浅谈Lifecycle的原理，以及liveData在Lifecy

前言扯淡

我们知道jetpack出现给android开发带来很大的便利，尤其是Lifecycle带来的方便，作为一个有志向的码农，就应该了解一下Lifecycle的原理

正题

Lifecycle是生命周期感知型组件是可执行操作来响应另一个组件（如 Activity 和 Fragment）的生命周期状态的变化。并将变化通知到已注册的观察者。有助于更好地组织代码，让代码逻辑符合生命周期规范，减少内存泄漏，增强稳定性。
Lifecycle整个就是一个观察者模式，来监听activity或者fragment的生命周期。

Lifecycle的分析

• LifecycleOwner（生命周期持有者）接口，activity和fragment的实现接口
  该接口只返回了一个Lifecycle对象

public interface LifecycleOwner {
    Lifecycle getLifecycle();
}

AppcompatActivity继承了FragmentActivity， FragmentActivity继承了SupportActivity的父类， SupportActivity和 Fragment实现了LifecycleOwner 接口， 所以我们在平常的开发中，使用到AppcompatActivity和Fragment可以使用到LifecycleOwner 。
在SupportActivity和 Fragment， getLifecycle()返回的是Lifecycle的父类LifecycleRegistry.
Tip:这边提到一点supportActivity，中是用Fragment (ReportFragment)的生命周期函数中调用LifecycleRegistry.handleLifecycleEvent()方法改变State。，一个Fragment对应着一个LifecycleRegistry对象。

LifecycleRegistry mLifecycleRegistry = new LifecycleRegistry(this);  // this 就是LifecycleOwner的对象
@Override
public Lifecycle getLifecycle() {
   return mLifecycleRegistry;
}

• Lifecycle抽象类：

public abstract class Lifecycle {
    
    @MainThread
    public abstract void addObserver(@NonNull LifecycleObserver observer);

  
    @MainThread
    public abstract void removeObserver(@NonNull LifecycleObserver observer);

  
    @MainThread
    @NonNull
    public abstract State getCurrentState();

    @SuppressWarnings("WeakerAccess")
    public enum Event {
        /**
         * Constant for onCreate event of the {@link LifecycleOwner}.
         */
        ON_CREATE,
        /**
         * Constant for onStart event of the {@link LifecycleOwner}.
         */
        ON_START,
        /**
         * Constant for onResume event of the {@link LifecycleOwner}.
         */
        ON_RESUME,
        /**
         * Constant for onPause event of the {@link LifecycleOwner}.
         */
        ON_PAUSE,
        /**
         * Constant for onStop event of the {@link LifecycleOwner}.
         */
        ON_STOP,
        /**
         * Constant for onDestroy event of the {@link LifecycleOwner}.
         */
        ON_DESTROY,
        /**
         * An {@link Event Event} constant that can be used to match all events.
         */
        ON_ANY
    }

    @SuppressWarnings("WeakerAccess")
    public enum State {

        DESTROYED,

        INITIALIZED,

        CREATED,

        STARTED,

        RESUMED;

        public boolean isAtLeast(@NonNull State state) {
            return compareTo(state) >= 0;
        }
    }
}

Lifecycle的内部方法说明

方法	说明
addObserver	添加观察者LifecycleObserver，将LifecycleOwner的状态更改时得到通知
removeObserver	从观察员列表中移除给定的观察员
getCurrentState	返回当前的声明周期状态

Event：是lifecycle一共提供了所有的生命周期事件，只要
通过注解进行声明，就能够使LifecycleObserver观察到对应的生命周期事件，activity或者fragment就是通过LifecycleRegistry 的handleLifecycleEvent(Event event)来告知LifecycleRegistry 当前的声明周期事件。

事件	说明
ON_CREATE	对应Activity的onCreate的事件
ON_START	对应Activity的onStart的事件
ON_RESUME	对应Activity的onResume的事件
ON_PAUSE	对应Activity的onPause的事件
ON_STOP	对应Activity的onStop的事件
ON_DESTROY	对应Activity的onDestroy的事件
ON_ANY	可以匹配任何事件

State：状态，是Lifecycle中对应Activity生命周期的一种状态标识

状态	说明
INITIALIZED	对应Activity的onCreate之前的生命周期
DESTROYED	对应Activity的onDestroy
CREATED	对应Activity的onCreate到onStop之间的生命周期
STARTED	对应Activity的onStart到onPause之间的生命周期
RESUMED	对应Activity的onResume

• LifecycleRegistry实现类：
  LifecycleRegistry实现了addObserver方法，添加观察者LifecycleObserver,

public class LifecycleRegistry extends Lifecycle {

    private static final String LOG_TAG = "LifecycleRegistry";

    private FastSafeIterableMap<LifecycleObserver, ObserverWithState> mObserverMap =
            new FastSafeIterableMap<>();

    /** Current state*/
    private State mState;
   
    private final WeakReference<LifecycleOwner> mLifecycleOwner;

    private int mAddingObserverCounter = 0;

    private boolean mHandlingEvent = false;
    private boolean mNewEventOccurred = false;

    private ArrayList<State> mParentStates = new ArrayList<>();

    public LifecycleRegistry(@NonNull LifecycleOwner provider) {
        mLifecycleOwner = new WeakReference<>(provider);
        mState = INITIALIZED;
    }

    @SuppressWarnings("WeakerAccess")
    @MainThread
    public void markState(@NonNull State state) {
        moveToState(state);
    }

    public void handleLifecycleEvent(@NonNull Lifecycle.Event event) {
        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;
    }

    private boolean isSynced() {
        if (mObserverMap.size() == 0) {
            return true;
        }
        State eldestObserverState = mObserverMap.eldest().getValue().mState;
        State newestObserverState = mObserverMap.newest().getValue().mState;
        return eldestObserverState == newestObserverState && mState == newestObserverState;
    }

    private State calculateTargetState(LifecycleObserver observer) {
        Entry<LifecycleObserver, ObserverWithState> previous = mObserverMap.ceil(observer);

        State siblingState = previous != null ? previous.getValue().mState : null;
        State parentState = !mParentStates.isEmpty() ? mParentStates.get(mParentStates.size() - 1)
                : null;
        return min(min(mState, siblingState), parentState);
    }

    @Override
    public void addObserver(@NonNull LifecycleObserver observer) {
        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++;
        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--;
    }

    private void popParentState() {
        mParentStates.remove(mParentStates.size() - 1);
    }

    private void pushParentState(State state) {
        mParentStates.add(state);
    }

    @Override
    public void removeObserver(@NonNull LifecycleObserver observer) {
        mObserverMap.remove(observer);
    }

    @SuppressWarnings("WeakerAccess")
    public int getObserverCount() {
        return mObserverMap.size();
    }

    @NonNull
    @Override
    public State getCurrentState() {
        return mState;
    }

    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);
    }

    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);
    }

    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);
    }

    private void forwardPass(LifecycleOwner lifecycleOwner) {
        Iterator<Entry<LifecycleObserver, ObserverWithState>> ascendingIterator =
                mObserverMap.iteratorWithAdditions();
        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);
                observer.dispatchEvent(lifecycleOwner, upEvent(observer.mState));
                popParentState();
            }
        }
    }

    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));
                observer.dispatchEvent(lifecycleOwner, event);
                popParentState();
            }
        }
    }

    // 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) {
            Log.w(LOG_TAG, "LifecycleOwner is garbage collected, you shouldn't try dispatch "
                    + "new events from it.");
            return;
        }
        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) {
                backwardPass(lifecycleOwner);
            }
            Entry<LifecycleObserver, ObserverWithState> newest = mObserverMap.newest();
            if (!mNewEventOccurred && newest != null
                    && mState.compareTo(newest.getValue().mState) > 0) {
                forwardPass(lifecycleOwner);
            }
        }
        mNewEventOccurred = false;
    }

    static State min(@NonNull State state1, @Nullable State state2) {
        return state2 != null && state2.compareTo(state1) < 0 ? state2 : state1;
    }

    static class ObserverWithState {
        State mState;
        GenericLifecycleObserver mLifecycleObserver;

        ObserverWithState(LifecycleObserver observer, State initialState) {
            mLifecycleObserver = Lifecycling.getCallback(observer);
            mState = initialState;
        }

        void dispatchEvent(LifecycleOwner owner, Event event) {
            State newState = getStateAfter(event);
            mState = min(mState, newState);
            mLifecycleObserver.onStateChanged(owner, event);
            mState = newState;
        }
    }
}

主要两个内容
1、根据observer和initialState构造ObserverWithState对象statefulObserver，然后将该对象存入mObserverMap，可以简单的把它理解成用来保存观察者的Map。
2、上面我们讲到了SupportActivity也是通过Fragment来向LifecycleRegistry传递声明周期事件，通sync()来更新同步通知mObserverMap 中的所有observer当前的声明周期的事件是处于哪个生命周期阶段。

• Fragment声明周期的对LifecycleRegistry的handleLifecycleEvent(event)调用

public class ReportFragment extends Fragment {
    .........................省略了代码.........................

    @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(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);  
            }
        }
    }
}

• LifecycleObserver接口：
  LifecycleObserver接口（ Lifecycle观察者）：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。

总结 ：整个流程就是Lifecycle（LifecycleRegistry）、LifecycleOwner(Fragment)、LifecycleObserver三者之间的关系：Fragment生命周期--通过handleLifecycleEvent(event)将生命周期通知给 LifecycleRegistry，LifecycleRegistry通知观察者LifecycleObserver当前生命周期。livedata 就是一个很好的例子

• 举个栗子：
  先自定义LifecycleObserver

public interface IObserver extends LifecycleObserver {

    @OnLifecycleEvent(Lifecycle.Event.ON_CREATE)
    void onCreate(@NotNull LifecycleOwner owner);

    @OnLifecycleEvent(Lifecycle.Event.ON_DESTROY)
    void onDestroy(@NotNull LifecycleOwner owner);

    @OnLifecycleEvent(Lifecycle.Event.ON_ANY)
    void onLifecycleChanged(@NotNull LifecycleOwner owner,
                            @NotNull Lifecycle.Event event);
}

在activity中添加往Lifecycle中添加observer观察者对象

public class MainActivity extends AppCompatActivity { //AppCompatActivity 是就是LifecycleOwner
   
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        Log.d("tag", "onCreate" + this.getClass().toString());
        setContentView(R.layout.activity_main);
        MyObserver observer = new MyObserver();
        getLifecycle().addObserver(observer );//添加LifecycleObserver
    }

     class MyObserver implements IObserver {

          @Override
          public void onLifecycleChanged(@NotNull LifecycleOwner owner, @NotNull Lifecycle.Event event) {

          }

          @Override
          public void onCreate(@NotNull LifecycleOwner owner) {
              Log.d("MyObserver ", "onCreate" + this.getClass().toString());
          }

        @Override
        public void onDestroy(@NotNull LifecycleOwner owner) {
            Log.d("MyObserver ", "onDestroy" + this.getClass().toString());
        }
    }

    @Override
    protected void onDestroy() {
        Log.d("tag", "onDestroy" + this.getClass().toString());
        super.onDestroy();
    }
}

上面的demo很简单，就是在AppcompatActivity中通过getLifecycle()添加一个观察一个观察者，然后activity的生命周期变动后，Lifecycle通知观察者。

LiveData的Observe方法

public void observe(@NonNull LifecycleOwner owner, @NonNull Observer<T> observer) {
        if (owner.getLifecycle().getCurrentState() == DESTROYED) {
            // ignore
            return;
        }
        LifecycleBoundObserver wrapper = new LifecycleBoundObserver(owner, observer);
        ObserverWrapper existing = mObservers.putIfAbsent(observer, wrapper);
        if (existing != null && !existing.isAttachedTo(owner)) {
            throw new IllegalArgumentException("Cannot add the same observer"
                    + " with different lifecycles");
        }
        if (existing != null) {
            return;
        }
        //核心代码添加观察者
        owner.getLifecycle().addObserver(wrapper);
    }

• LifecycleBoundObserver

class LifecycleBoundObserver extends ObserverWrapper implements GenericLifecycleObserver {
        @NonNull final LifecycleOwner mOwner;

        LifecycleBoundObserver(@NonNull LifecycleOwner owner, Observer<T> observer) {
            super(observer);
            mOwner = owner;
        }

        @Override
        boolean shouldBeActive() {
            return mOwner.getLifecycle().getCurrentState().isAtLeast(STARTED);
        }

        @Override
        public void onStateChanged(LifecycleOwner source, Lifecycle.Event event) {
            if (mOwner.getLifecycle().getCurrentState() == DESTROYED) {
                removeObserver(mObserver);
                return;
            }
            activeStateChanged(shouldBeActive());
        }

        @Override
        boolean isAttachedTo(LifecycleOwner owner) {
            return mOwner == owner;
        }

        @Override
        void detachObserver() {
            mOwner.getLifecycle().removeObserver(this);
        }
    }

GenericLifecycleObserver其实就是 LifecycleObserver 的子类

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

也就是说liveData通过observe的方法传进来LifecycleOwner参数, 创建一个LifecycleBoundObserver 就是一个观察者，被添加到Lifecycle中，进行观察activity或者fragment的生命周期。其实LiveData其他添加观察者到Lifecycle的管理者。

参考& 感谢

https://blog.csdn.net/u013651026/article/details/80822441
https://blog.csdn.net/zhuzp_blog/article/details/78871374
https://www.jianshu.com/p/2c9bcbf092bc