Touchscreen class for the PiTFT screen for Raspberry Pi

touch.js

var fs = require('fs');
var Struct = require('ref-struct');
var inherits = require('util').inherits;
var EventEmitter = require('events').EventEmitter;

// https://www.kernel.org/doc/Documentation/input/event-codes.txt
// https://www.kernel.org/doc/Documentation/input/input.txt
// /usr/include/linux/input.h

/*
 * Event types
 */

var EV_SYN = 0x00;
var EV_KEY = 0x01;
var EV_ABS = 0x03;

/*
 * Absolute axes
 */

var ABS_X = 0x00;
var ABS_Y = 0x01;
var ABS_PRESSURE = 0x18;

/*
 * Keys and buttons
 */

var BTN_TOUCH = 0x14a;



// struct input_event {
//   struct timeval time;
//   unsigned short type;
//   unsigned short code;
//   unsigned int value;
// };
var time_t = 'long';
var suseconds_t = 'long';
var timeval = Struct({
  tv_sec: time_t,
  tv_usec: suseconds_t
})
var input_event = Struct({
  time: timeval,
  type: 'ushort',
  code: 'ushort',
  value: 'uint'
});

// Expose as a nice JavaScript API
function Touchscreen (filename) {
  this.onOpen = this.onOpen.bind(this);
  this.onRead = this.onRead.bind(this);
  this.input = new input_event;
  this.buffer = this.input.ref();
  this.filename = filename || '/dev/input/touchscreen';
  this.touch = {
    identifier: 1,
    timeStamp: 0,
    type: 'touchstart',
    screenX: 0,
    screenY: 0,
    force: 0,
  };
  fs.open(this.filename, 'r', this.onOpen);
}
inherits(Touchscreen, EventEmitter);

Touchscreen.prototype.onOpen = function (err, fd) {
  if (err) return this.emit('error', err);
  this.fd = fd;
  this.emit('open');
  this.startRead();
};

Touchscreen.prototype.startRead = function () {
  fs.read(this.fd, this.buffer, 0, input_event.size, null, this.onRead);
};

Touchscreen.prototype.onRead = function (err, bytesRead) {
  if (err) return this.emit('error', err);
  if (bytesRead !== input_event.size) {
    return this.emit('error', new Error('only read ' +
       bytesRead + ' bytes, expected ' + input_event.size));
  }
  switch (this.input.type) {
    case EV_SYN:
      this.touch.timeStamp = (this.input.time.tv_sec * 1000) + ((this.input.time.tv_usec / 1000) | 0);
      this.emit(this.touch.type, this.touch);
      if (this.touch.type === 'touchstart') {
        this.touch.type = 'touchmove';
      }
      break;

    case EV_KEY:
      switch (this.input.code) {
        case BTN_TOUCH:
          if (this.input.value) {
            // finger down
            this.touch.type = 'touchstart';
          } else {
            // finger up
            this.touch.type = 'touchend';
          }
          break;
        default:
          return this.emit('error', new Error('bad EV_KEY code! ' + this.input.code));
      }
      break;

    case EV_ABS:
      switch (this.input.code) {
        // X and Y axes are rotated 90 degrees on PiTFT, so
        // ABS_X is really the Y-axis and ABS_Y is the X-axis
        // (when the PiTFT is rotated horizontally)
        case ABS_X:
          this.touch.screenY = (this.input.value / 4095) * 240;
          break;
        case ABS_Y:
          // X axis origin is to the right for some reason, so we have to
          // swap the value to begin on the left side by taking 320 - value
          this.touch.screenX = 320 - ((this.input.value / 4095) * 320);
          break;
        case ABS_PRESSURE:
          this.touch.force = this.input.value / 255;
          break;
        default:
          return this.emit('error', new Error('bad EV_ABS code! ' + this.input.code));
      }
      break;

    default:
      console.error(this.input);
      break;
  }
  if (this.fd) this.startRead();
};

Touchscreen.prototype.close = function (callback) {
  fs.close(this.fd, callback);
  this.fd = undefined;
};

usage.js

// Sample usage
var ts = new Touchscreen();
ts.once('open', function () {
  console.log('Touch me!');
});
ts.on('touchstart', console.log.bind(console, 'touchstart'));
ts.on('touchmove', console.log.bind(console, 'touchmove'));
ts.on('touchend', console.log.bind(console, 'touchend'));