Async raspberry pi gpio events in nodejs

Question

Async raspberry pi gpio events in nodejs

I connected a button to the Raspberry Pi GPIO ports:

The pi-gpio node js module allows only to pull the current status but not waiting for a state change:

gpio.read(16, function(err, value) {
    if(err) throw err;
    console.log(value);    // The current state of the pin
});

Is there any solution to bind events asynchronously?

In this example this would be whenever the button is pressed.

node.js
raspberry-pi

Answer 1

unfortunately there is no event generated when using the hardware GPIO. You can create an event by using an emitter and setInterval to create a timer that checks the status every so often and then emits an event out to your application.

var ee = new process.EventEmitter(),
    buttonState;

ee.on('stateChange', function(previousValue, value){
  console.log('button state changed from', previousValue, 'to', value);
});

setInterval(function(){
  gpio.read(16, function(err, value) {
    if(err){
      ee.emit('error', err);
    } else{
      if(buttonState !== value){
        var previousState = buttonState;
        buttonState = value;
        ee.emit('stateChange', previousState, value);
      }
    }        
  });
}, 50); //check button state every 50ms

You can tune the timer to your requirements. I think 50ms is a good number as it is not possible for the human eye to detect the difference if it is faster.

Answer 2

I'd suggest using the module called onoff.

It has the ability to watch a value-file while not taxing the cpu with polling in intervals.

I made a small test-app with node.js to try out leds and buttons with it, you can check it out here.

Answer 3

I just came accross this modulle

https://github.com/EnotionZ/GpiO

I have not looked in depth to it but is seems it uses the setInterval method in the background.

Answer 4

As an aside, there is a project very similar to node.js (however much more cut-down) called Espruino.

This runs on Raspberry Pi (and many low power ARMs microcontrollers too), supports HTTP, and has a function called setWatch which behaves in exactly the way you require.