Exploring collections in Dart

John Ryan
John Ryan
Sep 15 · 7 min read
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If you have ever called add(), addAll(), map(), or toList() to build a list or map, you might want to check out collection if, collection for, and spreads. Last year, Dart added these features in version 2.3.

In this article, we’ll look at collections, explore these new features, and see some interesting examples. By mastering these features, you can make your code more succinct and easier to read.

Collections

First, we need to understand what a collection is. A collection is an object that contains other objects. For example:

  • List: an ordered collection of objects with a length (also called an array)

These types are available in the dart:collection package. For even more collection types, check out package:collection on pub.dev.

Each of these collection types implements Iterable, which provides common behavior like running a function on each object in the collection, getting the first object, determining the length of the collection, and more.

Collection literals

Dart supports syntax for constructing three types of collections: list literals ([]), map literals ({}) and set literals (also {}).

Here’s a list literal:

List<String> getArtists() {
return [
'Picasso',
'Warhol',
'Monet',
];
}

Here’s a map literal:

Map<String, String> getArtistsByPainting {
return {
'The Old Guitarist': 'Picasso',
'Orange Prince': 'Warhol',
'The Water Lily Pond': 'Monet',
};
}

And here’s a set literal, added in Dart 2.3:

Set<String> getArtistsSet() {
return {
'Picasso',
'Warhol',
'Monet',
};
}

If you’re wondering why maps and sets can use the same {} syntax, it’s because Dart uses type inference to differentiate. The type system determines the type based on the type of parameters a and b. It can usually determine this based on the contents—for example, {1} is obviously a Set, and {1: 2} is obviously a Map.

Note: Using {} constructs a map by default. To create a set, you can use a generic type annotation: <String>{}. Using two generic type parameters creates a map: <String, String>{}.

Types of elements

Each item in a collection literal is usually a value or an expression, but can also be one of these new features: a collection if, collection for, or spread. All of these are called elements.

Each element unpacks zero or more items and puts them in the surrounding collection. For example, a string literal (say, "oatmeal") results in one item, but a collection for unpacks 0 or more items. These features can also be combined in interesting ways, as we’ll explore below.

Spreads

A spread takes a collection (for example, a list) and puts its contents into the surrounding collection:

List<String> combineLists(List<String> a, List<String> b) {
return [
...a,
...b,
];
}

The preceding code is equivalent to this:

List<String> combineLists(List<String> a, List<String> b) {
var list = [];
list.addAll(a);
list.addAll(b);
return list;
}

You can also use spreads in map and set literals:

Map<String, String> combineMaps(Map<String, String> a, Map<String, String> b) {
return {
...a,
...b,
};
}
Set<String> combineSets(Set<String> a, Set<String> b) {
return {
...a,
...b,
};
}

In both maps and sets, the contents of b overwrite the contents in a when there’s a conflict. For example, calling combineMaps({'foo': 'bar'}, {'foo': 'baz'}) results in a map containing {'foo': 'baz'}.

Null-aware spreads (…?)

A null-aware spread adds the contents to the collection only if the expression after the operator is non-null:

List<String> combineIfExists(List<String> a, List<String> b) {
return [
...?a,
...?b,
];
}
void main() {
var result = combineIfExists(['foo'], null);
print(result); // [foo]
}

Collection if

Use the if, else, and else if keywords to add something to a collection based on a condition. Here’s an example of using collection if:

class Article {
String title;
DateTime date;
Article(this.title, this.date); String toString() {
return [
if (title != null) title,
date.toString(),
].join(', ');
}
}

The else keyword can be added on the end:

String toString() {
return [
if (title != null) title else '(no title)',
].join(', ');
}

Notice where the comma is. The comma can’t be after title because the else is part of the same element. Keeping the if and else together before the comma is what keeps them separate from the next element in the collection.

Adding an else if works too:

String toString() {
return [
if (title != null) title else '(no title)',
if (date == null)
'(no date)'
else if (date.year == DateTime.now().year)
'this year'
else
'${date.year}',
].join(', ');
}

Collection for

Lastly, use the for keyword to insert a sequence into the collection:

class Article {
String title;
DateTime date;
List<String> tags;
Article(this.title, this.date, this.tags); String toString() {
return [
title,
date.toString(),
for (var tag in tags) 'tag: $tag'
].join(', ');
}
}

In this example, the for expression adds a string for each item in the tags list. Just like a normal for loop in Dart, the tags expression can be any Iterable.

Collections in Flutter code

If you’re using Dart, there’s a good chance you’re using it to build Flutter apps. Since the features described here were designed with Flutter in mind, let’s take a look at some Flutter code.

Refactoring a build() method

In Flutter, it’s common to build up a list of widgets in a build() method:

@override
Widget build(BuildContext context) {
var articleWidgets = articles
.map<Widget>((article) => ArticleWidget(article: article))
.toList();
return ListView(
children: articleWidgets,
);
}

This could be rewritten with a spread:

Widget build(BuildContext context) {  return ListView(
children: [
...articles.map((article) => ArticleWidget(article:article))
],
);
}

Or with a collection for:

Widget build(BuildContext context) {
return ListView(
children: [
for (var article in articles)
ArticleWidget(article: article)
],
);
}

The first snippet converts the Article class into a collection of ArticleWidget objects using map(), and then applies the spread operator to expand them into the surrounding list. In the second example, the collection for operator lets you express this a little more succinctly.

A larger build() method

Here’s a more complex example:

Widget build(BuildContext context) {
var headerStyle = Theme.of(context).textTheme.headline4;
return Column(
children: [
if (article.title != null)
Text(article.title, style: headerStyle),
if (article.date != null)
Text(article.date.toString()),
Text('Tags:'),
for (var tag in article.tags)
Text(tag),
],
);
}

The logic to place widgets in the Column is right where a reader might expect, and saves a lot of code. Before these features, the most common way to achieve the same behavior was to create a variable and use normal if statements that call add().

Combining these features

These features can be combined in interesting ways, as the examples in this section show. Here are a few things to keep in mind:

  • Syntactically a collection if, collection for, or spread is a single element — even if it ends up creating multiple objects.

Using if and for together

Here’s some code that creates a list using a collection if inside of a collection for. Here, each article is added to the list if it’s after a certain date:

List<Article> recentArticles(List<Article> allArticles) {
var ninetyDaysAgo = DateTime.now().subtract(Duration(days: 90));
return [
for (var article in allArticles)
if (article.date.isAfter(ninetyDaysAgo))
article
];
}

If you prefer spreads, the return value could instead be written as ...allArticles.where((article) => article.date.isAfter(ninetyDaysAgo)).

Using collection if and spreads together

A collection if takes a single element, but if you want to include more than one you can use a spread:

Widget build(BuildContext context) {
return Column(
children: [
if (article.date != null) ...[
Icon(Icons.calendar_today),
Text('${article.date}'),
],
],
);
}

Using collection features with async-await

You can also combine asynchronous calls with collection literals. For example, a common pattern is to use Future.wait() to trigger a group of asynchronous calls:

Future<Article> fetchArticle(String id);Future<List<Article>> fetchArticles() async {
return Future.wait([
fetchArticle('1'),
fetchArticle('2'),
fetchArticle('3'),
]);
}

That code can be improved using a collection for:

Future<List<Article>> fetchArticles(List<String> ids) async {
return Future.wait([
for (var id in ids)
fetchArticle(id),
]);
}

It’s also possible to put an await in a collection literal, although it will wait for each future in sequence:

Future<List<Article>> fetchArticles(List<String> ids) async {
return [
// fetches one at a time
for (var id in ids)
await fetchArticle(id),
];
}

The preceding code waits in sequence because it’s equivalent to the following:

Future<List<Article>> fetchArticles() async {
return <Article>[
// fetches one at a time
await fetchArticle('1'),
await fetchArticle('2'),
await fetchArticle('3'),
];
}

You can also expand a Stream using await for:

Stream<String> get idStream => Stream.fromIterable(['1','2','3']);
Future<List<String>> gatherIds(Stream<String> ids) async {
return [
await for (var id in ids)
id
];
}
void main() async {
print(await gatherIds(idStream)); // [1, 2, 3]
}

This is another example of how collection if, collection for, and spreads work with other parts of the language. If you’ve used an await for statement, you might be able to guess the behavior: it listens to the stream for new values and puts the body into the surrounding list.

Exploring further

Hopefully these tips help you write cleaner Dart code. There are even more ways to use these features than are mentioned here. If you find a nice technique, share it with the community or mention @dart_lang on Twitter. For more details, check out Making Dart a Better Language for UI or the initial language proposal on GitHub.

Dart

Dart is a client-optimized language for fast apps on any platform.

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