diff --git a/src/reader.ts b/src/reader.ts
index 0f859f7..c9bd2a3 100644
--- a/src/reader.ts
+++ b/src/reader.ts
@@ -54,6 +54,7 @@ export class Reader {
 			}
 		}
 
+		this.source.onread?.({ start, end });
 		const bytesPromise = this.source._read(start, end);
 		const loadingSegment: LoadingSegment = { start, end, promise: bytesPromise };
 		this.loadingSegments.push(loadingSegment);
diff --git a/src/source.ts b/src/source.ts
index dde1ab0..9357b76 100644
--- a/src/source.ts
+++ b/src/source.ts
@@ -12,6 +12,8 @@ export abstract class Source {
 	_getSize() {
 		return this._sizePromise ??= this._retrieveSize();
 	}
+
+	onread: ((range: { start: number; end: number }) => unknown) | null = null;
 }
 
 /** @public */
diff --git a/todo.txt b/todo.txt
index 1100ef6..ddfc2ea 100644
--- a/todo.txt
+++ b/todo.txt
@@ -3,6 +3,5 @@
 - Add the new audio codecs to muxers
 - Mov muxer!!! Only mov can hold PCM audio, MP4 cannot
 - A stream source?? Or like a callback-driven source
-- onRead callback for Source
 - onHeader, etc callbacks for Matroska
 - https://github.com/Vanilagy/mp4-muxer/issues/83 tell him it's possible now
\ No newline at end of file