preemptive-animation-frame/lib/preemptive-animation-frame.js

Preemptive requestAnimationFrame

'use strict';

Object.defineProperty(exports, "__esModule", {
  value: true
});
exports.forceAnimationFrame = exports.cancelAnimationFrame = exports.requestAnimationFrame = exports.now = undefined;

var _requestPolyfilledAnimationFrame = require('request-polyfilled-animation-frame');

var RAF = _interopRequireWildcard(_requestPolyfilledAnimationFrame);

function _interopRequireWildcard(obj) { if (obj && obj.__esModule) { return obj; } else { var newObj = {}; if (obj != null) { for (var key in obj) { if (Object.prototype.hasOwnProperty.call(obj, key)) newObj[key] = obj[key]; } } newObj.default = obj; return newObj; } }

// Invariants:
// 1. In the NO_REQUEST state, there is never a scheduled animation frame.
// 2. In the PENDING_REQUEST and EXTRA_REQUEST states, there is always exactly
// one scheduled animation frame.
var NO_REQUEST = 0;

var PENDING_REQUEST = 1;
var EXTRA_REQUEST = 2;

var requestID = null;
var nextID = 0;
var state = NO_REQUEST;
var requests = [];
var ids = [];

var absent = new Error('absent');

var now = exports.now = RAF.now;

var requestAnimationFrame = exports.requestAnimationFrame = function requestAnimationFrame(request) {
  if (state === NO_REQUEST) {
    requestID = RAF.requestAnimationFrame(performAnimationFrame);
  }

  var id = ++nextID;
  requests.push(request);
  ids.push(id);
  state = PENDING_REQUEST;
  return id;
};

var cancelAnimationFrame = exports.cancelAnimationFrame = function cancelAnimationFrame(id) {
  var index = ids.indexOf(id);
  if (index >= 0) {
    ids.splice(index, 1);
    requests.splice(index, 1);
  }
};

var forceAnimationFrame = exports.forceAnimationFrame = function forceAnimationFrame() {
  var time = arguments.length > 0 && arguments[0] !== undefined ? arguments[0] : now();

  switch (state) {
    case NO_REQUEST:
      break;
    default:
      if (requestID != null) {
        RAF.cancelAnimationFrame(requestID);
        requestID = null;
      }
      performAnimationFrame(time);
  }
};

var performAnimationFrame = function performAnimationFrame(time) {
  switch (state) {
    case NO_REQUEST:
      // This state should not be possible. How can there be no
      // request, yet somehow we are actively fulfilling a
      // request?
      throw Error('Unexpected frame request');
    case PENDING_REQUEST:
      // At this point, we do not *know* that another frame is
      // needed, but we make an extra frame request just in
      // case. It's possible to drop a frame if frame is requested
      // too late, so we just do it preemptively.
      requestID = RAF.requestAnimationFrame(performAnimationFrame);
      state = EXTRA_REQUEST;
      ids.splice(0);
      dispatchAnimationFrame(requests.splice(0), 0, time);
      break;
    case EXTRA_REQUEST:
      // Turns out the extra request was not needed, so we will
      // stop requesting. No reason to call it all the time if
      // no one needs it.
      state = NO_REQUEST;
      break;
  }
};

var dispatchAnimationFrame = function dispatchAnimationFrame(requests, index, time) {
  var exception = absent;
  var count = requests.length;
  try {
    while (index < count) {
      var request = requests[index];
      index = index + 1;
      request(time);
    }
  } catch (error) {
    exception = error;
  }

  if (index < count) {
    dispatchAnimationFrame(requests, index, time);
  }

  if (exception !== absent) {
    throw exception;
  }
};
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