Take a look at the documentation and code postValue:

    /** * Posts a task to a main thread to set the given value. So if you have a following code * executed in the main thread: * <pre class="prettyprint"> * liveData.postValue("a"); * liveData.setValue("b"); * </pre> * The value "b" would be set at first and later the main thread would override it with * the value "a". * <p> *  If you called this method multiple times before a main thread executed a posted task, only * the last value would be dispatched. * *@param value The new value
     */
    protected void postValue(T value) {
        boolean postTask;
        synchronized (mDataLock) {
            postTask = mPendingData == NOT_SET;
            mPendingData = value;
        }
        if(! postTask) {return;
        }
        ArchTaskExecutor.getInstance().postToMainThread(mPostValueRunnable);
    }
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setValue:

    /**
     * Sets the value. If there are active observers, the value will be dispatched to them.
     * <p>
     * This method must be called from the main thread. If you need set a value from a background
     * thread, you can use {@link #postValue(Object)}
     *
     * @param value The new value
     */
    @MainThread
    protected void setValue(T value) {
        assertMainThread("setValue");
        mVersion++;
        mData = value;
        dispatchingValue(null);
    }
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So setValue() must be called on the main thread and postValue() can be called on the worker thread but ！！！！！ If multiple calls postValue, may only update the data in the last post code says it all, ArchTaskExecutor. GetInstance () postToMainThrea cut back to the main thread before setValue mPendingData can be modified