class MainActivity : AppCompatActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        //訂閱生命周期
        lifecycle.addObserver(MyLifecycleEventObserver())
        lifecycle.addObserver(MyLifecycleObserver())
    }
    /**
     * 方式一（個人比較推薦）
     */
    class MyLifecycleEventObserver : LifecycleEventObserver {
        override fun onStateChanged(source: LifecycleOwner, event: Lifecycle.Event) {
            when (event) {
                Lifecycle.Event.ON_CREATE -> println("onCreate")
                Lifecycle.Event.ON_START -> println("onStart")
                Lifecycle.Event.ON_RESUME -> println("onResume")
                Lifecycle.Event.ON_PAUSE -> println("onPause")
                Lifecycle.Event.ON_STOP -> println("onStop")
                Lifecycle.Event.ON_DESTROY -> println("onDestroy")
            }
        }
    }
    /**
     * 方式二
     */
    class MyLifecycleObserver : LifecycleObserver {
        @OnLifecycleEvent(Lifecycle.Event.ON_CREATE)
        fun onCreate(event: Lifecycle.Event) {
            println("onCreate")
        }
        @OnLifecycleEvent(Lifecycle.Event.ON_START)
        fun onStart(event: Lifecycle.Event) {
            println("onStart")
        }
        @OnLifecycleEvent(Lifecycle.Event.ON_RESUME)
        fun onResume(event: Lifecycle.Event) {
            println("onResume")
        }
    }
}

public class ComponentActivity extends androidx.core.app.ComponentActivity implements
        LifecycleOwner,
        ViewModelStoreOwner,
        SavedStateRegistryOwner,
        OnBackPressedDispatcherOwner {
    //創(chuàng)建Lifecycle實例
    private final LifecycleRegistry mLifecycleRegistry = new LifecycleRegistry(this);
    @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        mSavedStateRegistryController.performRestore(savedInstanceState);
        //往Activity添加一個ReportFragment來達到下發(fā)生命周期的目的
        ReportFragment.injectIfNeededIn(this);
        if (mContentLayoutId != 0) {
            setContentView(mContentLayoutId);
        }
    }
}

public class ReportFragment extends Fragment {
    public static void injectIfNeededIn(Activity activity) {
        //往Activity中存放一個ReportFragment
        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();
        }
    }
    @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;
    }
    /**
     * 下發(fā)生命周期事件
     */   
    private void dispatch(Lifecycle.Event event) {
        //獲取Activity的Lifecycle實例，下發(fā)生命周期事件
        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);
            }
        }
    }
}

public class FragmentActivity extends ComponentActivity implements
        ActivityCompat.OnRequestPermissionsResultCallback,
        ActivityCompat.RequestPermissionsRequestCodeValidator {
    //創(chuàng)建Lifecycle實例
    final LifecycleRegistry mFragmentLifecycleRegistry = new LifecycleRegistry(this);
    @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        //下發(fā)生命周期事件
        mFragmentLifecycleRegistry.handleLifecycleEvent(Lifecycle.Event.ON_CREATE);
    }
        @Override
    protected void onDestroy() {
        super.onDestroy();
        mFragments.dispatchDestroy();
        //下發(fā)生命周期事件
        mFragmentLifecycleRegistry.handleLifecycleEvent(Lifecycle.Event.ON_DESTROY);
    }
    @Override
    protected void onPause() {
        super.onPause();
        mResumed = false;
        mFragments.dispatchPause();
        //下發(fā)生命周期事件
        mFragmentLifecycleRegistry.handleLifecycleEvent(Lifecycle.Event.ON_PAUSE);
    }
    protected void onResumeFragments() {
        //下發(fā)生命周期事件
        mFragmentLifecycleRegistry.handleLifecycleEvent(Lifecycle.Event.ON_RESUME);
        mFragments.dispatchResume();
    }
}

public class Fragment implements ComponentCallbacks, OnCreateContextMenuListener, LifecycleOwner,
        ViewModelStoreOwner, SavedStateRegistryOwner {
    LifecycleRegistry mLifecycleRegistry;
    public Fragment() {
        initLifecycle();
    }
    private void initLifecycle() {
        //創(chuàng)建Lifecycle實例
        mLifecycleRegistry = new LifecycleRegistry(this);
    }
    void performCreate(Bundle savedInstanceState) {
        onCreate(savedInstanceState);
        //下發(fā)生命周期事件給觀察者
        mLifecycleRegistry.handleLifecycleEvent(Lifecycle.Event.ON_CREATE);
    }
    void performStart() {
        onStart();
        //下發(fā)生命周期事件給觀察者
        mChildFragmentManager.dispatchStart();
    }
    void performResume() {
        onResume();
        //下發(fā)生命周期事件給觀察者
        mLifecycleRegistry.handleLifecycleEvent(Lifecycle.Event.ON_RESUME);
    }
}

public class LifecycleRegistry extends Lifecycle {
    @Override
    public void addObserver(@NonNull LifecycleObserver observer) {
        //首次添加觀察者，如果宿主狀態(tài)不是DESTROYED，那么觀察者初始狀態(tài)都是INITIALIZED
        State initialState = mState == DESTROYED ? DESTROYED : INITIALIZED;
        //把傳進去的生命周期觀察者以及初始狀態(tài)包裝成ObserverWithState對象
        ObserverWithState statefulObserver = new ObserverWithState(observer, initialState);
        //把包裝好的ObserverWithState對象存放到觀察者集合中，如果此前已經(jīng)添加過則會返回此前添加的值，否則返回null
        ObserverWithState previous = mObserverMap.putIfAbsent(observer, statefulObserver);
        //判斷此前是否添加過，如果添加過則直接結(jié)束方法
        if (previous != null) {
            return;
        }
        //獲取宿主實例
        LifecycleOwner lifecycleOwner = mLifecycleOwner.get();
        if (lifecycleOwner == null) {
            // it is null we should be destroyed. Fallback quickly
            return;
        }
        //計算觀察者的目標狀態(tài)
        State targetState = calculateTargetState(observer);
        //循環(huán)比對觀察者的狀態(tài)和宿主的狀態(tài)，如果沒有對齊則下發(fā)對應(yīng)的生命周期事件
        //拿觀察者的狀態(tài)和宿主的狀態(tài)做比較 如果小于0代表狀態(tài)還沒有對齊，需要繼續(xù)下發(fā)生命周期狀態(tài)給觀察者
        //假設(shè)是在Activity的onResume()方法中注冊的觀察者，那么就需要給觀察者下發(fā)onCreate,onStart,onResume這些事件
        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);
        }
    }
}

public class LifecycleRegistry extends Lifecycle {
    public void handleLifecycleEvent(@NonNull Lifecycle.Event event) {
        //根據(jù)生命周期事件推算出其狀態(tài)
        State next = getStateAfter(event);
        //移動到新狀態(tài)
        moveToState(next);
    }
    private void moveToState(State next) {
        //如果當前狀態(tài)和新狀態(tài)相等 結(jié)束方法
        if (mState == next) {
            return;
        }
        //記錄新狀態(tài)
        mState = next;
        //如果當前正在下發(fā)生命周期事件 或 當前正在添加觀察者 結(jié)束方法
        if (mHandlingEvent || mAddingObserverCounter != 0) {
            mNewEventOccurred = true;
            return;
        }
        //同步新狀態(tài)給觀察者
        mHandlingEvent = true;
        sync();
        mHandlingEvent = false;
    }
    private void sync() {
        //獲取宿主實例
        LifecycleOwner lifecycleOwner = mLifecycleOwner.get();
        //停止循環(huán)的條件是已經(jīng)同步狀態(tài)完成 或 沒有觀察者
        while (!isSynced()) {
            mNewEventOccurred = false;
            //獲取觀察者集合中最先添加的那個元素 拿當前狀態(tài)和觀察者狀態(tài)作比較 判斷當前是不是向后移動狀態(tài) STARTED -> ON_STOP
            if (mState.compareTo(mObserverMap.eldest().getValue().mState) < 0) {
                backwardPass(lifecycleOwner);
            }
            //獲取觀察者集合中最新添加的那個元素 拿當前狀態(tài)和觀察者狀態(tài)作比較 判斷當前是不是向前移動狀態(tài) STARTED -> ON_RESUME
            Entry<LifecycleObserver, ObserverWithState> newest = mObserverMap.newest();
            if (!mNewEventOccurred && newest != null
                    && mState.compareTo(newest.getValue().mState) > 0) {
                forwardPass(lifecycleOwner);
            }
        }
        mNewEventOccurred = false;
    }
    /**
     * 向前移動狀態(tài)
     */
    private void forwardPass(LifecycleOwner lifecycleOwner) {
        Iterator<Entry<LifecycleObserver, ObserverWithState>> ascendingIterator =
                mObserverMap.iteratorWithAdditions();
        //遍歷所有觀察者
        while (ascendingIterator.hasNext() && !mNewEventOccurred) {
            Entry<LifecycleObserver, ObserverWithState> entry = ascendingIterator.next();
            ObserverWithState observer = entry.getValue();
            //拿觀察者的狀態(tài)和宿主的狀態(tài)做比較 如果小于0代表狀態(tài)還沒有對齊
            //假設(shè)當前宿主在RESUMED狀態(tài) 觀察者在CREATED狀態(tài) 則需要下發(fā)：ON_START,ON_RESUME生命周期事件 需要循環(huán)兩次
            while ((observer.mState.compareTo(mState) < 0 && !mNewEventOccurred
                    && mObserverMap.contains(entry.getKey()))) {
                pushParentState(observer.mState);
                //根據(jù)觀察者狀態(tài)反推向前移動事件 下發(fā)生命周期事件
                observer.dispatchEvent(lifecycleOwner, upEvent(observer.mState));
                popParentState();
            }
        }
    }
    /**
     * 通過觀察者狀態(tài)反推向前移動事件
     */
    private static Event upEvent(State state) {
        switch (state) {
            case INITIALIZED:
            case DESTROYED:
                return ON_CREATE;
            case CREATED:
                return ON_START;
            case STARTED:
                return ON_RESUME;
            case RESUMED:
                throw new IllegalArgumentException();
        }
        throw new IllegalArgumentException("Unexpected state value " + state);
    }
    /**
     * 向后移動狀態(tài)
     */
    private void backwardPass(LifecycleOwner lifecycleOwner) {
        Iterator<Entry<LifecycleObserver, ObserverWithState>> descendingIterator =
                mObserverMap.descendingIterator();
        //遍歷所有觀察者
        while (descendingIterator.hasNext() && !mNewEventOccurred) {
            Entry<LifecycleObserver, ObserverWithState> entry = descendingIterator.next();
            ObserverWithState observer = entry.getValue();
            //拿觀察者的狀態(tài)和宿主的狀態(tài)做比較 如果大于0代表狀態(tài)還沒有對齊
            //假設(shè)當前觀察者在RESUMED狀態(tài) 宿主在DESTROYED狀態(tài) 那么需要下發(fā)：ON_PAUSE，ON_STOP，ON_DESTROY這些生命周期事件 循環(huán)三次
            while ((observer.mState.compareTo(mState) > 0 && !mNewEventOccurred
                    && mObserverMap.contains(entry.getKey()))) {
                //根據(jù)觀察者狀態(tài)反推出向后移動事件
                Event event = downEvent(observer.mState);
                pushParentState(getStateAfter(event));
                //下發(fā)該生命周期事件給觀察者
                observer.dispatchEvent(lifecycleOwner, event);
                popParentState();
            }
        }
    }
    /**
     * 通過觀察者狀態(tài)反推向后移動的事件
     */
    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);
    }
}

static class ObserverWithState {
        //記錄當前狀態(tài)
        State mState;
        //生命周期觀察者
        LifecycleEventObserver mLifecycleObserver;
        ObserverWithState(LifecycleObserver observer, State initialState) {
            mLifecycleObserver = Lifecycling.lifecycleEventObserver(observer);
            mState = initialState;
        }
        //下發(fā)生命周期事件給生命周期觀察者
        void dispatchEvent(LifecycleOwner owner, Event event) {
            //根據(jù)生命周期事件推算出生命周期狀態(tài)
            State newState = getStateAfter(event);
            mState = min(mState, newState);
            //調(diào)用生命周期觀察者的onStateChanged()方法通知生命周期觀察者生命周期發(fā)生變化
            mLifecycleObserver.onStateChanged(owner, event);
            mState = newState;
        }
}
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);
}