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https://github.com/google/built_collection.dart

Immutable Dart collections via the builder pattern.
https://github.com/google/built_collection.dart

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Immutable Dart collections via the builder pattern.

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## Introduction



Built Collections are immutable collections using the

[builder pattern](https://en.wikipedia.org/wiki/Builder_pattern).



Each of the core SDK collections is split in two: a mutable builder class

and an immutable "built" class. Builders are for computation,

"built" classes are for safely sharing with no need to copy defensively.



Immutable collections work particularly well with immutable values. See

[built_value](https://github.com/google/built_value.dart#built-values-for-dart).



You can read more about built_collection

[on medium](https://medium.com/@davidmorgan_14314/darts-built-collection-for-immutable-collections-db662f705eff).



## Design



Built Collections:



* are immutable, if the elements/keys/values used are immutable;

* are comparable;

* are hashable;

* use copy-on-write to avoid copying unnecessarily.



See below for details on each of these points.



## Recommended Style



A project can benefit greatly from using Built Collections throughout.

Methods that will not mutate a collection can accept the "built" version,

making it clear that no mutation will happen and completely avoiding

the need for defensive copying.



For code that is public to other projects or teams not using

Built Collections, prefer to accept `Iterable` where possible. That way

your code is compatible with SDK collections, Built Collections and any

other collection implementation that builds on `Iterable`.



It's okay to accept `List`, `Set` or `Map` if needed. Built Collections

provide efficient conversion to their SDK counterparts via

`BuiltList.toList`, `BuiltListMultimap.toMap`, `BuiltSet.toSet`,

`BuiltMap.toMap` and `BuiltSetMultimap.toMap`.



## Built Collections are Immutable



Built Collections do not offer any methods that modify the collection. In

order to make changes, first call `toBuilder` to get a mutable builder.



In particular, Built Collections do not implement or extend their mutable

counterparts. `BuiltList` implements `Iterable`, but not `List`. `BuiltSet`

implements `Iterable`, but not `Set`. `BuiltMap`, `BuiltListMultimap` and

`BuiltSetMultimap` share no interface with the SDK collections.



Built Collections can contain mutable elements. However, this use is not

recommended, as mutations to the elements will break comparison and

hashing.



## Built Collections are Comparable



Core SDK collections do not offer equality checks by default.



Built Collections do a deep comparison against other Built Collections

of the same type using Dart's equality operator (`==`). A Built Collection

cannot be compared to another Built Collection when they are of different

types, such as a `BuiltList` and a `BuiltSet`. Hashing is used to make

repeated comparisons fast.



```dart

// true: same contents according to `int.operator==`, `MyClass.operator==`

BuiltList([1, 2, 3]) == BuiltList([1, 2, 3]);

BuiltList([MyClass(1), MyClass(2)]) == BuiltList([MyClass(1), MyClass(2)]);



// false: BuiltList and BuiltSet are never equal

BuiltList([1, 2, 3]) == BuiltSet([1, 2, 3]);

// false: different contents according to `int.operator==`

BuiltList([1, 2, 3]) == BuiltList([2, 3, 4]);

```



## Built Collections are Hashable



Core SDK collections do not compute a deep hashCode.



Built Collections do compute, and cache, a deep hashCode. That means they

can be stored inside collections that need hashing, such as hash sets and

hash maps. They also use the cached hash code to speed up repeated

comparisons.



## Built Collections Avoid Copying Unnecessarily



Built Collections and their builder and helper types collaborate to avoid

copying unless it's necessary.



In particular, `BuiltList.toList`, `BuiltListMultimap.toMap`,

`BuiltSet.toSet`, `BuiltMap.toMap` and `BuiltSetMultimap.toMap` do not make

a copy, but return a copy-on-write wrapper. So, Built Collections can be

efficiently and easily used with code that needs core SDK collections but

does not mutate them.



When you want to provide a collection that explicitly _throws_ when a

mutation is attempted, use `BuiltList.asList`,

`BuiltListMultimap.asMap`, `BuiltSet.asSet`, `BuiltSetMultimap.asMap`

and `BuiltMap.asMap`.



## Features and bugs



Please file feature requests and bugs at the [issue tracker][tracker].



[tracker]: https://github.com/google/built_collection.dart/issues