Language: Support spread syntax like Javascript.

[liudonghua123]

I find it's difficult to do a shallow copy and modify some attributes at the same time like spread syntax in javascript.

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Spread_syntax

The following are some js example.

const parts = ['shoulders', 'knees']; 
const lyrics = ['head', ...parts, 'and', 'toes']; 
//  ["head", "shoulders", "knees", "and", "toes"]

const obj1 = { foo: 'bar', x: 42 };
const obj2 = { foo: 'baz', y: 13 };

const clonedObj = { ...obj1 };
// Object { foo: "bar", x: 42 }

const mergedObj = { ...obj1, ...obj2 };
// Object { foo: "baz", x: 42, y: 13 }

The releated topic is rest parameters.
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/rest_parameters

[mateusfccp]

Dart does have spread operator. Only for lists, tho.

[liudonghua123]

@mateusfccp Thanks for your tips. I think object spread is more useful. For example I just want to copy the theme data object or some other object, and modify some instance variables. Without this features, maybe I need to write an extra function to do it.
Hope dart could support object spread operator too.

[rrousselGit]

You can use code-generators to help until Dart has a proper syntax for it.

One of them would be: https://github.com/rrousselGit/freezed
(made by me)

It has support for a pretty good "copyWith"

[mateusfccp]

Yes, it would be useful.

Currently, a common pattern in Flutter (don't know if it is also for Dart) is to make a function copyWith, where you provide only the fields you want to be different, and the rest is copied from the source.

For example, it's used in Flutter's themes/styles.
https://api.flutter.dev/flutter/painting/TextStyle/copyWith.html

[munificent]

Dart does have spread operator. Only for lists, tho.

Spreads work with all collection types in Dart: lists, maps, and sets. This is a valid Dart program:

main() {
  const parts = ['shoulders', 'knees'];
  const lyrics = ['head', ...parts, 'and', 'toes'];
  //  ["head", "shoulders", "knees", "and", "toes"]
  const obj1 = { 'foo': 'bar', 'x': 42 };
  const obj2 = { 'foo2': 'baz', 'y': 13 };

  const clonedObj = { ...obj1 };
  // Object { foo: "bar", x: 42 }

  const mergedObj = { ...obj1, ...obj2 };
  // Object { foo: "baz", x: 42, y: 13 }
}

All I did was take that JS, put it inside main(), quote the map keys, and rename the second foo so that it doesn't collide with the first. :)

[liudonghua123]

@munificent Hi, if i have a identity key in both maps, then this behavior is error in dart, how can I achieve it in dart with a clean and simple way?

Update I need to change const to var to make above works.

main() {
  const obj1 = {'foo': 'bar', 'x': 42};
  const obj2 = {'foo': 'baz', 'y': 13};

  var clonedObj = {...obj1, 'x': 43};
  // var clonedObj = {...obj1}..['x'] = 43;

  var mergedObj = {...obj1, ...obj2, 'x': 43};
  // var mergedObj = {}..addAll(obj1)..addAll(obj2);
}