Limit progress callbacks (#30)

Set it to a reasonable value by default that gives somewhat smooth
progress bars.

lib/usb.ts

@@ -204,6 +204,7 @@ export class MicrobitWebUSBConnection
     options: {
       partial: boolean;
       progress: (percentage: number | undefined) => void;
+      miniumProgressIncrement: number;
     },
   ): Promise<void> {
     this.flashing = true;
@@ -232,6 +233,7 @@ export class MicrobitWebUSBConnection
     options: {
       partial: boolean;
       progress: (percentage: number | undefined, partial: boolean) => void;
+      miniumProgressIncrement: number;
     },
   ): Promise<void> {
     this.log("Stopping serial before flash");
@@ -243,7 +245,10 @@ export class MicrobitWebUSBConnection
     }
 
     const partial = options.partial;
-    const progress = options.progress || (() => {});
+    const progress = rateLimitProgress(
+      options.miniumProgressIncrement ?? 0.0025,
+      options.progress || (() => {}),
+    );
 
     const boardId = this.connection.boardSerialInfo.id;
     const boardVersion = boardId.toBoardVersion();
@@ -516,3 +521,21 @@ const enrichedError = (err: any): DeviceError => {
     }
   }
 };
+
+const rateLimitProgress = (
+  miniumProgressIncrement: number,
+  callback: (value: number | undefined, partial: boolean) => void,
+) => {
+  let lastCallValue = -1;
+  return (value: number | undefined, partial: boolean) => {
+    if (
+      value === undefined ||
+      value === 0 ||
+      value === 1 ||
+      value >= lastCallValue + miniumProgressIncrement
+    ) {
+      lastCallValue = value ?? -1;
+      callback(value, partial);
+    }
+  };
+};

src/demo.ts

@@ -209,12 +209,14 @@ const createFlashSection = (): Section => {
           if (file) {
             const text = await file.text();
             if (connection.flash) {
+              console.time("flash");
               await connection.flash(createUniversalHexFlashDataSource(text), {
                 partial: true,
                 progress: (percentage: number | undefined) => {
                   console.log(percentage);
                 },
               });
+              console.timeEnd("flash");
             }
           }
         },