Attempt to use web bluetooth from chrome mobile to stream cadence rather then dedicated hardware

static/js/BLE.js

@@ -0,0 +1,68 @@
+async function connect(props) {
+  const device = await navigator.bluetooth.requestDevice({
+    filters: [{ services: ["cycling_speed_and_cadence"] }],
+    acceptAllDevices: false,
+  });
+  console.log(`%c\n👩🏼‍⚕️`, "font-size: 82px;", "Starting CSC...\n\n");
+  const server = await device.gatt.connect();
+  const service = await server.getPrimaryService("cycling_speed_and_cadence");
+  const char = await service.getCharacteristic("csc_measurement");
+  char.oncharacteristicvaluechanged = props.onChange;
+  char.startNotifications();
+  return char;
+}
+
+function parseCSC({ value }) {
+  const flags = value.getUint8(0, true);
+  const hasWheel = !!(flags & 0x01);
+  const hasCrank = !!(flags & 0x02);
+  let index = 1;
+  const res = {
+    wheelRevs: null,
+    wheelTime: null,
+    crankRevs: null,
+    crankTime: null,
+  };
+  if (hasWheel) {
+    res.wheelRevs = value.getUint32(index, true);
+    index += 4;
+    res.wheelTime = value.getUint16(index, true);
+    index += 2;
+  }
+  if (hasCrank) {
+    res.crankRevs = value.getUint16(index, true);
+    index += 2;
+    res.crankTime = value.getUint16(index, true);
+    index += 2;
+  }
+  console.log("CSC", res);
+  const pre = document.createElement("pre");
+  pre.style.border = "1px solid red";
+  pre.textContent = JSON.stringify(res, null, 2);
+  document.body.appendChild(pre);
+  return res;
+}
+
+function revsToRPM(t1, t2) {
+  const deltaRevs = t2.revs - t1.revs;
+  if (deltaRevs === 0) {
+    // no rotation
+    return 0;
+  }
+
+  let deltaTime = (t2.time - t1.time) / 1024;
+  if (deltaTime < 0) {
+    // time counter wraparound
+    deltaTime += Math.pow(2, 16) / 1024;
+  }
+  deltaTime /= 60; // seconds to minutes
+
+  const rpm = deltaRevs / deltaTime;
+  return rpm;
+}
+
+document.querySelector("#ble-connect").addEventListener("click", () =>
+  connect({
+    onChange: parseCSC,
+  }).catch(console.error)
+);

templates/base.html

@@ -51,8 +51,18 @@
                     </div>
                 </div>
             </div>
+            <button
+                class="p-0 w-16 h-16 bg-red-600 rounded-full hover:bg-red-700 active:shadow-lg mouse shadow transition ease-in duration-200 focus:outline-none"
+                id="ble-connect">
+                <svg viewBox="0 0 20 20" enable-background="new 0 0 20 20" class="w-6 h-6 inline-block">
+                    <path fill="#FFFFFF" d="M16,10c0,0.553-0.048,1-0.601,1H11v4.399C11,15.951,10.553,16,10,16c-0.553,0-1-0.049-1-0.601V11H4.601
+                                            C4.049,11,4,10.553,4,10c0-0.553,0.049-1,0.601-1H9V4.601C9,4.048,9.447,4,10,4c0.553,0,1,0.048,1,0.601V9h4.399
+                                            C15.952,9,16,9.447,16,10z" />
+                </svg>
+            </button>
         </main>
     </div>
+    <script src="/static/js/BLE.js"></script>
 </body>
 
 </html>