asyncExpand

Learn stream chaining with asyncExpand.

asyncExpand

asyncExpand(fn) turns each element into a new stream and concatenates them in order. The next stream is not started before the previous one completes.

Unlike expand, which synchronously maps each element to an Iterable, asyncExpand maps each element to a Stream and waits asynchronously.

Usage: make an API call per element and process results in order. Example: ids.asyncExpand((id) => fetchUserStream(id)).

asyncExpand example

Stream.fromIterable([1, 2]).asyncExpand((x) => Stream.fromIterable([x, x * 10])) yields: 1, 10, 2, 20. Each inner stream is fully consumed before moving to the next element.

This is ideal for sequential async chains. For example, take page numbers from a stream and load an API page for each.

If you want parallelism, use a Future list with Future.wait; asyncExpand is sequential.

Exercise Duplicate elements with asyncExpand

Make main async. Add var s = Stream.fromIterable([1, 2]);. Add await s.asyncExpand((x) => Stream.fromIterable([x, x * 10])).forEach(print);.

Show example solution

void main() async {
  var s = Stream.fromIterable([1, 2]);
  await s.asyncExpand((x) => Stream.fromIterable([x, x * 10])).forEach(print);
}

Test yourself

asyncExpand call

var t = s.____((x) => Stream.fromIterable([x, x]));

Show answer

var t = s.asyncExpand((x) => Stream.fromIterable([x, x]));

Match the methods

asyncExpand

Element → Stream
expand

Element → Iterable
asyncMap

Element → Future
map

Element → value
How does asyncExpand behave?
- Concatenates inner streams in order
- Takes only the first element
- Processes all inner streams in parallel
- Ignores errors
Show answer

Concatenates inner streams in order — asyncExpand concatenates inner streams in order.

Next lesson: Summary - Advanced streams

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