class JokesDetailViewModel : ViewModel() {
    //創(chuàng)建 LiveData
    private val _state by lazy { MutableLiveData<JokesUIState>() }
    val state : LiveData<JokesUIState> = _state
    private fun loadChildComment(page: Int, commentId: Int, parentPos: Int, curPos: Int) {
        viewModelScope.launch {
            launchHttp {
                jokesApi.jokesCommentListItem(commentId, page)//請求數(shù)據(jù)
            }.toData {
                //通知觀察者
                _state.value = JokesUIState.LoadMoreChildComment(it.data, parentPos, curPos)
            }
        }
     }
}    
//觀察 LiveData
viewModel.state.observe(this, Observer {
    //更新 UI 
})

    @MainThread
    public void observe(@NonNull LifecycleOwner owner, @NonNull Observer<? super T> observer) {
        //如果已經(jīng)銷毀，直接退出
        if (owner.getLifecycle().getCurrentState() == DESTROYED) {
            return;
        }
        //包裝類（下面源碼）
        LifecycleBoundObserver wrapper = new LifecycleBoundObserver(owner, observer);
        //判斷是否已添加
        ObserverWrapper existing = mObservers.putIfAbsent(observer, wrapper);
        if (existing != null && !existing.isAttachedTo(owner)) {
           //....
        }
        //之前添加過
        if (existing != null) {
            return;
        }
        //注冊 lifecycle，生命周期改變時會回調(diào)
        owner.getLifecycle().addObserver(wrapper);
    }

class LifecycleBoundObserver 
                extends ObserverWrapper implements LifecycleEventObserver {
    @NonNull
    final LifecycleOwner mOwner;
    LifecycleBoundObserver(@NonNull LifecycleOwner owner, Observer<? super T> observer) {
        super(observer);
        mOwner = owner;
    }
    //當前狀態(tài)大于或者等于 STARTED 返回 true
    @Override
    boolean shouldBeActive() {
        return mOwner.getLifecycle().getCurrentState().isAtLeast(STARTED);
    }
    //生命周期相關回調(diào)
    @Override
    public void onStateChanged(@NonNull LifecycleOwner source,
            @NonNull Lifecycle.Event event) {
        Lifecycle.State currentState = mOwner.getLifecycle().getCurrentState();
        //如果已經(jīng)銷毀，移除觀察者 
        if (currentState == DESTROYED) {
            removeObserver(mObserver);
            return;
        }
        Lifecycle.State prevState = null;
       //循環(huán)更新狀態(tài)
        while (prevState != currentState) {
            prevState = currentState;
            //修改活躍狀態(tài)
            activeStateChanged(shouldBeActive());
            currentState = mOwner.getLifecycle().getCurrentState();
        }
    }
    @Override
    boolean isAttachedTo(LifecycleOwner owner) {
        return mOwner == owner;
    }
    @Override
    void detachObserver() {
        mOwner.getLifecycle().removeObserver(this);
    }
}

void activeStateChanged(boolean newActive) {
  //如果等于之前狀態(tài)
  if (newActive == mActive) {
    return;
  }
  mActive = newActive;
  //活躍 +1，不活躍 -1 （下面有源碼）
  changeActiveCounter(mActive ? 1 : -1);
  //如果狀態(tài)變成了活躍狀態(tài)，直接調(diào)用 dispatchingValue，傳入當前的觀察者
  if (mActive) {
    dispatchingValue(this); //（下面有源碼）
  }
}
//修改活躍數(shù)量
void changeActiveCounter(int change) {
    int previousActiveCount = mActiveCount;//之前活躍的數(shù)量
    mActiveCount += change;//總活躍數(shù)量
    if (mChangingActiveState) {//如果正在更改，退出
        return;
    }
    mChangingActiveState = true;//更改中
    try {
        while (previousActiveCount != mActiveCount) {
            boolean needToCallActive = previousActiveCount == 0 && mActiveCount > 0;
            boolean needToCallInactive = previousActiveCount > 0 && mActiveCount == 0;
            previousActiveCount = mActiveCount;
            //如果當前是第一個激活的，調(diào)用 onActive
            if (needToCallActive) {
                onActive();//當活動的觀察者從0 變成 1的時候調(diào)用
            //如果沒有激活的為 0 ，調(diào)用 onInactive  
            } else if (needToCallInactive) {
                onInactive();//活躍的觀察者變成 0 時調(diào)用
            }
        }
    } finally {
        mChangingActiveState = false;
    }
}
//分發(fā)數(shù)據(jù)
void dispatchingValue(@Nullable ObserverWrapper initiator) {
  if (mDispatchingValue) {
        mDispatchInvalidated = true;
        return;  //如果正在分發(fā)，退出  
    }
    mDispatchingValue = true;
    do {
        mDispatchInvalidated = false;
        //如果觀察者不為空 
        if (initiator != null) {
            considerNotify(initiator);//對數(shù)據(jù)進行派發(fā)，通知觀察者
            initiator = null;
        } else {
          //如果為空，遍歷所有的觀察者，將數(shù)據(jù)發(fā)送給所有觀察者
            for (Iterator<Map.Entry<Observer<? super T>, ObserverWrapper>> iterator =
                    mObservers.iteratorWithAdditions(); iterator.hasNext(); ) {
                considerNotify(iterator.next().getValue());
                if (mDispatchInvalidated) {
                    break;
                }
            }
        }
    } while (mDispatchInvalidated);
    mDispatchingValue = false;
}
//數(shù)據(jù)進行派發(fā)，通知觀察者
private void considerNotify(ObserverWrapper observer) {
    if (!observer.mActive) {
        return;
    }
    //再次進行判斷，如果不活躍，則會更新狀態(tài)，然后退出
    if (!observer.shouldBeActive()) {
        observer.activeStateChanged(false);
        return;
    }
    //觀察者版本是否低于當前的版本
    //初始值 mLastVersion 為 -1，mVersion 為 0
    //小于等于表示沒有需要更新的數(shù)據(jù)
    if (observer.mLastVersion >= mVersion) {
        return;
    }
    //更新版本
    observer.mLastVersion = mVersion;
    //通知觀察者
    observer.mObserver.onChanged((T) mData);
}

class StockLiveData(url: String) : LiveData<String>() {
    private val downloadManager = DownloadManager(url)
    override fun onActive() {
      if(!downloadManager.isStart()){
          downloadManager.start()
      }
    }
    override fun onInactive() {
         downloadManager.stop()
    }
}

public class MutableLiveData<T> extends LiveData<T> {
    /**
     * Creates a MutableLiveData initialized with the given {@code value}.
     */
    public MutableLiveData(T value) {
        super(value);
    }
    /**
     * Creates a MutableLiveData with no value assigned to it.
     */
    public MutableLiveData() {
        super();
    }
    @Override
    public void postValue(T value) {
        super.postValue(value);
    }
    @Override
    public void setValue(T value) {
        super.setValue(value);
    }
}

val strLiveData = MutableLiveData<String>()
 val strLengthLiveData = Transformations.map(strLiveData) {
    it.length
 }
 strLiveData.observe(this) {
   Log.e("---345---> str：", "$it");
 }
 strLengthLiveData.observe(this) {
  Log.e("---345---> strLength：", "$it");
 }
strLiveData.value = "hello word"
E/---345---> str：: hello word
E/---345---> strLength：: 10

val idLiveData = MutableLiveData<Int>()
val userLiveData = Transformations.switchMap(idLiveData) { id->
      getUser(id)
}

val live1 = MutableLiveData<String>()
val live2 = MutableLiveData<String>()
val mediator = MediatorLiveData<String>()
mediator.addSource(live1) {
  mediator.value  = it
  Log.e("---345---> live1", "$it");
}
mediator.addSource(live2){
  mediator.value  = it
  Log.e("---345---> live2", "$it");
}
mediator.observe(this, Observer {
  Log.e("---345---> mediator", "$it");
})
live1.value = "hello"
E/---345---> mediator: hello
E/---345---> live1: hello