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val mutableLiveData = MutableLiveData<String>()
mutableLiveData.observe(this){
  // ...
}

// 更新值
mutableLiveData.postValue(...)
mutableLiveData.setValue(...)

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   void dispatchingValue(@Nullable ObserverWrapper initiator) {
        if (mDispatchingValue) {
            mDispatchInvalidated = true;
            return;
        }
        mDispatchingValue = true;
        do {
            mDispatchInvalidated = false;
            if (initiator != null) {
                considerNotify(initiator);
                initiator = null;
            } else {
                for (Iterator<Map.Entry<Observer<? super T>, ObserverWrapper>> iterator =
                        mObservers.iteratorWithAdditions(); iterator.hasNext(); ) {
                    considerNotify(iterator.next().getValue());
                    if (mDispatchInvalidated) {
                        break;
                    }
                }
            }
        } while (mDispatchInvalidated);
        mDispatchingValue = false;
    }

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private void considerNotify(ObserverWrapper observer) {
        if (!observer.mActive) {
            return;
        }
        // Check latest state b4 dispatch. Maybe it changed state but we didn't get the event yet.
        //
        // we still first check observer.active to keep it as the entrance for events. So even if
        // the observer moved to an active state, if we've not received that event, we better not
        // notify for a more predictable notification order.
        if (!observer.shouldBeActive()) {
            observer.activeStateChanged(false);
            return;
        }
        if (observer.mLastVersion >= mVersion) {
            return;
        }
        observer.mLastVersion = mVersion;
        observer.mObserver.onChanged((T) mData);
    }

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public class MutableLiveData<T> extends LiveData<T> {

    /**
     * Creates a MutableLiveData initialized with the given {@code value}.
     *
     * @param value initial 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);
    }
}

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/**
 * {@link LiveData} subclass which may observe other {@code LiveData} objects and react on
 * {@code OnChanged} events from them.
 * <p>
 * This class correctly propagates its active/inactive states down to source {@code LiveData}
 * objects.
 * <p>
 * Consider the following scenario: we have 2 instances of {@code LiveData}, let's name them
 * {@code liveData1} and {@code liveData2}, and we want to merge their emissions in one object:
 * {@code liveDataMerger}. Then, {@code liveData1} and {@code liveData2} will become sources for
 * the {@code MediatorLiveData liveDataMerger} and every time {@code onChanged} callback
 * is called for either of them, we set a new value in {@code liveDataMerger}.
 *
 * <pre>
 * LiveData&lt;Integer&gt; liveData1 = ...;
 * LiveData&lt;Integer&gt; liveData2 = ...;
 *
 * MediatorLiveData&lt;Integer&gt; liveDataMerger = new MediatorLiveData&lt;&gt;();
 * liveDataMerger.addSource(liveData1, value -&gt; liveDataMerger.setValue(value));
 * liveDataMerger.addSource(liveData2, value -&gt; liveDataMerger.setValue(value));
 * </pre>
 * <p>
 * Let's consider that we only want 10 values emitted by {@code liveData1}, to be
 * merged in the {@code liveDataMerger}. Then, after 10 values, we can stop listening to {@code
 * liveData1} and remove it as a source.
 * <pre>
 * liveDataMerger.addSource(liveData1, new Observer&lt;Integer&gt;() {
 *      private int count = 1;
 *
 *      {@literal @}Override public void onChanged(@Nullable Integer s) {
 *          count++;
 *          liveDataMerger.setValue(s);
 *          if (count &gt; 10) {
 *              liveDataMerger.removeSource(liveData1);
 *          }
 *      }
 * });
 * </pre>
 *
 * @param <T> The type of data hold by this instance
 */