在开发小程序展开全文组件时需要用到节点查询API - wx.createSelectorQuery() 来查询全文内容的高度。

• wx.createSelectorQuery() 返回一个 SelectorQuery 对象实例。
• SelectorQuery 有五个方法（in，select，selectAll，selectViewport，exec），第一个返回 SelectorQuery，后四个返回 NodesRef。
• NodesRef 有四个方法（fields，boundingClientRect，scrollOffset，context），第一个返回 NodesRef，后三个返回 SelectorQuery。

对照官方提供的示例代码来看

const query = wx.createSelectorQuery() // query 是 SelectorQuery 对象
query.select('#the-id').boundingClientRect() // select 后是 NodesRef 对象，然后 boundingClientRect 返回 SelectorQuery 对象
query.selectViewport().scrollOffset() // selectViewport 后是 NodesRef 对象，然后 scrollOffset 返回 SelectorQuery 对象
query.exec(function (res) { // exec 返回 NodesRef 对象
  res[0].top // #the-id节点的上边界坐标
  res[1].scrollTop // 显示区域的竖直滚动位置
})

问题：每行执行返回的 SelectorQuery 对象是相同的吗？
答案：是的，都是同一个对象。

问题：直接执行 query.select('#the-id').boundingClientRect().exec 也可以吗？
答案：可以，boundingClientRect() 返回就是 query。

问题：这样连写 query.select('#the-id').boundingClientRect().selectViewport().scrollOffset() 算两条查询请求吗？
答案：是两条请求。

问题：query.exec 执行后会清空前面的查询请求吗？再次执行还能拿到结果吗？
答案：可以，query 不会清空请求。

问题：boundingClientRect 和 scrollOffset 可以接受 callback 参数，它与 query.exec 执行顺序是怎样，修改 res 结果会影响到后面的 callback 吗？
答案：先执行 boundingClientRect 和 scrollOffset 的 callback，再执行 query.exec 的 callback；修改 res 结果会影响到后面 exec 的结果。

上面的问题通过小程序开发者工具中的 WAService.js 源码简单美化还原后可以了解 SelectorQuery 的代码逻辑

SelectorQuery.js

import NodesRef from 'NodesRef';
import requestComponentInfo from 'requestComponentInfo';

export default function(pluginId) {
  return class SelectorQuery {
    constructor(plugin) {
      if (plugin && plugin.page) {
        this._component = this._defaultComponent = plugin.page;
        this._webviewId = this._defaultComponent.__wxWebviewId__;
      } else {
        var pages = __internalGlobal__.getCurrentPagesByDomain('');
        this._defaultComponent = pages[pages.length - 1],
        this._component = null;
        this._webviewId = null;
      }
      this._queue = [];
      this._queueCb = [];
    }
    in(component) {
      if (!this._webviewId) {
        this._webviewId = component.__wxWebviewId__;
        this._component = component;
      } else if (this._webviewId !== component.__wxWebviewId__) {
        console.error('A single SelectorQuery could not work in components in different pages. A SelectorQuery#in call has been ignored.');
      } else {
        this._component = component;
      }
      return this;
    }
    select(selector) {
      return new NodesRef(this, this._component, selector, true);
    }
    selectAll(selector) {
      return new NodesRef(this, this._component, selector, false);
    }
    selectViewport() {
      return new NodesRef(this, 0, '', true);
    }
    _push(selector, component, single, fields, callback) {
      if (!this._webviewId) {
        this._webviewId = this._defaultComponent ? this._defaultComponent.__wxWebviewId__ : undefined;
      }
      const rootNodeId = pluginId ? '' : r.getRootNodeId(this._webviewId);
      this._queue.push({
        component: null != component ? (0 === component ? 0 : component.__wxExparserNodeId__) : rootNodeId,
        selector,
        single,
        fields,
      });
      this._queueCb.push(callback || null);
    }
    exec(callback) {
      requestComponentInfo(this._webviewId, {
        pluginId,
        queue: this._queue,
      }, (results) => {
        const queueCb = this._queueCb;
        results.forEach((res, index) => {
          if ('function' == typeof queueCb[index]) {
            queueCb[index].call(this, res);
          }
        });
        if ('function' == typeof callback) {
          callback.call(this, results);
        }
      })
    }
  }
}

requestComponentInfo.js

const subscribe = function(eventType, callback) {
  const _callback = function(event, webviewId, nativeInfo = {}) {
    const { data = {}, options } = event;
    const startTime = options && options.timestamp || 0;
    const endTime = Date.now();
    if ('function' == typeof callback) {
      callback(data, webviewId);
      Reporter.speedReport({
        key: 'webview2AppService',
        data,
        timeMark: {
          startTime,
          endTime,
          nativeTime: nativeInfo.nativeTime || 0,
        }
      });
    }
  };
  __safeway__.bridge.subscribe(eventType, _callback);
}

const publish = function(eventType, data, webviewIds) {
  const event = {
    data,
    options: {
      timestamp: Date.now(),
    }
  };
  __safeway__.bridge.publish(eventType, event, webviewIds);
}

const requestCb = {};
let requestId = 1;
subscribe('responseComponentInfo', function(data) {
  const reqId = data.reqId;
  const callback = requestCb[reqId];
  if (callback) {
    delete requestCb[reqId];
    callback(data.res);
  }
});
export default function requestComponentInfo(webviewId, reqs, callback) {
  const reqId = requestId++;
  if (!webviewId) {
    console.warn('An SelectorQuery call is ignored because no proper page or component is found. Please considering using `SelectorQuery.in` to specify a proper one.');
    return;
  }
  requestCb[reqId] = callback,
  publish('requestComponentInfo',
    {
      reqId,
      reqs,
    }, 
    [webviewId],
  );
}

NodesRef.js

export default class NodesRef {
  constructor(selectorQuery, component, selector, single) {
    this._selectorQuery = selectorQuery;
    this._component = component;
    this._selector = selector;
    this._single = single;
  }
  fields(fields, callback) {
    this._selectorQuery._push(this._selector, this._component, this._single,
      fields,
      callback,
    );
    return this._selectorQuery;
  }
  boundingClientRect(callback) {
    this._selectorQuery._push( this._selector, this._component, this._single,
      {
        id: true,
        dataset: true,
        rect: true,
        size: true,
      },
      callback,
    );
    return this._selectorQuery;
  }
  scrollOffset(callback) {
    this._selectorQuery._push( this._selector, this._component, this._single,
      {
        id: true,
        dataset: true,
        scrollOffset: true,
      },
      callback,
    );
    return this._selectorQuery;
  }
}