General help with node / promises

thex

Hi guys,
could anyone help me with better understanding promises?

I guess I understood the general concept but I really struggle applying it in some places.

Could you reference any good tutorials?

Use case that currently makes my brain hurt:

I want to send a packet via UDP. Getting a promise that is fulfilled when this is sent is no problem but I actually want this to only be fulfilled if n UDP answer has been received and is matching the request.

Hope someone can help.

thex

leader21

Guess @mwittig or @sweetpi can surely help or maybe another @Developer

also have a look here
http://forum.pimatic.org/topic/332/plugin-for-wifi-controlled-led-light-iwy-light/77

and
https://github.com/philip1986/pimatic-led-light

don’t know how far the development is actually regarding Philips hue

thex

Finally I think I got it sorted, could someone give some feedback please?

var Promise = require ('bluebird');

var EventEmitter = require("events").EventEmitter;
 
var ee = new EventEmitter();

ee.on("someEvent", function (id) {
    console.log("event " + id + " has occured");
});
 
setTimeout(function emitEvent(){
  ee.emit("someEvent", 1);
  },
  1000
);

setTimeout(function emitEvent(){
  ee.emit("someEvent", 2);
  },
  2000
);

setTimeout(function emitEvent(){
  ee.emit("someEvent", 3);
  },
  3000
);

function waitForEvent(id){
  var def = Promise.pending();
  ee.once('someEvent', function (eventId){
    if(id == eventId){
      //the right event has happened
      console.log('right event');
      def.resolve();
    }
    else{
      //not the right event, keep waiting
      console.log('not the right event');
      def.resolve(waitForEvent(id));
    }
  });
  return def.promise;
};

var waitingPromise = waitForEvent(2).timeout(5000, 'timed out').then(
  function(){
    console.log('promise fulfilled because of right event');
  },
  function(err){
    console.log('promise failed:' + err);
  }
);

console.log('waiting...');

mwittig

@thex Regarding Promises I really like the following article http://javascriptplayground.com/blog/2015/02/promises/ Besides this, the Bluebird project provides an API reference and various useful examples - https://github.com/petkaantonov/bluebird

Regarding you code post I am happy to have a look at it later today.

mwittig

@thex I have reviewed your code and have a few changes to suggest (see revised code below).

For error cases, i.e. ‘not the right event’, you should reject the promise. This way you can handle the failed promise later on (see next point)
To handle failed promises you should use .catch() as shown below. The code will call .catch() if the timeout occurs (raises an TimeoutError) or if ‘not the right event’ Error is raised. For the latter you can define your own Error class.

One side note. Plugins for pimatic shall be written using Coffeescript. This may be a bit of a hurdle in the beginning, but also has lot of benefits once you are familiar with it.

var Promise = require('bluebird');

var EventEmitter = require("events").EventEmitter;

var ee = new EventEmitter();

ee.on("someEvent", function (id) {
    console.log("event " + id + " has occured");
});

setTimeout(function emitEvent() {
        ee.emit("someEvent", 1);
    },
    1000
);

setTimeout(function emitEvent() {
        ee.emit("someEvent", 2);
    },
    2000
);

setTimeout(function emitEvent() {
        ee.emit("someEvent", 3);
    },
    3000
);

function waitForEvent(id) {
    var def = Promise.pending();
    ee.once('someEvent', function (eventId) {
        if (id == eventId) {
            //the right event has happened
            console.log('right event');
            def.resolve();
        }
        else {
            //not the right event, keep waiting
            console.log('not the right event');
            def.reject(new Error('not the right event'));
        }
    });
    return def.promise;
}

var waitingPromise = waitForEvent(2).timeout(5000, 'timed out').then(
    function () {
        console.log('promise fulfilled because of right event');
    }).catch(function (err) {
        console.log('promise failed:' + err);
    });

console.log('waiting...');

EDIT: The code should work as is. I have doe some test runs at least . Regarding the use of the timeout() see also https://github.com/petkaantonov/bluebird/blob/master/API.md#timeoutint-ms–string-message—promise

EDIT2: I didn’t pay enough attention to your first as you wrote you want the Promise to be only be “fulfilled if an UDP answer has been received and is matching the request”. If you want handle multiple send/receive activities at the same time it does not make sense to reject the Promise if the wrong event has been received as replies may be in a different order than sent requests. For this case simply remove the “def.reject” code line for the code above. This way the Promise will be rejected only if the expected reply to a given request has not been received with the given timeout period.