Streaming Large Files
Process files chunk by chunk.
Streaming Large Files is a free Dart Academy lesson on CoddyKit — lesson 3 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Dart Academy learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.
Why Not Read It All
readAsString loads the whole file into memory. For a huge log or video that can crash your program, so you stream it instead.
Open a Read Stream
Call openRead on a File to get a stream of byte chunks. You process each chunk and let the old ones go.
final stream = File('big.log').openRead();Walk the Chunks
Use await for to receive each chunk as it arrives. Memory stays flat no matter how large the file grows.
await for (final chunk in stream) { use(chunk); }Bytes Need Decoding
Each chunk is raw bytes. To read text, pipe the stream through utf8.decoder so you get String pieces instead.
stream.transform(utf8.decoder);Split Into Lines
Chain the LineSplitter after the decoder and the stream emits one tidy line at a time, ready to handle.
stream.transform(utf8.decoder).transform(LineSplitter());Count Lines Cheaply
Now counting a million-line file is trivial and light. Increment a counter inside the loop, never holding the whole file.
var n = 0; await for (final l in lines) n++;Open a Write Stream
Writing big output? openWrite returns an IOSink you feed gradually, flushing data to disk as you go.
final sink = File('out.txt').openWrite();Feed the Sink
Send text with sink.write or sink.writeln inside a loop. Each call streams more data without rebuilding a giant String.
for (final row in rows) sink.writeln(row);Always Close the Sink
When done, call close and await it. This flushes the last bytes and frees the file handle cleanly.
await sink.close();Pipe Stream to Sink
To copy a file efficiently, pipe a read stream straight into a write sink. Dart handles backpressure for you.
await source.openRead().pipe(target.openWrite());Handle Stream Errors
Streams can fail mid-flow. Wrap the loop in try/catch so a read error does not silently corrupt your results.
try { await for (final c in stream) {} } catch (e) {}Quick Check
You must process a 5 GB log on a small machine. What is the right approach?
Recap
You now stream reads with openRead, decode and split into lines, write gradually through an IOSink, and pipe for fast copies.
Frequently asked questions
Is the “Streaming Large Files” lesson free?
Yes — the full text of “Streaming Large Files” is free to read here on the web, and the Dart Academy course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Dart Academy course, upgrade to CoddyKit PRO.
What will I learn in “Streaming Large Files”?
Process files chunk by chunk. You practise Dart Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.
Do I need any experience to start Dart Academy?
No prior experience is required. Dart Academy on CoddyKit is structured for beginners through advanced learners; this is — lesson 3 of 4, so you can start here or from the beginning and move at your own pace.
How long does the “Streaming Large Files” lesson take?
Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.
Can I write and run code in this Dart Academy lesson?
Yes. Every Dart Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.