NodeJS请求对象何时发出事件？

Question

It seems I can attach multiple handlers to a request and they will all receive events in the right order/time. When exactly does a request , which has already been received and emitted by the server - when does it emit the data/end events?

function handler1(req, res){
   req.on('data', function(data){{
       console.log("Handler 1 data");
   });
   req.on('end', function(){{
       console.log("Handler 1 end");
   });
}

function handler2(req, res){
   req.on('data', function(data){{
       console.log("Handler 2 data");
   });
   req.on('end', function(){{
       console.log("Handler 2 end");
   });
}

http.on('request', function(req, res){
   handler1(req, res);
   handler2(req, res);
});

// Handler1 data
// Handler2 data
// Handler1 end
// Handler2 end

Answer 1

The request object is an EventEmitter object. As such, it emits events and anyone can add listeners for those events.

In the case of the request object, it emits events that correspond with networking. The data event is emitted when some data has arrived (from the request body). If you have more than one listener for the data event, then each listener will see each data event with the exact same data.

The end event is emitted where there is no more data to from the request body (all data has arrived).

Until you attach some means of reading the data from the incoming request (such as a data event listener), the data is queued and will be held in memory. This gives you a chance to attach the listener even after the data has already started arriving without any worry about missing any of the data. In node.js, many streams work this way. Data doesn't start flowing through the listeners until you attach at least one listener for the data.