@OnLifecycleEvent(Lifecycle.Event.ON_CREATE)
    private void fun1(){
        //on_create時需要做的事情
    }

getLifecycle().addObserver(new MyUtil());

//對應的狀態(tài)轉移函數(shù)
@NonNull
public State getTargetState() {
  	//枚舉類，this標識的是某種事件，例如ON_CREATE、ON_STOP等等。
    switch (this) {
        //對于ON_CREATE和ON_STOP事件，都對應CREATED狀態(tài)。（圖上二者都指向CREATED）
        case ON_CREATE:
        case ON_STOP:
            return State.CREATED;
        case ON_START:
        case ON_PAUSE:
            return State.STARTED;
        case ON_RESUME:
            return State.RESUMED;
        case ON_DESTROY:
            return State.DESTROYED;
        case ON_ANY:
            break;
    }
    throw new IllegalArgumentException(this + " has no target state");
}

public class ComponentActivity extends androidx.core.app.ComponentActivity implements
        LifecycleOwner

public interface LifecycleOwner {
    @NonNull
    Lifecycle getLifecycle();
}
private final LifecycleRegistry mLifecycleRegistry = new LifecycleRegistry(this);

//LifecycleRegistry.java
@Override
public void addObserver(@NonNull LifecycleObserver observer) {
    enforceMainThreadIfNeeded("addObserver");//主線程斷言
    State initialState = mState == DESTROYED ? DESTROYED : INITIALIZED;//如果mState是已經(jīng)銷毀的則就標記為已經(jīng)銷毀；如果是其他的（所有有效的），就標記為已初始化過的。
    ObserverWithState statefulObserver = new ObserverWithState(observer, initialState);//傳入自定義的觀察者（非被觀察者）和狀態(tài)。
    ObserverWithState previous = mObserverMap.putIfAbsent(observer, statefulObserver);//嘗試放入被觀察者維護的觀察者對象集合中（如果不存在的話，防止多次注冊）。如果存在，返回既有值的value；如果不存在put，并返回null。
  	//如果之前存在一個相同的observer，那么直接return；
    if (previous != null) {
        return;
    }
  	//取得lifecycleOwner對象，通常是Activity或者是Fragment，注意，此處的LifecycleOwner是被WeakReference<LivecycleOwner>引用住的。是在LifecycleRegistry的構造函數(shù)中，被注入了值。
   	//private final WeakReference<LifecycleOwner> mLifecycleOwner;

    LifecycleOwner lifecycleOwner = mLifecycleOwner.get();

  	//如果為空，說明對象已經(jīng)被回收了，return掉。
    if (lifecycleOwner == null) {
        // it is null we should be destroyed. Fallback quickly
        return;
    }

    boolean isReentrance = mAddingObserverCounter != 0 || mHandlingEvent;
    State targetState = calculateTargetState(observer);
    mAddingObserverCounter++;
		//正在注冊的觀察者數(shù)量+1

  	//ENUM Class之間的比較，二者相等會返回0。這個循環(huán)作用就是追加之前的生命周期事件派發(fā)（粘性）
    while ((statefulObserver.mState.compareTo(targetState) < 0
            && mObserverMap.contains(observer))) {
      	//該行代碼的內(nèi)部實現(xiàn)是：mParentStates.add(state);其中的mParentStates是一個ArrayList，用于存儲一些“舊”的消息。例如我們在running（create、start、resume都走完了）狀態(tài)通過點擊按鈕注冊一個監(jiān)聽，此時的pushParentState中的參數(shù)：statefulObserver.mState就是“INITIALIZED”。
        pushParentState(statefulObserver.mState);
      	//接著上面的情況，傳入的參數(shù)是：INITIALIZED，從upFrom中取出的Event是“ON_CREATE”，但是此時onCreate狀態(tài)實際上早已走完了。
        final Event event = Event.upFrom(statefulObserver.mState);
        if (event == null) {
            throw new IllegalStateException("no event up from " + statefulObserver.mState);
        }
      	//注意，我們在實例中的ON_CREATE等等回調(diào)都是在這步里面執(zhí)行的【圖1】，主要是更新狀態(tài)，和派發(fā)事件，走完這代碼，statefulObserver.mState就變成了CREATED
        statefulObserver.dispatchEvent(lifecycleOwner, event);

      	//和pushParentState對應，此時刪除后parentState長度為0。
        popParentState();

        targetState = calculateTargetState(observer);
      	//此時statefulObserver.mState為CREATED，而targetState的值被更新為：RESUME。
    }

    if (!isReentrance) {
        // we do sync only on the top level.
        sync();
    }
    mAddingObserverCounter--;
}

private State calculateTargetState(LifecycleObserver observer) {

  	//mObserverMap即上面addObserver主干代碼中的第七行：
  	//ObserverWithState previous = mObserverMap.putIfAbsent(observer, statefulObserver);
  	//此處是對該Map唯一的加入調(diào)用。

  	//取出臨近的ObserverWithState,FastSafeIterableMap，注釋中說是：Poor's man LinkedHashMap
  	//LifecycleObserver在androidx.core.app.ComponentActivity中可以看到，ComponentActivity實現(xiàn)了該接口。
  	//所以，該Map是一個Activity對應ObserverWithState(一個State、一個LifecycleEventObserver)的結構：

    Map.Entry<LifecycleObserver, ObserverWithState> previous = mObserverMap.ceil(observer);

  	//如果不為空，那么就設置siblingState為previous中存儲對象的mState else null
    State siblingState = previous != null ? previous.getValue().mState : null;
  	//如果有parentState，那么獲取其中的最后一個的State else null
    State parentState = !mParentStates.isEmpty() ? mParentStates.get(mParentStates.size() - 1)
            : null;
  	//取mState、siblingState和parentState中最小值。
    return min(min(mState, siblingState), parentState);
}


//statefulObserver.dispatchEvent(lifecycleOwner, event);
void dispatchEvent(LifecycleOwner owner, Event event)
   //根據(jù)event（ENUM）獲取其狀態(tài)，這個和上面的狀態(tài)圖是一樣的。
   State newState = event.getTargetState();
	 //取最小狀態(tài)
   mState = min(mState, newState);
   //更新狀態(tài)，響應回調(diào)
   mLifecycleObserver.onStateChanged(owner, event);
	 //設置新狀態(tài)
   mState = newState;
}

private void sync() {
  	//即Activity、Fragment這類生命周期持有者。
    LifecycleOwner lifecycleOwner = mLifecycleOwner.get();
  	//判斷是否已經(jīng)被回收
    if (lifecycleOwner == null) {
        throw new IllegalStateException("LifecycleOwner of this LifecycleRegistry is already"
                + "garbage collected. It is too late to change lifecycle state.");
    }
  	//如果mObserverMap中的元素還沒有完全同步就進行循環(huán),進行所有保存著的觀察者的狀態(tài)同步。
    while (!isSynced()) {
      	//標記是否有新的事件出現(xiàn)，當moveToState被調(diào)用時，滿足條件（mHandlingEvent || mAddingObserverCounter != 0）時，該屬性會被置為true。
      	//這兩個條件是：1是否正在處理事件，2是是否正在處理新加入的觀察者。
        mNewEventOccurred = false;
        //利用mState和mObserverMap中最老的元素狀態(tài)相比較，如果更小（最小的是DESTROY，INITIALIZED，一直到RESUMED），那么久從最早被加入的元素，順著這個LinkedHashMap進行后向的更新。例如CREATED < RESUMED
        if (mState.compareTo(mObserverMap.eldest().getValue().mState) < 0) {
          	//后向傳播
            backwardPass(lifecycleOwner);
        }
        Map.Entry<LifecycleObserver, ObserverWithState> newest = mObserverMap.newest();
      	//如果沒有新的事件出現(xiàn)（期間沒有moveToState被觸發(fā)），最新的觀察者不為空，并且有當前組件的狀態(tài)更先于最新組件的狀態(tài)。
      	//例如mState = RESUMED和newest.mState = CREATED，那么就觸發(fā)前向傳播。
        if (!mNewEventOccurred && newest != null
                && mState.compareTo(newest.getValue().mState) > 0) {
          	//前向傳播
            forwardPass(lifecycleOwner);
        }
      	//在這個循環(huán)中，直到所有Observer狀態(tài)全部同步為止。
    }
    mNewEventOccurred = false;
}

private boolean isSynced() {
  	if (mObserverMap.size() == 0) {
    	return true;
  	}
  	State eldestObserverState = mObserverMap.eldest().getValue().mState;
  	State newestObserverState = mObserverMap.newest().getValue().mState;
  	//判斷最晚加入的 是否等于 最早加入的 是否等于 當前的狀態(tài)。
  	return eldestObserverState == newestObserverState && mState == newestObserverState;
}

//forwardPass同理；
private void backwardPass(LifecycleOwner lifecycleOwner) {
  	//迭代器
    Iterator<Map.Entry<LifecycleObserver, ObserverWithState>> descendingIterator =
            mObserverMap.descendingIterator();
  	//在forwardPass中：
    //Iterator<Map.Entry<LifecycleObserver, ObserverWithState>> ascendingIterator =
    //			mObserverMap.iteratorWithAdditions();
  	//在沒有新的事件（moveToState觸發(fā)）的情況下進行迭代。
    while (descendingIterator.hasNext() && !mNewEventOccurred) {
        Map.Entry<LifecycleObserver, ObserverWithState> entry = descendingIterator.next();
        ObserverWithState observer = entry.getValue();
        while ((observer.mState.compareTo(mState) > 0 && !mNewEventOccurred
                && mObserverMap.contains(entry.getKey()))) {
          	//和addObserver中相似的邏輯，同步時主動去回調(diào)其他滯后的監(jiān)聽者的狀態(tài)。
            Event event = Event.downFrom(observer.mState);
            if (event == null) {
                throw new IllegalStateException("no event down from " + observer.mState);
            }
            pushParentState(event.getTargetState());
            observer.dispatchEvent(lifecycleOwner, event);
            popParentState();
        }
    }
}

    @MainThread
    public void setCurrentState(@NonNull State state) {
        enforceMainThreadIfNeeded("setCurrentState");//斷言主線程
        moveToState(state);
    }

    /**
     * Sets the current state and notifies the observers.
     * <p>
     * Note that if the {@code currentState} is the same state as the last call to this method,
     * calling this method has no effect.（如果在相同的State下調(diào)用此方法無效。）
     *
     * @param event The event that was received
     */
    public void handleLifecycleEvent(@NonNull Lifecycle.Event event) {
        enforceMainThreadIfNeeded("handleLifecycleEvent");//斷言主線程
        moveToState(event.getTargetState());
    }

    private void moveToState(State next) {
      	//相同狀態(tài)下調(diào)用無效。
        if (mState == next) {
            return;
        }
        mState = next;
      	//如果正在處理事件，或者含有某個剛剛加入的Observer（數(shù)量不為0），那么久標記當前有新的事件出現(xiàn)了。
        if (mHandlingEvent || mAddingObserverCounter != 0) {
            mNewEventOccurred = true;
            // we will figure out what to do on upper level.
            return;
        }
        mHandlingEvent = true;//標記正在處理事件，有點像鎖
        sync();//同步（前后向傳播）
        mHandlingEvent = false;//結束
    }

@Deprecated
@MainThread
public void markState(@NonNull State state) {
    enforceMainThreadIfNeeded("markState");
    setCurrentState(state);
}

@MainThread
public void setCurrentState(@NonNull State state) {
    enforceMainThreadIfNeeded("setCurrentState");
    moveToState(state);
}
public void handleLifecycleEvent(@NonNull Lifecycle.Event event) {
    enforceMainThreadIfNeeded("handleLifecycleEvent");
    moveToState(event.getTargetState());
}

package androidx.core.app;

@CallSuper
@Override
protected void onSaveInstanceState(@NonNull Bundle outState) {
    mLifecycleRegistry.markState(Lifecycle.State.CREATED);//
    super.onSaveInstanceState(outState);
}

package androidx.activity;

@CallSuper
@Override
protected void onSaveInstanceState(@NonNull Bundle outState) {
    Lifecycle lifecycle = getLifecycle();
    if (lifecycle instanceof LifecycleRegistry) {
        ((LifecycleRegistry) lifecycle).setCurrentState(Lifecycle.State.CREATED);//
    }
    super.onSaveInstanceState(outState);
    mSavedStateRegistryController.performSave(outState);
}

    protected void onCreate(@Nullable Bundle savedInstanceState) {
        mFragments.attachHost(null /*parent*/);

        if (savedInstanceState != null) {
           //如果是重建的Activity進行的操作，以下省略
           ......
        }

        if (mPendingFragmentActivityResults == null) {
            mPendingFragmentActivityResults = new SparseArrayCompat<>();
            mNextCandidateRequestIndex = 0;
        }

        super.onCreate(savedInstanceState);

        mFragmentLifecycleRegistry.handleLifecycleEvent(Lifecycle.Event.ON_CREATE);//派發(fā)事件
        mFragments.dispatchCreate();
    }

@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    mSavedStateRegistryController.performRestore(savedInstanceState);
    ReportFragment.injectIfNeededIn(this);//
    if (mContentLayoutId != 0) {
        setContentView(mContentLayoutId);
    }
}

public static void injectIfNeededIn(Activity activity) {
    if (Build.VERSION.SDK_INT >= 29) {
        // On API 29+(Android 10.0 +), 我們可以直接注冊正確的lifecycle監(jiān)聽,內(nèi)部將會直接實現(xiàn)監(jiān)聽：在Activity類中的mActivityLifecycleCallbacks中，加入一個Callback：mActivityLifecycleCallbacks.add(callback);
        activity.registerActivityLifecycleCallbacks(
                new LifecycleCallbacks());
    }

    android.app.FragmentManager manager = activity.getFragmentManager();
    if (manager.findFragmentByTag(REPORT_FRAGMENT_TAG) == null) {
      	//注意這里，我們向當前的Activity中新加入了一個new ReportFragment()。
        manager.beginTransaction().add(new ReportFragment(), REPORT_FRAGMENT_TAG).commit();
        manager.executePendingTransactions();
    }
}

@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);
}

@SuppressWarnings("deprecation")
static void dispatch(@NonNull Activity activity, @NonNull Lifecycle.Event event) {
    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);//
        }
    }
}

public static void injectIfNeededIn(Activity activity) {
    if (Build.VERSION.SDK_INT >= 29) {
        // On API 29+, we can register for the correct Lifecycle callbacks directly
        LifecycleCallbacks.registerIn(activity);
    }
    //……
}

static class LifecycleCallbacks implements Application.ActivityLifecycleCallbacks {

  	//有做省略
    static void registerIn(Activity activity) {
        activity.registerActivityLifecycleCallbacks(new LifecycleCallbacks());
    }
    @Override
    public void onActivityPostCreated(@NonNull Activity activity,
            @Nullable Bundle savedInstanceState) {
        dispatch(activity, Lifecycle.Event.ON_CREATE);
    }
    @Override
    public void onActivityPostStarted(@NonNull Activity activity) {
        dispatch(activity, Lifecycle.Event.ON_START);
    }
    @Override
    public void onActivityPostResumed(@NonNull Activity activity) {
        dispatch(activity, Lifecycle.Event.ON_RESUME);
    }
    @Override
    public void onActivityPrePaused(@NonNull Activity activity) {
        dispatch(activity, Lifecycle.Event.ON_PAUSE);
    }
    @Override
    public void onActivityPreStopped(@NonNull Activity activity) {
        dispatch(activity, Lifecycle.Event.ON_STOP);
    }
    @Override
    public void onActivityPreDestroyed(@NonNull Activity activity) {
        dispatch(activity, Lifecycle.Event.ON_DESTROY);
    }
}

private void dispatch(@NonNull Lifecycle.Event event) {
    if (Build.VERSION.SDK_INT < 29) {
        dispatch(getActivity(), event);//TAG
    }
}

static void dispatch(@NonNull Activity activity, @NonNull Lifecycle.Event event) {
    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);
        }
    }
}

//API29中的調(diào)用，走的是上面的靜態(tài)方法。
@Override
public void onActivityPostStarted(@NonNull Activity activity) {
	  dispatch(activity, Lifecycle.Event.ON_START);
}

@Override
public void onActivityResumed(@NonNull Activity activity) {
}
@Override
public void onActivityPostResumed(@NonNull Activity activity) {
    dispatch(activity, Lifecycle.Event.ON_RESUME);//向監(jiān)聽者（MyUtils類等等）派發(fā)事件
}