套接字io内部的回调

Question

I am having a problem with socket when the executing data takes a long time. For example:

socket.on('start', function(input){
    var output = foo(input); //taking a very long time to get data, networking etc.
    console.log("this is ending", output);
    socket.emit('end',output);
});

It seems that if it takes a long time for foo(input) to execute, nodejs would actually emit output first while it is still null.

How can I make sure to finish executing first before emit?

I modified like this

    socket.on('several',function(meterIDArray){
    console.log("I have receiever meterIDArray",meterIDArray);
    meterIDs = meterIDArray.meterIDs;
    foo(meterIDs,function(Datasets){
        console.log("This is datasets",Datasets);
        socket.emit('Datasets',Datasets);
    });

});

And I modified my foo like this

function foo(meterIDs,callback){
//this is what I meant by a long call. 
var Datasets = [];
    for (var i = meterIDs.length - 1; i >= 0; i--) {
        datapoint.setMeterID(meterIDs[i],function(err, results){
            datapoint.doRequest(null, function(err, results){
                var dataAfter = [];
                var step = 1;
                for(var i = 0; i < results.timeseries.length; i = i + step) {
                    var item = results.timeseries[i];
                    for(date in item) {
                        dataAfter.push({x: date, y: item[date]})
                    }
                }
                console.log("This is data After",dataAfter);
                Datasets.push(dataAfter);
            });
        });
    }
callback(Datasets);
}

But it is still the same effect: socket is still emitting null. Sorry if it is a naive bug. Thanks!

Answer 1

If you can modify foo all you need is this:

socket.on('start', function(input){
  foo(input, function (output) {
    console.log("this is ending", output)
    socket.emit('end', output)
  ))
})

You need to fire the callback when your server request returns.

UPDATE: Can't use the for loop i to keep track of how many requests have been made, have to keep track once the request returns.

function foo(meterIDs, callback) {
  var Datasets = [];
  var numberOfRequestsToMake = meterIDs.length;
  for (var i = meterIDs.length - 1; i >= 0; i--) {
    datapoint.setMeterID(meterIDs[i],function(err, results){
      datapoint.doRequest(null, function(err, results){
        var dataAfter = [];
        var step = 1;
        for(var i = 0; i < results.timeseries.length; i = i + step) {
          var item = results.timeseries[i];
          for(var date in item) {
            dataAfter.push({x: date, y: item[date]})
          }
        }
        console.log("This is data After",dataAfter);
        Datasets.push(dataAfter);

        numberOfRequestsToMake--;

        // if this is the final return from the server
        if (numberOfRequestsToMake === 0) {
          callback(Datasets)
        }
      });
    });
  }
}

Promises are awesome though

Answer 2

You can redefine your foo function by adding a callback to guaranty that the output will be emited after executing what foo is doing.

For example

    socket.on('start', function(input){
        foo(input, function(output){
            // This code will be executed after foo performs the main job
            console.log("this is ending", output);
            socket.emit('end',output);
        }); //taking a very long time to get data, networking etc.
    });

Learn more about callbacks