lens_base

Description:

lens base

Functional lenses for Dart #

Developing a Flutter app with BLoC? Check out the bloc_lens
package for a more convenient way to manage complex app states!

Rationale #
Sometimes, you want to allow other parts of your application to read and modify
a certain value, but you don't want to expose the entire object or its internals.
In short, you want two operations:

get the current value
set a new value

This is where lenses come in. They encapsulate the value and provide a way to
read and modify it, without exposing the object itself. THey are also composable,
so you can easily modify e.g. a specific item in a list or an entry in a map.
Usage #
Predefined lenses #
This package provides a basic implementation of the Lens interface, called DirectLens.
It allows you to combine a getter and a setter into a single object.
final lens = DirectLens(
get: () => /* get the value */,
set: (value) => /* set the value */,
);
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Then, this object can be passed around and used to read and modify the value.
Specialized lenses #
Aside from the standard Lens class, this package also offers more specializes classes,
which provide additional utility methods:


NumberLens – for (possibly bounded) numeric values.
Configurable with optional minimum and maximum values, and an increment.
Adds two methods: increment and decrement.
NumberLens<int> lens;
lens.get(); // 42
lens.increment(); // 43
lens.decrement(5); // 38
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BoolLens – for boolean values.
Adds a toggle method.
BoolLens lens;
lens.get(); // true
lens.toggle(); // false
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EnumLens – for enumerable values. Not just for actual enums, but for
any type that has a finite number of values.
Adds a next method, which cycles through the values.
MyEnum { a, b, c }
EnumLens<MyEnum> lens;
lens.get(); // MyEnum.a
lens.next(); // MyEnum.b
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ListLens – for lists of values.
Adds methods for adding, removing, and modifying elements. It also provides
a way to "look deeper" and get a lens for a specific element of the list.
ListLens<int> lens;
lens.get(); // [1, 2, 3]
lens.add(4); // [1, 2, 3, 4]
lens.removeAt(1); // [1, 3, 4]
lens[1].set(100); // [1, 100, 4]
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MapLens – for maps of values.
Adds methods for adding, removing, and modifying specific entries, as well as
modifying all entries at once.
MapLens<String, int> lens;
lens.get(); // {'a': 1, 'b': 2}
lens.updateAll((key, value) => value * 2); // {'a': 2, 'b': 4}
lens['a'].set(42); // {'a': 42, 'b': 4}
lens['c'].set(3); // {'a': 42, 'b': 4, 'c': 3}
lens['b'].remove(); // {'a': 42, 'c': 3}
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