feat: Isolate MJPEG helpers in the driver

lib/driver.ts

@@ -11,10 +11,10 @@ import type {
 import {DEFAULT_ADB_PORT, type ADB} from 'appium-adb';
 import {AndroidDriver, utils} from 'appium-android-driver';
 import {BaseDriver, DeviceSettings} from 'appium/driver.js';
-import {mjpeg, util} from 'appium/support.js';
+import {util} from 'appium/support.js';
 import UIAUTOMATOR2_CONSTRAINTS, {type Uiautomator2Constraints} from './constraints.js';
 import {newMethodMap} from './method-map.js';
-import {assignDefaults, memoize} from './utils/index.js';
+import {assignDefaults, memoize, MJpegStream} from './utils/index.js';
 import type {
   Uiautomator2Settings,
   Uiautomator2DeviceDetails,
@@ -233,7 +233,7 @@ class AndroidUiautomator2Driver
 
   _originalIme: string | null;
 
-  mjpegStream?: mjpeg.MJpegStream;
+  mjpegStream?: MJpegStream;
 
   override caps: Uiautomator2DriverCaps;
 
@@ -457,7 +457,7 @@ class AndroidUiautomator2Driver
 
       if (this.opts.mjpegScreenshotUrl) {
         this.log.info(`Starting MJPEG stream reading URL: '${this.opts.mjpegScreenshotUrl}'`);
-        this.mjpegStream = new mjpeg.MJpegStream(this.opts.mjpegScreenshotUrl);
+        this.mjpegStream = new MJpegStream(this.opts.mjpegScreenshotUrl);
         await this.mjpegStream.start();
       }
       return [sessionId, result];

lib/utils/index.ts

@@ -1,4 +1,5 @@
-export * from './app.js';
-export * from './lang.js';
-export * from './memoize.js';
-export * from './object.js';
+export {signApp} from './app.js';
+export {escapeRegExp, isEmpty} from './lang.js';
+export {memoize} from './memoize.js';
+export {MJpegStream} from './mjpeg.js';
+export {assignDefaults} from './object.js';

lib/utils/mjpeg.ts

@@ -0,0 +1,303 @@
+import {
+  Transform,
+  Writable,
+  type Readable,
+  type TransformCallback,
+  type WritableOptions,
+} from 'node:stream';
+
+import {logger} from 'appium/support.js';
+import axios from 'axios';
+import type sharp from 'sharp';
+
+const log = logger.getLogger('MJPEG');
+
+const DEFAULT_SERVER_TIMEOUT_MS = 10000;
+const JPEG_SOI = Buffer.from([0xff, 0xd8]);
+const JPEG_EOI = Buffer.from([0xff, 0xd9]);
+const CONTENT_LENGTH_RE = /Content-Length:\s*(\d+)/i;
+
+/**
+ * Extracts individual JPEG frames out of a multipart MJPEG-over-HTTP byte stream by
+ * scanning for JPEG start/end-of-image markers and the multipart `Content-Length` header.
+ */
+export class MjpegFrameParser extends Transform {
+  private buffer: Buffer | null = null;
+  private expectedLength = 0;
+  private bytesWritten = 0;
+  private isReading = false;
+
+  /* eslint-disable promise/prefer-await-to-callbacks -- Transform._transform is callback-based */
+  override _transform(chunk: Buffer, _encoding: BufferEncoding, callback: TransformCallback): void {
+    const startIdx = chunk.indexOf(JPEG_SOI);
+    const endIdx = chunk.indexOf(JPEG_EOI);
+    const lengthMatch = CONTENT_LENGTH_RE.exec(chunk.toString('latin1'));
+
+    if (this.buffer && (this.isReading || startIdx > -1)) {
+      this.appendChunk(chunk, endIdx);
+    }
+    if (lengthMatch) {
+      this.startFrame(Number(lengthMatch[1]), chunk, startIdx, endIdx);
+    }
+    callback();
+  }
+  /* eslint-enable promise/prefer-await-to-callbacks */
+
+  private startFrame(length: number, chunk: Buffer, start: number, end: number): void {
+    this.expectedLength = length;
+    this.buffer = Buffer.alloc(length);
+    this.bytesWritten = 0;
+    this.isReading = false;
+
+    if (start < 0) {
+      return;
+    }
+    const hasEnd = end > start;
+    const copyEnd = hasEnd ? end + JPEG_EOI.length : chunk.length;
+    // Buffer.copy() silently truncates if the destination has less room than requested,
+    // so bytesWritten must track what was actually copied, not the requested range size.
+    this.bytesWritten = chunk.copy(this.buffer, 0, start, copyEnd);
+
+    if (hasEnd) {
+      this.emitFrame();
+    } else {
+      this.isReading = true;
+    }
+  }
+
+  private appendChunk(chunk: Buffer, end: number): void {
+    if (!this.buffer) {
+      return;
+    }
+    // We are continuing a frame that is already open, so the whole chunk belongs to it
+    // and must be copied starting at offset 0. A JPEG SOI marker found anywhere in this
+    // chunk can only belong to the *next* frame (it necessarily comes after our own EOI),
+    // so it must never be used as the copy start here, or the current frame's tail bytes
+    // between offset 0 and that marker would be skipped, truncating it.
+    const copyEnd = end > -1 ? end + JPEG_EOI.length : chunk.length;
+    // Buffer.copy() silently truncates if the destination has less room than requested,
+    // so bytesWritten must track what was actually copied, not the requested range size.
+    this.bytesWritten += chunk.copy(this.buffer, this.bytesWritten, 0, copyEnd);
+
+    if (end > -1 || this.bytesWritten === this.expectedLength) {
+      this.emitFrame();
+    } else {
+      this.isReading = true;
+    }
+  }
+
+  private emitFrame(): void {
+    this.isReading = false;
+    if (this.buffer) {
+      // Only push what was actually copied: the buffer is allocated to `expectedLength`
+      // up front, so pushing it as-is would leak trailing zero bytes whenever fewer
+      // bytes were actually written (e.g. a mismatched/truncated Content-Length).
+      this.push(this.buffer.subarray(0, this.bytesWritten));
+    }
+    // Clear the frame state so an unrelated later chunk (e.g. one with a JPEG SOI
+    // marker but no Content-Length header) cannot be appended onto a buffer that
+    // has already been pushed downstream.
+    this.buffer = null;
+    this.expectedLength = 0;
+    this.bytesWritten = 0;
+  }
+}
+
+let sharpModule: typeof sharp | null = null;
+
+async function requireSharp(): Promise<typeof sharp> {
+  if (sharpModule) {
+    return sharpModule;
+  }
+  try {
+    sharpModule = (await import('sharp')).default;
+    return sharpModule;
+  } catch (err) {
+    const message = err instanceof Error ? err.message : String(err);
+    throw new Error(
+      `Cannot load the 'sharp' module needed for MJPEG frame processing. ` +
+        `Consider visiting https://sharp.pixelplumbing.com/install for troubleshooting. ` +
+        `Original error: ${message}`,
+      {cause: err},
+    );
+  }
+}
+
+const noop = () => {};
+
+/**
+ * Connects to an MJPEG-over-HTTP stream and keeps track of the last JPEG frame received,
+ * so that it can be used as a cheap, low-latency screenshot source.
+ */
+export class MJpegStream extends Writable {
+  readonly errorHandler: (err: Error) => void;
+  readonly url: string;
+  private updateCount = 0;
+  private lastChunk: Buffer | null = null;
+  private registerStartSuccess: (() => void) | null = null;
+  private registerStartFailure: ((err: Error) => void) | null = null;
+  private responseStream: Readable | null = null;
+  private consumer: MjpegFrameParser | null = null;
+
+  /**
+   * @param mJpegUrl - URL of the MJPEG-over-HTTP stream
+   * @param errorHandler - additional function that will be called in the case of any errors
+   * @param options - Options to pass to the Writable constructor
+   */
+  constructor(
+    mJpegUrl: string,
+    errorHandler: (err: Error) => void = noop,
+    options: WritableOptions = {},
+  ) {
+    super(options);
+    this.errorHandler = errorHandler;
+    this.url = mJpegUrl;
+    this.clear();
+  }
+
+  get lastChunkBase64(): string | null {
+    const lastChunk = this.lastChunk;
+    return lastChunk && lastChunk.length > 0 ? lastChunk.toString('base64') : null;
+  }
+
+  async lastChunkPNG(): Promise<Buffer | null> {
+    const chunk = this.lastChunk;
+    if (!chunk || chunk.length === 0) {
+      return null;
+    }
+    try {
+      const sharp = await requireSharp();
+      return await sharp(chunk).png().toBuffer();
+    } catch (err: any) {
+      log.warn(`Cannot convert MJPEG chunk to PNG: ${err.message}`);
+      return null;
+    }
+  }
+
+  async lastChunkPNGBase64(): Promise<string | null> {
+    const png = await this.lastChunkPNG();
+    return png ? png.toString('base64') : null;
+  }
+
+  clear(): void {
+    this.registerStartSuccess = null;
+    this.registerStartFailure = null;
+    this.responseStream = null;
+    this.consumer = null;
+    this.lastChunk = null;
+    this.updateCount = 0;
+  }
+
+  async start(serverTimeout = DEFAULT_SERVER_TIMEOUT_MS): Promise<void> {
+    this.stop();
+
+    this.consumer = new MjpegFrameParser();
+    const url = this.url;
+    // Bound only the connect phase with an abort signal; axios's own `timeout` option would
+    // otherwise keep ticking for the whole request lifetime and race with the "first frame"
+    // watchdog below, since both would share the same deadline.
+    const connectController = new AbortController();
+    const connectTimeoutId = setTimeout(() => connectController.abort(), serverTimeout);
+    try {
+      try {
+        this.responseStream = (
+          await axios({
+            url,
+            responseType: 'stream',
+            signal: connectController.signal,
+          })
+        ).data as Readable;
+      } catch (e) {
+        let message: string;
+        if (e && typeof e === 'object' && 'response' in e) {
+          message = JSON.stringify((e as {response: unknown}).response);
+        } else if (e instanceof Error) {
+          message = e.message;
+        } else {
+          message = String(e);
+        }
+        throw new Error(
+          `Cannot connect to the MJPEG stream at ${url}. Original error: ${message}`,
+          {
+            cause: e,
+          },
+        );
+      }
+    } finally {
+      clearTimeout(connectTimeoutId);
+    }
+
+    const onErr = (err: Error) => {
+      this.lastChunk = null;
+      log.error(`Error getting MJPEG screenshot chunk: ${err.message}`);
+      this.errorHandler(err);
+      this.registerStartFailure?.(err);
+    };
+    const onClose = () => {
+      log.debug(`The connection to MJPEG server at ${url} has been closed`);
+      this.lastChunk = null;
+      // No-op if start() has already resolved; only rejects a still-pending start().
+      this.registerStartFailure?.(
+        new Error(
+          `The connection to the MJPEG stream at ${url} has been closed before any frame was received`,
+        ),
+      );
+    };
+
+    let timeoutId: NodeJS.Timeout | undefined;
+    const startPromise = new Promise<void>((resolve, reject) => {
+      this.registerStartSuccess = resolve;
+      this.registerStartFailure = reject;
+      timeoutId = setTimeout(
+        () =>
+          reject(new Error(`Waited ${serverTimeout}ms but the MJPEG server never sent any images`)),
+        serverTimeout,
+      );
+    });
+
+    (this.responseStream as Readable & {pipe<T extends Writable>(dest: T): T})
+      .once('close', onClose)
+      .on('error', onErr)
+      .pipe(this.consumer)
+      .pipe(this);
+
+    try {
+      await startPromise;
+    } catch (err) {
+      // Do not leak the underlying HTTP connection/pipes if we never reached a usable state.
+      this.stop();
+      throw err;
+    } finally {
+      clearTimeout(timeoutId);
+    }
+  }
+
+  stop(): void {
+    if (this.consumer) {
+      this.consumer.unpipe(this);
+    }
+    if (this.responseStream) {
+      if (this.consumer) {
+        this.responseStream.unpipe(this.consumer);
+      }
+      this.responseStream.destroy();
+    }
+    this.clear();
+  }
+
+  /* eslint-disable promise/prefer-await-to-callbacks -- Writable._write is callback-based */
+  override _write(
+    chunk: Buffer | string,
+    _encoding: BufferEncoding,
+    callback: (error?: Error | null) => void,
+  ): void {
+    this.lastChunk = Buffer.isBuffer(chunk) ? chunk : Buffer.from(chunk);
+    this.updateCount++;
+    if (this.registerStartSuccess) {
+      this.registerStartSuccess();
+      this.registerStartSuccess = null;
+    }
+    callback();
+  }
+  /* eslint-enable promise/prefer-await-to-callbacks */
+}

package.json

@@ -100,6 +100,9 @@
   "peerDependencies": {
     "appium": "^3.0.0-rc.2"
   },
+  "optionalDependencies": {
+    "sharp": "^0.x"
+  },
   "engines": {
     "node": "^20.19.0 || ^22.12.0 || >=24.0.0",
     "npm": ">=10"