Lifecycle原理

墨蓝 2022-09-12 09:49 182阅读 0赞

## 1、原理解析 ##

### 1.1 从注册观察者的入口方法开始 ###

#### 1.1.1 LifecycleOwner ####

/** * 接口，用来获取Lifecycle对象 */
    public interface LifecycleOwner { 
        @NonNull
        Lifecycle getLifecycle();
    }

#### 1.1.2 ComponentActivity、androidx包中的Fragment自身实现了LifecycleOwner接口，并创建了继承了Lifecycle类的LifecycleRegistry对象，lifecycleOwner.getLifecycle()获取的就是LifecycleRegistry对象，通过LifecycleRegistry来进行注册和反注册观察者 ####

lifecycleOwner.getLifecycle().addObserver(this);

#### 1.1.3 Lifecycle ####

/** * 抽象类，提供注册/反注册方法和记录组件生命周期事件和状态的枚举 */
     public abstract class Lifecycle { 
        @MainThread
        public abstract void addObserver(@NonNull LifecycleObserver observer);
    
        @MainThread
        public abstract void removeObserver(@NonNull LifecycleObserver observer);
    
        @SuppressWarnings("WeakerAccess")
        public enum Event { 
            ON_CREATE,
            ON_START,
            ON_RESUME,
            ON_PAUSE,
            ON_STOP,
            ON_DESTROY,
            ON_ANY;
        }
    
        @SuppressWarnings("WeakerAccess")
        public enum State { 
            DESTROYED,
            INITIALIZED,
            CREATED,
            STARTED,
            RESUMED;
    
            /** * DESTROYED<INITIALIZED<CREATED<STARTED<RESUMED */
            public boolean isAtLeast(@NonNull State state) { 
                return compareTo(state) >= 0;
            }
        }
    }

#### 1.1.4 LifecycleRegistry ####

public class LifecycleRegistry extends Lifecycle { 
    
        /** * 是一个可以在遍历中删除数据的链表,用来保存ObserverWithState */
        private FastSafeIterableMap<LifecycleObserver, ObserverWithState> mObserverMap =
                new FastSafeIterableMap<>();
    
     	/** * 注册观察者的地方 */
        @Override
        public void addObserver(@NonNull LifecycleObserver observer) { 
        	//是否是在主线程，不是，抛异常，终止。
            enforceMainThreadIfNeeded("addObserver");
            //新注册的观察者的 initialState 一般是INITIALIZED
            State initialState = mState == DESTROYED ? DESTROYED : INITIALIZED;
            //LifecycleObserver 和 State 创建ObserverWithState对象
            ObserverWithState statefulObserver = new ObserverWithState(observer, initialState);
            //查看要注册的观察者是否已经存在，存在，终止，不存在，添加到mObserverMap中。
            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;
    	    }
    	    // 下面的逻辑就是：把之前优先分发的事件，一个个分发给后添加的观察者
    	    // 比如在Activity的onResume生命周期中调用LifecycleRegistry.addObserver的时候
    	    // 因为新添加的statefulObserver的mState为INITIALIZED
    	    // 而targetState已经发生了变化，在LifecycleRegistry中的mState变成了RESUMED
    	    // 这里就会满足小于0的条件，并且在map中也有了新加入的这个Observer
    	    // 分发一次之后，就会改变targetState的值，如果新的值还是比当前的大，则继续分发
    	    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) { 
    	        // we do sync only on the top level.
    	        sync();
    	    }
    	    mAddingObserverCounter--;
        }
    
        @Override
        public void removeObserver(@NonNull LifecycleObserver observer) { 
            enforceMainThreadIfNeeded("removeObserver");
            mObserverMap.remove(observer);
        }
    }

### 1.2 ComponentActivity ###

**ComponentActivity 实现了LifecycleOwner接口，内部创建了继承Lifecycle类的LifecycleRegistry，add一个空的ReportFragment，并通过ReportFragment来分管理生命周期事件**

public class ComponentActivity extends Activity implements LifecycleOwner { 
    
        private LifecycleRegistry mLifecycleRegistry = new LifecycleRegistry(this);
    
        @SuppressLint("RestrictedApi")
        @Override
        protected void onCreate(@Nullable Bundle savedInstanceState) { 
            super.onCreate(savedInstanceState);
            ReportFragment.injectIfNeededIn(this);
        }
        @NonNull
        @Override
        public Lifecycle getLifecycle() { 
            return mLifecycleRegistry;
        }
    }

### 1.3 ReportFragment 在对应的生命周期方法中分发事件，最后其实是调用LifecycleRegistry的handleLifecycleEvent方法来分发事件 ###

**ReportFragment**

public class ReportFragment extends android.app.Fragment { 
      
        public static void injectIfNeededIn(Activity activity) { 
        	//Sdk >= 29的时候会，通过Application.ActivityLifecycleCallbacks来监听
        	//生命周期方法分发事件，具体的自己可以去看一下，这也是我们上面例子，代码中打出的log
        	//页面中的onPause、onStop、onDestory 后调用的原因
            if (Build.VERSION.SDK_INT >= 29) { 
                LifecycleCallbacks.registerIn(activity);
            }
            ...
            android.app.FragmentManager manager = activity.getFragmentManager();
            if (manager.findFragmentByTag(REPORT_FRAGMENT_TAG) == null) { 
                manager.beginTransaction().add(new ReportFragment(), REPORT_FRAGMENT_TAG).commit();
                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);
        }
    
        private void dispatch(@NonNull Lifecycle.Event event) { 
            if (Build.VERSION.SDK_INT < 29) { 
                dispatch(getActivity(), event);
            }
        }
    
          @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);
                }
            }
        }
    }

### 1.4 LifecycleRegistry 对事件进行分发，最后会调用子类ObserverWithState的dispatchEvent() ###

public class LifecycleRegistry extends Lifecycle { 
    
        public void handleLifecycleEvent(@NonNull Lifecycle.Event event) { 
            enforceMainThreadIfNeeded("handleLifecycleEvent");
            moveToState(event.getTargetState());
        }
        
     	private void moveToState(State next) { 
            if (mState == next) { 
                return;
            }
            mState = next;
            if (mHandlingEvent || mAddingObserverCounter != 0) { 
                mNewEventOccurred = true;
                return;
            }
            mHandlingEvent = true;
            sync();
            mHandlingEvent = false;
        }
        
        // 把生命周期状态同步给所有观察者
     	private void sync() { 
     		...
            while (!isSynced()) { 
                mNewEventOccurred = false;
                if (mState.compareTo(mObserverMap.eldest().getValue().mState) < 0) { 
                    backwardPass(lifecycleOwner);
                }
                Map.Entry<LifecycleObserver, ObserverWithState> newest = mObserverMap.newest();
                if (!mNewEventOccurred && newest != null
                        && mState.compareTo(newest.getValue().mState) > 0) { 
                    forwardPass(lifecycleOwner);
                }
            }
            mNewEventOccurred = false;
        }
        
        //正序遍历，分发事件
       private void forwardPass(LifecycleOwner lifecycleOwner) { 
            Iterator<Map.Entry<LifecycleObserver, ObserverWithState>> ascendingIterator =
                    mObserverMap.iteratorWithAdditions();
            while (ascendingIterator.hasNext() && !mNewEventOccurred) { 
                Map.Entry<LifecycleObserver, ObserverWithState> entry = ascendingIterator.next();
                ObserverWithState observer = entry.getValue();
                while ((observer.mState.compareTo(mState) < 0 && !mNewEventOccurred
                        && mObserverMap.contains(entry.getKey()))) { 
                    pushParentState(observer.mState);
                    final Event event = Event.upFrom(observer.mState);
                    if (event == null) { 
                        throw new IllegalStateException("no event up from " + observer.mState);
                    }
                    observer.dispatchEvent(lifecycleOwner, event);
                    popParentState();
                }
            }
        }
          //倒序遍历，分发事件
    	 private void backwardPass(LifecycleOwner lifecycleOwner) { 
            Iterator<Map.Entry<LifecycleObserver, ObserverWithState>> descendingIterator =
                    mObserverMap.descendingIterator();
            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()))) { 
                    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();
                }
            }
        }
    
    	//最后会调用ObserverWithState的dispatchEvent来分发事件
    	static class ObserverWithState { 
            State mState;
            LifecycleEventObserver mLifecycleObserver;
    
            ObserverWithState(LifecycleObserver observer, State initialState) { 
                mLifecycleObserver = Lifecycling.lifecycleEventObserver(observer);
                mState = initialState;
            }
    
            void dispatchEvent(LifecycleOwner owner, Event event) { 
                State newState = event.getTargetState();
                mState = min(mState, newState);
                mLifecycleObserver.onStateChanged(owner, event);
                mState = newState;
            }
        }
    }

### 1.5 ObserverWithState的dispatchEvent 方法分发事件过程解析 ###

ObserverWithState对象创建的时候，会通过Lifecycling.lifecycleEventObserver(observer)方法创建一个LifecycleEventObserver对象，事件分发的dispatchEvent()方法中最终也是调用了  
LifecycleEventObserver对象的onStateChanged方法

//LifecycleRegistry 子类
    	static class ObserverWithState { 
            State mState;
            LifecycleEventObserver mLifecycleObserver;
    
            ObserverWithState(LifecycleObserver observer, State initialState) { 
                mLifecycleObserver = Lifecycling.lifecycleEventObserver(observer);
                mState = initialState;
            }
    
            void dispatchEvent(LifecycleOwner owner, Event event) { 
                State newState = event.getTargetState();
                mState = min(mState, newState);
                mLifecycleObserver.onStateChanged(owner, event);
                mState = newState;
            }
        }

public interface LifecycleEventObserver extends LifecycleObserver { 
        /** * Called when a state transition event happens. * * @param source The source of the event * @param event The event */
        void onStateChanged(@NonNull LifecycleOwner source, @NonNull Lifecycle.Event event);
    }

看一下Lifecycling类的lifecycleEventObserver方法

static LifecycleEventObserver lifecycleEventObserver(Object object) { 
            boolean isLifecycleEventObserver = object instanceof LifecycleEventObserver;
            boolean isFullLifecycleObserver = object instanceof FullLifecycleObserver;
             //如果观察者是通过两种接口接口实现的，接口回掉触发事件
            if (isLifecycleEventObserver && isFullLifecycleObserver) { 
                return new FullLifecycleObserverAdapter((FullLifecycleObserver) object,
                        (LifecycleEventObserver) object);
            }
             //如果观察者是通过FullLifecycleObserver接口实现的，接口回调出发事件
            if (isFullLifecycleObserver) { 
                return new FullLifecycleObserverAdapter((FullLifecycleObserver) object, null);
            }
            //如果观察者是通过LifecycleEventObserver接口实现的，接口回调出发事件
            if (isLifecycleEventObserver) { 
                return (LifecycleEventObserver) object;
            }
    		
    		//如果是LifecycleObserver接口+注解形式实现
    		//反射来判断类型
    		//如果是GENERATED_CALLBACK类型，根据constructors数据的size来判断是
    		//返回SingleGeneratedAdapterObserver 或 CompositeGeneratedAdaptersObserver
    		//否则返回ReflectiveGenericLifecycleObserver
            final Class<?> klass = object.getClass();
            int type = getObserverConstructorType(klass);
            if (type == GENERATED_CALLBACK) { 
                List<Constructor<? extends GeneratedAdapter>> constructors =
                        sClassToAdapters.get(klass);
                if (constructors.size() == 1) { 
                    GeneratedAdapter generatedAdapter = createGeneratedAdapter(
                            constructors.get(0), object);
                    return new SingleGeneratedAdapterObserver(generatedAdapter);
                }
                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);
            }
            return new ReflectiveGenericLifecycleObserver(object);
        }

1.如果观察者实现的是FullLifecycleObserver接口，会返回FullLifecycleObserverAdapter对象，最终调用FullLifecycleObserverAdapter的onStateChanged方法，并通过接口回掉来处理事件

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);
    }

class FullLifecycleObserverAdapter implements LifecycleEventObserver { 
    
        private final FullLifecycleObserver mFullLifecycleObserver;
        private final LifecycleEventObserver mLifecycleEventObserver;
    
        FullLifecycleObserverAdapter(FullLifecycleObserver fullLifecycleObserver,
                LifecycleEventObserver lifecycleEventObserver) { 
            mFullLifecycleObserver = fullLifecycleObserver;
            mLifecycleEventObserver = lifecycleEventObserver;
        }
    
        @Override
        public void onStateChanged(@NonNull LifecycleOwner source, @NonNull 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);
            }
        }
    }

2.如果观察者实现的是LifecycleEventObserver接口那么直接返回，并调用onStateChanged()方法通过接口回调来分发事件，这种方式在ComponentActivity类中有实现，如下：

public ComponentActivity() { 
            Lifecycle lifecycle = getLifecycle();
            ...
            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) { 
                        // Clear out the available context
                        mContextAwareHelper.clearAvailableContext();
                        // And clear the ViewModelStore
                        if (!isChangingConfigurations()) { 
                            getViewModelStore().clear();
                        }
                    }
                }
            });
            getLifecycle().addObserver(new LifecycleEventObserver() { 
                @Override
                public void onStateChanged(@NonNull LifecycleOwner source,
                        @NonNull Lifecycle.Event event) { 
                    ensureViewModelStore();
                    getLifecycle().removeObserver(this);
                }
            });
        }

3.如果观察者的实现方式不是继承的上面的两个接口，而是通过LifecycleObserver接口+注解的  
方式实现的话，就会走到下面的逻辑。  
首先根据反射判断是什么type，type定义了两种

public class Lifecycling { 
        private static final int REFLECTIVE_CALLBACK = 1;
        private static final int GENERATED_CALLBACK = 2;
        
     	private static Map<Class<?>, Integer> sCallbackCache = new HashMap<>();
        private static Map<Class<?>, List<Constructor<? extends GeneratedAdapter>>> sClassToAdapters =
                new HashMap<>();
    }

方法getObserverConstructorType()返回的type若等于GENERATED\_CALLBACK则会采用SingleGeneratedAdapterObserver或者CompositeGeneratedAdaptersObserver来实现观察者，这个方法有是通过resolveObserverCallbackType()来解析type的：

private static int getObserverConstructorType(Class<?> klass) { 
     		//看Map中是否已经有这个类的类型，有直接返回
            Integer callbackCache = sCallbackCache.get(klass);
            if (callbackCache != null) { 
                return callbackCache;
            }
            //没有，判断是什么类型并保存到map中
            int type = resolveObserverCallbackType(klass);
            sCallbackCache.put(klass, type);
            return type;
        }

上面函数主要是在判断该观察者对象是否有相应的GeneratedAdapter类生成，是通过generatedConstructor方法实现的，若有相应的GeneratedAdapter类生成，则通过反射生成相应的构造器：

private static int resolveObserverCallbackType(Class<?> klass) { 
            // 1.根据本类klass对象来判断是否有APT生成相应的GeneratedAdapter对象
            if (klass.getCanonicalName() == null) { 
                return REFLECTIVE_CALLBACK;
            }
    
            Constructor<? extends GeneratedAdapter> constructor = generatedConstructor(klass);
            if (constructor != null) { 
                sClassToAdapters.put(klass, Collections
                        .<Constructor<? extends GeneratedAdapter>>singletonList(constructor));
                return GENERATED_CALLBACK;
            }
    
            boolean hasLifecycleMethods = ClassesInfoCache.sInstance.hasLifecycleMethods(klass);
            if (hasLifecycleMethods) { 
                return REFLECTIVE_CALLBACK;
            }
    		// 2.根据父类superclass对象来判断是否有APT生成相应的GeneratedAdapter对象
            Class<?> superclass = klass.getSuperclass();
            List<Constructor<? extends GeneratedAdapter>> adapterConstructors = null;
            if (isLifecycleParent(superclass)) { 
                if (getObserverConstructorType(superclass) == REFLECTIVE_CALLBACK) { 
                    return REFLECTIVE_CALLBACK;
                }
                adapterConstructors = new ArrayList<>(sClassToAdapters.get(superclass));
            }
    		//3.根据其实现接口来判断是否有APT生成相应的GeneratedAdapter对象
            for (Class<?> intrface : klass.getInterfaces()) { 
                if (!isLifecycleParent(intrface)) { 
                    continue;
                }
                if (getObserverConstructorType(intrface) == REFLECTIVE_CALLBACK) { 
                    return REFLECTIVE_CALLBACK;
                }
                if (adapterConstructors == null) { 
                    adapterConstructors = new ArrayList<>();
                }
                adapterConstructors.addAll(sClassToAdapters.get(intrface));
            }
            if (adapterConstructors != null) { 
                sClassToAdapters.put(klass, adapterConstructors);
                return GENERATED_CALLBACK;
            }
    
            return REFLECTIVE_CALLBACK;
        }

private static Constructor<? extends GeneratedAdapter> generatedConstructor(Class<?> klass) { 
            try { 
                Package aPackage = klass.getPackage();
                String name = klass.getCanonicalName();
                final String fullPackage = aPackage != null ? aPackage.getName() : "";
                final String adapterName = getAdapterName(fullPackage.isEmpty() ? name :
                        name.substring(fullPackage.length() + 1));
    
                @SuppressWarnings("unchecked") final Class<? extends GeneratedAdapter> aClass =
                        (Class<? extends GeneratedAdapter>) Class.forName(
                                fullPackage.isEmpty() ? adapterName : fullPackage + "." + adapterName);
                Constructor<? extends GeneratedAdapter> constructor =
                        aClass.getDeclaredConstructor(klass);
                if (!constructor.isAccessible()) { 
                    constructor.setAccessible(true);
                }
                return constructor;
            } catch (ClassNotFoundException e) { 
                return null;
            } catch (NoSuchMethodException e) { 
                // this should not happen
                throw new RuntimeException(e);
            }
        }

调用GeneratedAdapter的callMethods()的方法来进行事件分发。

@RestrictTo(RestrictTo.Scope.LIBRARY_GROUP_PREFIX)
    public interface GeneratedAdapter { 
    
        /** * Called when a state transition event happens. * * @param source The source of the event * @param event The event * @param onAny approveCall onAny handlers * @param logger if passed, used to track called methods and prevent calling the same method * twice */
        void callMethods(LifecycleOwner source, Lifecycle.Event event, boolean onAny,
                MethodCallsLogger logger);
    }

其实，调用resolveObserverCallbackType()方法，该过程中已经生成了相应GeneratedAdapter的构造器，并返回一个SingleGeneratedAdapterObserver对象或CompositeGeneratedAdaptersObserver对象，在SingleGeneratedAdapterObserver中onStateChanged()方法中调用相应GeneratedAdapter对象的callMethods()来分发生命周期事件（通过Apt生成一个相应的Adapter对象，最后调用这个对象的callMethods的方法）。

4.以上情况都不是，最后会返回ReflectiveGenericLifecycleObserver对象，并调用LifecycleObserver.onStateChanged(owner, event)方法 。  
创建CallBackInfo时候，通过反射来解析具有OnLifecycleEvent注解的方法，最后通过CallBackInfo.invokeCallbacks 反射调用注解方法

**ReflectiveGenericLifecycleObserver**

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(@NonNull LifecycleOwner source, @NonNull Event event) { 
            mInfo.invokeCallbacks(source, event, mWrapped);
        }
    }

**CallbackInfo对象的创建过程**

private CallbackInfo createInfo(Class<?> klass, @Nullable Method[] declaredMethods) { 
    		//1.递归解析父类
            Class<?> superclass = klass.getSuperclass();
            Map<MethodReference, Lifecycle.Event> handlerToEvent = new HashMap<>();
            if (superclass != null) { 
            	//因为会有多重继承，因此会递归解析父类，getInfo()方法还会调用createInfo()方法
                CallbackInfo superInfo = getInfo(superclass);
                if (superInfo != null) { 
                    handlerToEvent.putAll(superInfo.mHandlerToEvent);
                }
            }
    		//2.递归解析接口。Java中接口也可以继承，这里getInterfaces()方法处理两种情况：a. klass对应一个类，b. kclass对应一个接口
            Class<?>[] interfaces = klass.getInterfaces();
            for (Class<?> intrfc : interfaces) { 
                for (Map.Entry<MethodReference, Lifecycle.Event> entry : getInfo(
                        intrfc).mHandlerToEvent.entrySet()) { 
                    //接口中解析的方法可能与父类中解析的是重复定义的，会做去重后再添加到handlerToEvent中
                    verifyAndPutHandler(handlerToEvent, entry.getKey(), entry.getValue(), klass);
                }
            }
    		//3. 解析本类对象具有OnLifecycleEvent注解的方法
            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();
                //OnLifecycleEvent注解的方法最多允许两个Event参数，定义参数超过两个会抛出异常
                int callType = CALL_TYPE_NO_ARG;
                if (params.length > 0) { 
                    callType = CALL_TYPE_PROVIDER;
                    //第一个参数类型期望为LifecycleOwner类型
                    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;
                    //第二个参数类型期望为Event类型
                    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);
                //去重添加到handlerToEvent中
                verifyAndPutHandler(handlerToEvent, methodReference, event, klass);
            }
            CallbackInfo info = new CallbackInfo(handlerToEvent);
            mCallbackMap.put(klass, info);
            mHasLifecycleMethods.put(klass, hasLifecycleMethods);
            return info;
        }

**CallbackInfo通过反射调用注解方法**

static class CallbackInfo { 
            final Map<Lifecycle.Event, List<MethodReference>> mEventToHandlers;
            final Map<MethodReference, Lifecycle.Event> mHandlerToEvent;
    
            CallbackInfo(Map<MethodReference, Lifecycle.Event> handlerToEvent) { 
                mHandlerToEvent = handlerToEvent;
                mEventToHandlers = new HashMap<>();
                for (Map.Entry<MethodReference, Lifecycle.Event> entry : handlerToEvent.entrySet()) { 
                    Lifecycle.Event event = entry.getValue();
                    List<MethodReference> methodReferences = mEventToHandlers.get(event);
                    if (methodReferences == null) { 
                        methodReferences = new ArrayList<>();
                        mEventToHandlers.put(event, methodReferences);
                    }
                    methodReferences.add(entry.getKey());
                }
            }
    
            @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);
                    }
                }
            }
    
         
        }

--------------------

**上面这些是根据ComponentActivity的实现方式来分析的，Fragment的实现方式与之类似，却别就是没有用ReportFragment来管理事件的分发，而是在对应的生命周期方法中通过LifecycleRegistry直接来分发的。**

## 总结 ##

### 关键类和接口 ###

**1.1 、Lifecycle**  
用于有关组件（如 Activity 或 Fragment）的生命周期状态的信息，并允许其他对象观察此状态。

**1.2、LifecycleOwner ：**  
实现该接口的类持有生命周期(Lifecycle对象)，该接口的生命周期(Lifecycle对象)的改变会被其注册的观察者LifecycleObserver观察到并触发其对应的事件。

**1.3、LifecycleObserver：**  
实现该接口的类，通过注解的方式，可以通过被LifecycleOwner类的addObserver(LifecycleObserver o)方法注册,被注册后，LifecycleObserver便可以观察到LifecycleOwner的生命周期事件。

**1.4、LifecycleRegistry：**  
Lifecycle的具体实现类，注册和反注册观察者，并进行事件的分发。

**1.5、ObserverWithState：**  
持有观察者，并进行事件分发

**1.6、Lifecycling：**  
处理具体的分发事件逻辑。

## 原理概括 ##

Activity/Fragment实现LifecycleOwner接口，通过LifecycleRegistry在对应生命周期分发事件，最后回调观察者中对应订阅方法。