rxjs中do和subscribe之间的区别

Question

在这个JS Bin中，我有一些可加载模拟的分页数据源的observable。 getPageFromServer从给定索引开始从服务器加载一页数据：

function getPageFromServer( index ) {

  index = index || 0;

  var values = [];
  var nextIndex;

  for(var i = 0; i< 3; i++) {
    var newValue = index + i;
    if(newValue < 10){
      values.push( newValue );
      nextIndex = newValue + 1;
    }
    else {
      nextIndex = undefined;
    }
  }

  data = { values: values, nextIndex: nextIndex };

  return Rx.Observable.return(data).delay(500);
}

然后getPagedItems加载页面并将多个observable连接成一个，这样我们就可以获得所有页面中所有值的可观察值：

function getPagedItems(index) {
  return getPageFromServer(index)
    .flatMap(function (response) {
      var result = Rx.Observable.from(response.values);

      if(response.nextIndex !== undefined){
        return result.concat( getPagedItems(response.nextIndex) );
      } else {
        return result;
      }

  });
}

这给了我预期的结果：

"page received: 1"
"page received: 2"
"page received: 3"
"page received: 4"
"page received: 5"
"page received: 6"
"page received: 7"
"page received: 8"
"page received: 9"
"complete"

但是，如果我在getPagedItems的末尾添加一个do：

function getPagedItems(index) {
  return getPageFromServer(index)
    .flatMap(function (response) {
      var result = Rx.Observable.from(response.values);

      if(response.nextIndex !== undefined){
        return result.concat( getPagedItems(response.nextIndex) );
      } else {
        return result;
      }
  })
  .do( function ( result) { console.log( "do: " + result); } );
}

我得到每个值的次数等于我们在flatmap中创建的可观察数量：

"starting..."
"do: 0"
"page received: 0"
"do: 1"
"page received: 1"
"do: 2"
"page received: 2"
"do: 3"
"do: 3"
"page received: 3"
"do: 4"
"do: 4"
"page received: 4"
"do: 5"
"do: 5"
"page received: 5"
"do: 6"
"do: 6"
"do: 6"
"page received: 6"
"do: 7"
"do: 7"
"do: 7"
"page received: 7"
"do: 8"
"do: 8"
"do: 8"
"page received: 8"
"do: 9"
"do: 9"
"do: 9"
"do: 9"
"page received: 9"
"complete"

请向我解释为什么“do”多次触发，但订阅中的“onNext”只触发一次。

非常感谢

Answer 1

在我发布问题后约5分钟，我想出了这个答案！

do多次触发的原因是因为我以递归方式调用loadCommentThreads，因此每个对loadComment线程的嵌套调用在消息返回到父调用函数实例之前都在管道中有do。