blobreader.ts

class BlobReader {
  public static toBytes = (ab: ArrayBuffer) => new Uint8Array(ab);
  public static EOF = new Error('EOF');
  public static LOCK = new Error('LOCK');

  private _remain!: Blob;

  private _offset = 0;
  private _end = false;

  private _mutex = false;

  public constructor(public blob: Blob) {
    this._check();
  }

  public async read(buffer: Uint8Array) {
    if (this._mutex) throw BlobReader.LOCK;

    this._check();

    if (this._end) throw BlobReader.EOF;

    if (buffer instanceof ArrayBuffer) buffer = BlobReader.toBytes(buffer);

    const chunk = this._remain.slice(0, buffer.length);

    this._lock();

    try {
      buffer.set(BlobReader.toBytes(await chunk.arrayBuffer()));
    } catch (e) {
      this._unlock();
      throw e;
    }

    this._unlock();

    this._offset += chunk.size;
  }

  private _check() {
    this._remain = this.blob.slice(this._offset);
    if (!this._remain.size) this._end = true;
  }

  private _lock() {
    this._mutex = true;
  }

  private _unlock() {
    this._mutex = false;
  }
}