This project is a port of Kotlin's Kotlin Standard library for Dart/Flutter projects. It's a useful addition to dart:core
and includes collections (KtList
, KtMap
, KtSet
) as well as other packages which can improve every Dart/Flutter app.
dependencies:
kt_dart: ^1.1.0
import 'package:kt_dart/kt.dart';
Dart's dart:core
package provides basic building blocks. But sometimes they are too low level and not as straightforward as Kotlin's kotlin-stdlib
.
Here are a few examples of what this project offers: (click to expand)
Immutable collections by default
Dart's List
is mutable by default. The immutable List.unmodifiable
is the same type, but the mutation methods throw at runtime.
final dartList = [1, 2, 3];
dartList.add(4); // mutation is by default possible
assert(dartList.length == 4);
final immutableDartList = List.unmodifiable(dartList);
immutableDartList.add(5); // throws: Unsupported operation: Cannot add to an unmodifiable list
Dart's mutable List
is indistinguishable from an immutable List
which might cause errors.
void addDevice(List<Widget> widgets, Device device) {
// no way to check whether widgets is mutable or not
// add might or might now throw
widgets.add(_deviceRow());
widgets.add(Divider(height: 1.0));
}
KtList
and KtMutableList
are two different Types. KtList
is immutable by default and has no mutation methods (such as add
). Methods like map((T)->R)
or plusElement(T)
return a new KtList
leaving the old one unmodified.
final ktList = listOf(1, 2, 3);
// The method 'add' isn't defined for the class 'KtList<int>'.
ktList.add(4); // compilation error
^^^
// Adding an item returns a new KtList
final mutatedList = ktList.plusElement(4);
assert(ktList.size == 3);
assert(mutatedList.size == 4);
KtMutableList
offers mutation methods where the content of that collection can be actually mutated.
I.e. with remove(T)
or add(T)
;
// KtMutableList allow mutation
final mutableKtList = mutableListOf(1, 2, 3);
mutableKtList.add(4); // works!
assert(mutableKtList.size == 4);
All collection types have mutable counterparts:
Immutable | Mutable |
---|---|
KtList |
KtMutableList |
KtSet |
KtMutableSet , KtHashSet , KtLinkedSet |
KtMap |
KtMutableMap , KtHashMap , KtLinkedMap |
KtCollection |
KtMutableCollection and all the above |
KtIterable |
KtMutableIterable and all the above |
Deep equals
Dart's List
works like a Array
in Java. Equals
doesn't compare the items; it only checks the identity.
To compare the contents you have to use helper methods methods from 'package:collection/collection.dart'
.
// Comparing two Dart Lists works only by identity
final a = [1, 2, 3, 4];
final b = [1, 2, 3, 4];
print(a == b); // false, huh?
// Content-based comparisons require unnecessary glue code
Function listEq = const ListEquality().equals;
print(listEq(a, b)); // true
// MapEquality isn't deep by default
final x = {1: ["a", "b", "c"], 2: ["xx", "yy", "zz"]};
final y = {1: ["a", "b", "c"], 2: ["xx", "yy", "zz"]};
Function mapEq = const MapEquality().equals;
print(mapEq(x, y)); // false, wtf?!
Function deepEq = const DeepCollectionEquality().equals;
print(deepEq(x, y)); // true, finally
KtList
and all other collection types implement equals
by deeply comparing all items.
final a = listOf(1, 2, 3, 4);
final b = listOf(1, 2, 3, 4);
print(a == b); // true, as expected
final x = mapFrom({1: listOf("a", "b", "c"), 2: listOf("xx", "yy", "zz")});
final y = mapFrom({1: listOf("a", "b", "c"), 2: listOf("xx", "yy", "zz")});
print(x == y); // deep equals by default
Common methods
Some of Dart's method names feel unfamiliar. That's because modern languages and frameworks (Kotlin, Swift, TypeScript, ReactiveExtensions) kind of agreed on naming methods when it comes to collections. This makes it easy to switch platforms and discuss implementations with coworkers working with a different language.
final dList = [[1, 2, 3], [4, 5, 6], [7, 8, 9]];
final kList = listOf(listOf(1, 2, 3), listOf(4, 5, 6), listOf(7, 8, 9));
// dart:core
final dFlat = dList.expand((l) => l).toList();
print(dFlat); // [1, 2, 3, 4, 5, 6, 7, 8, 9]
// kt_dart
final kFlat = kList.flatMap((l) => l);
print(kFlat); // [1, 2, 3, 4, 5, 6, 7, 8, 9]
final dNames = ["Chet", "Tor", "Romain", "Jake", "Dianne"];
final kNames = listFrom(dNames);
// dart:core
final dShortNames = dNames.where((name) => name.length <= 4).toList();
print(dShortNames); // [Chet, Tor, Jake]
// kt_dart
final kShortNames = kNames.filter((name) => name.length <= 4);
print(kShortNames); // [Chet, Tor, Jake]
final dNames = ["Chet", "Tor", "Romain", "Jake", "Dianne"];
final kNames = listFrom(dNames);
// dart:core
dNames.firstWhere((name) => name.contains("k")); // Jake
dNames.firstWhere((name) => name.contains("x"), orElse: () => null); // null
dNames.firstWhere((name) => name.contains("x"), orElse: () => "Nobody"); // Nobody
// kt_dart
kNames.first((name) => name.contains("k")); // Jake
kNames.firstOrNull((name) => name.contains("x")); // null
kNames.firstOrNull((name) => name.contains("x")) ?? "Nobody"; // Nobody
KtList
is a read-only list of elements. It is immutable because it doesn't offer mutation methods such as remove
or add
.
Use KtMutableMap
if you want to use a mutable list.
To create a KtList
/KtMutableList
use the KtList.of
constructor or convert an existing Dart List
to a KtList
with the list.toImmutableList()
extension.
// Create a KtList from scratch
final beatles = KtList.of("John", "Paul", "George", "Ringo");
// Convert a existing List to KtList
final abba = ["Agnetha", "Björn", "Benny", "Anni-Frid"];
final immutableAbba = abba.toImmutableList();
KtList
is immutable by default, which means it doesn't offer methods like add
or remove
.
To create mutable list with kt_dart use the KtMutableList
constructor.
// Create a KtMutableList from scratch
final beatles = KtMutableList.of("John", "Paul", "George", "Ringo");
beatles.removeAt(0);
print(beatles); // [Paul, George, Ringo]
Conversions between KtList
and KtMutableList
can be done with KtList.toMutableList()
and KtMutableList.toList()
;
final beatles = KtList.of("John", "Paul", "George", "Ringo");
final mutable = beatles.toMutableList();
mutable.removeAt(0);
print(mutable); // [Paul, George, Ringo]
print(beatles); // [John, Paul, George, Ringo]
kt_dart collections do not implement Iterable
.
It is therefore not possible to directly iterate over the entries of a KtList
.
All kt_dart collections offer a .iter
property which exposes a Dart Iterable
.
For-loops therefore don't look much different.
final beatles = KtList.of("John", "Paul", "George", "Ringo");
for (final member in beatles.iter) {
print(member);
}
Yes, alternatively you could use .asList()
instead which returns a Dart List
.
Kotlin users might be more familiar with the listOf()
and mutableListOf()
functions.
Use them if you like but keep in mind that the dart community is much more used to use constructors instead of top-level functions.
final beatles = listOf("John", "Paul", "George", "Ringo");
final abba = mutableListOf("Agnetha", "Björn", "Benny", "Anni-Frid");
A KtSet
is a unordered collection of elements without duplicates.
Creating a KtSet
/KtMutableSet
is very similar to the KtList
API.
// Create a KtSet from scratch
final beatles = KtSet.of("John", "Paul", "George", "Ringo");
// Convert a existing Set to KtSet
final abba = {"Agnetha", "Björn", "Benny", "Anni-Frid"};
final immutableAbba = abba.toImmutableSet();
To create a KtMap
/KtMutableMap
start with Dart Map
and then convert it to a KtMap
with either:
pokemon.toImmutableMap(): KtMap
(since Dart 2.7)KtMap.from(pokemon): KtMap
pokemon.kt: KtMutableMap
(since Dart 2.7)KtMutableMap.from(pokemon): KtMutableMap
// immutable
final pokemon = {
1: "Bulbasaur",
2: "Ivysaur",
3: "Stegosaur",
}.toImmutableMap();
final newPokemon = KtMap.from({
152: "Chikorita",
153: "Bayleef",
154: "Meganium",
});
// mutable
final mutablePokemon = {
1: "Bulbasaur",
2: "Ivysaur",
3: "Stegosaur",
}.kt;
final newMutablePokemon = KtMutableMap.from({
152: "Chikorita",
153: "Bayleef",
154: "Meganium",
});
You may want to use a specific Map
implementation. kt_dart offers:
KtLinkedMap
- based on DartsLinkedHashMap
where the insertion order of keys is remembered and keys are iterated in the order they were inserted into the mapKtHashMap
- based on DartsHashMap
where keys of aHashMap
must have consistent [Object.==] and [Object.hashCode] implementations. Iterating the map's keys, values or entries (through [forEach]) may happen in any order.
kt_dart offer two types of tuples, KtPair
with two elements and KtTriple
with three elements.
They are used by some collection APIs and prevent a 3rd party dependency.
final beatles = KtList.of("John", "Paul", "George", "Ringo");
final partitions = beatles.partition((it) => it.contains("n"));
print(partitions.first); // [John, Ringo]
print(partitions.second); // [Paul, George]
There won't be a KtQuadruple
or TupleN
in this library.
If you want to use tuples heavily in you application consider using the tuple
package.
Better, use freezed
to generated data classes which makes for a much better API.
Kotlin already has Non-Nullable types, something which is coming to Dart soon™.
kt_dart already makes use of Non-Nullable Types and never returns null
unless a method is annotated with @nullable
.
There isn't any tooling which will warn you about the wrong usage but at least it's documented. And once nnbd lands in Dart it will be fairly easy to convert.
This annotation annotates methods which never return null
.
Although this is the default in kt_dart, is makes it very obvious for methods which sometimes return null
in other languages.
A method/class annotated with @experimental
marks the method/class as experimental feature. Experimental APIs can be changed or removed at any time.
Copyright 2019 Pascal Welsch
Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at
http://www.apache.org/licenses/LICENSE-2.0
Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.