fix(worker-prop): cant instantiate arrow functions

When the function passed via the `worker` prop is an arrow function
than we can't construct instances of it with the `new` keyword.
The problem probably went unnoticed because in many implementations
arrow functions are transpiled to regular functions.

Issue: #197

src/components/QrcodeStream.vue

@@ -29,7 +29,7 @@ import { keepScanning } from "../misc/scanner.js";
 import { thinSquare } from "../misc/track-func.js";
 import Camera from "../misc/camera.js";
 import CommonAPI from "../mixins/CommonAPI.vue";
-import Worker from "../worker/jsqr.js";
+import spawnWorker from "../worker/jsqr.js";
 
 export default {
   name: "qrcode-stream",
@@ -58,7 +58,7 @@ export default {
 
     worker: {
       type: Function,
-      default: Worker
+      default: spawnWorker
     }
   },
 

src/misc/scanner.js

@@ -1,7 +1,7 @@
 import { eventOn } from "callforth";
 
-export async function scan(Worker, imageData) {
-  const worker = new Worker();
+export const scan = async (spawnWorker, imageData) => {
+  const worker = spawnWorker();
 
   worker.postMessage(imageData, [imageData.data.buffer]);
 
@@ -16,14 +16,14 @@ export async function scan(Worker, imageData) {
  * Continuously extracts frames from camera stream and tries to read
  * potentially pictured QR codes.
  */
-export function keepScanning(Worker, camera, options) {
+export const keepScanning = (spawnWorker, camera, options) => {
   const { detectHandler, locateHandler, minDelay } = options;
 
   let contentBefore = null;
   let locationBefore = null;
   let lastScanned = performance.now();
 
-  const worker = new Worker();
+  const worker = spawnWorker();
 
   // If worker can't process frames fast enough, memory will quickly full up.
   // Make sure to process only one frame at a time.