullLifecycleObserver;
mLifecycleEventObserver = lifecycleEventObserver;
}
//進(jìn)行生命周期事件轉(zhuǎn)發(fā)
@Override
public void onStateChanged(LifecycleOwner source, Lifecycle.Event event) {
switch (event) {
case ON_CREATE:
mFullLifecycleObserver.onCreate(source);
break;
case ON_START:
mFullLifecycleObserver.onStart(source);
break;
case ON_RESUME:
mFullLifecycleObserver.onResume(source);
break;
case ON_PAUSE:
mFullLifecycleObserver.onPause(source);
break;
case ON_STOP:
mFullLifecycleObserver.onStop(source);
break;
case ON_DESTROY:
mFullLifecycleObserver.onDestroy(source);
break;
case ON_ANY:
throw new IllegalArgumentException("ON_ANY must not been send by anybody");
}
if (mLifecycleEventObserver != null) {
mLifecycleEventObserver.onStateChanged(source, event);
}
}
}
- 以上對如何添加觀察者以及這些觀察者如何接受宿主生命周期變更事件做了分析特石，接下來分析宿主的生命周期變更是怎么分發(fā)出去的 還記得
ComponentActivity中添加的那個ReportFragment嗎
class ComponentActivity implements LifecycleOwner{
//LifecycleRegistry觀察者添加和事件分發(fā)
private final LifecycleRegistry mLifecycleRegistry = new LifecycleRegistry(this);
@NonNull
@Override
public Lifecycle getLifecycle() {
return mLifecycleRegistry;
}
@SuppressLint("RestrictedApi")
@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
//往Activity上添加一個fragment,用以監(jiān)聽生命周期的變化
//其目的是為了兼顧不是繼承自AppCompactActivity的場景.
ReportFragment.injectIfNeededIn(this);
}
}
-
ReportFragment 獲取到 AppCompatActivity 的事件變化通知盅蝗，最終調(diào)用
LifecycleRegistry的handleLifecycleEvent(Lifecycle.Event)方法將Lifecycle.Event傳遞出去。此時姆蘸，LifecycleRegistry就拿到了Lifecycle.Event
public class ReportFragment extends Fragment {
//添加到Activity
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();
// Hopefully, we are the first to make a transaction.
manager.executePendingTransactions();
}
}
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);
}
}
}
@Override
public void onActivityCreated(Bundle savedInstanceState) {
super.onActivityCreated(savedInstanceState);
dispatch(Lifecycle.Event.ON_CREATE);
}
@Override
public void onStart() {
super.onStart();
dispatch(Lifecycle.Event.ON_START);
}
@Override
public void onResume() {
super.onResume();
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);
}
}
- 分發(fā)宿狀態(tài)的
handleLifecycleEvent該方法會根據(jù)接收到的Event值換算出對應(yīng)的State值墩莫，然后更新本地的 mState，再向所有 Observer 進(jìn)行事件通知乞旦，最終還是會調(diào)用到ObserverWithState的dispatchEvent方法贼穆，所以后邊重點關(guān)注 dispatchEvent 方法即可
public class LifecycleRegistry extends Lifecycle {
private FastSafeIterableMap<LifecycleObserver, ObserverWithState>
mObserverMap =new FastSafeIterableMap<>();
public void handleLifecycleEvent(@NonNull Lifecycle.Event event) {
//根據(jù)接收到的 Event 值換算出對應(yīng)的 State 值
State next = getStateAfter(event);
moveToState(next);
}
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;
}
}
- sync() 宿主生命周期狀態(tài)同步和分發(fā)
private void sync() {
//mLifecycleOwner.get() 獲取到的是宿主（Activity/Fragment）
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;
//如果宿主當(dāng)前轉(zhuǎn)態(tài) 小于 mObserverMap集合中最先添加的那個觀察者的狀態(tài)
//則說明宿主可能發(fā)生了狀態(tài)回退，比如當(dāng)前是RESUMED狀態(tài)兰粉，執(zhí)行了onPause則回退到
//STARTED狀態(tài)
//此時調(diào)用backwardPass把集合中的每個一觀察者分發(fā)一個on_pause事件故痊，并同步它的狀態(tài)。
if (mState.compareTo(mObserverMap.eldest().getValue().mState) < 0) {
backwardPass(lifecycleOwner);
}
//如果宿主當(dāng)前轉(zhuǎn)態(tài) 大于 mObserverMap集合中最先添加的那個觀察者的狀態(tài)
//則說明宿主可能發(fā)生了狀態(tài)前進(jìn)玖姑，比如當(dāng)前是STARTED狀態(tài)愕秫，執(zhí)行了onResume則前進(jìn)到
//RESUMED狀態(tài)
//此時調(diào)用forwardPass把集合中的每個一觀察者分發(fā)一個on_resume事件，并同步它的狀態(tài)焰络。
Entry<LifecycleObserver, ObserverWithState> newest = mObserverMap.newest();
if (!mNewEventOccurred && newest != null
&& mState.compareTo(newest.getValue().mState) > 0) {
forwardPass(lifecycleOwner);
}
}
mNewEventOccurred = false;
}
-
backwardPass和forwardPass 遍歷Map中保存的觀察者 進(jìn)行事件分發(fā)
注意mNewEventOccurred這個boolean值開關(guān) 是在有新的事件過來時能及時終止
while循環(huán)分發(fā)沒有意義的事件
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()))) {
Event event = downEvent(observer.mState);
pushParentState(getStateAfter(event));
//生命周期事件分發(fā)
observer.dispatchEvent(lifecycleOwner, event);
popParentState();
}
}
}
二戴甩、 Fragment 實現(xiàn) Lifecycle Fragment的實現(xiàn)和add到 AppCompatActivity中的 ReportFragment就打同小異了 一看就明白
public class Fragment implements LifecycleOwner{
LifecycleRegistry mLifecycleRegistry;
public Fragment() {
initLifecycle();
}
private void initLifecycle() {
mLifecycleRegistry = new LifecycleRegistry(this);
if (Build.VERSION.SDK_INT >= 19) {
mLifecycleRegistry.addObserver(new LifecycleEventObserver() {
@Override
public void onStateChanged(@NonNull LifecycleOwner source,
@NonNull Lifecycle.Event event) {
if (event == Lifecycle.Event.ON_STOP) {
if (mView != null) {
mView.cancelPendingInputEvents();
}
}
}
});
}
}
@Override
@NonNull
public Lifecycle getLifecycle() {
return mLifecycleRegistry;
}
//事件分發(fā)
void performCreate(){
mLifecycleRegistry.handleLifecycleEvent(Lifecycle.Event.ON_CREATE);
}
void performStart(){
mLifecycleRegistry.handleLifecycleEvent(Lifecycle.Event.ON_START);
}
void performResume(){
mLifecycleRegistry.handleLifecycleEvent(Lifecycle.Event.ON_RESUME);
}
}
總結(jié)
- LifecycleOwner：
Activity/Fragment都實現(xiàn)了該接口，用以聲明它是一個能夠提供生命周期事件的宿主闪彼。同時必須復(fù)寫getLifecycle()方法提供一個Lifecycle對象 - Lifecycle：是一個抽象類甜孤，里面定義了兩個枚舉
State宿主的狀態(tài)协饲，Event需要分發(fā)的事件的類型 -
LifecycleRegistry: 是 Lifecycle 的唯一實現(xiàn)類，主要用來負(fù)責(zé)注冊 Observer(觀察者)缴川，以及分發(fā)宿主狀態(tài)事件給它們
1610167303751.jpg
使用場景介紹
對平時開發(fā)中需要感知（Fragment/Activity）生命周期邏輯的地方 可以繼承LifecycleEventObserver或者 DefaultLifecycleObserver 并添加到 （Fragment/Activity）的 LifecycleRegistry中來觀察 （Fragment/Activity）生命周期的變化 而做出對應(yīng)的業(yè)務(wù)邏輯變化 (Ps:以前我們可能是在BaseActivity 或者BaseFragment的onStart()和onStop()添加對應(yīng)生命周期相關(guān)的邏輯 )
class LocationObserver extends DefaultLifecycleObserver{
//宿主執(zhí)行了onstart時茉稠，會分發(fā)該事件
void onStart(@NotNull LifecycleOwner owner){
//開啟定位
}
//宿主執(zhí)行了onstop時 會分發(fā)該事件
void onStop(@NotNull LifecycleOwner owner){
//停止定位
}
}
//2. 注冊觀察者,觀察宿主生命周期狀態(tài)變化
class MyFragment extends Fragment{
public void onCreate(Bundle bundle){
LocationObserver observer =new LocationObserver()
//添加觀察者
getLifecycle().addObserver(observer);
}
}
