Exploring const keyword in Dart

const_keyword.dart

class Contact {
  // Constructor is marked 'const' so all fields must be final.
  final String name;
  final int age;

  // const at class level have to go along with static keyword
  static const alive = true;

  // const constructor
  const Contact(this.name, this.age);

  @override
  String toString() {
    return "$name: $age";
  }
}

void main(List<String> arguments) {
  const contacts = <Contact>[Contact("Tuan", 34), Contact("Duong", 26)];
  
  const tuan2 = Contact("Tuan", 34);

  // A const constructor is an optimization!
  // The compiler makes the object immutable, allocating the same portion of memory for all object which have the same values
  print("Equivalent? ${contacts[0] == tuan2}"); // true.
  
  //RUN-TIME error: Cannot add to an unmodifiable list
  contacts.add(Contact("Simon", 33)); 
  //RUN-TIME error: Cannot add to an unmodifiable list
  contacts[0] = Contact("Phuong", 35);

  //COMPILE-TIME Error: The setter 'name' isn't defined for the class 'Contact'.
  contacts[0].name = "Tuan Nguyen";
  
  // COMPILE-TIME  Can't assign to the final variable 'contacts'.
  contacts = <Contact>[Contact("Harry", 33)];
  
  // create const values. It looks complicated to me but works
  var nums = const [];
  final nums2 = const [];
  const nums3 = []; // equivalent `const []`
}

/*
 * Theory of const:
 * - The const keyword is used to represent a compile-time constant.
 * - once assigned a value, const variable's value cannot be changes.
 * - value of const variable is calculated at compile-time. (except the const collection element, it calculated at run-time)
 * - const is initialize at compile-time
 * - const means that the object's entire deep state can be determined entirely at compile time and that the object will be frozen and completely immutable.
 * - a const object and its fields cannot be changed: they’re immutable.
 * - if const variables have the same value, Dart will intial it once and reuse it instead of initialize multiple time to optimize cache
 *
 * - you can also use const to create constant values
 */

Best practices:

Define a meaningful class for holding const variables:

class Colors {
  static const String primary = 'red';
  static const String primaryDark = 'rose';
  static const String secondary = 'pink';
  static const String background = 'gray';
}

class PurchaseStatus {
  static const int SUCCESS = 10;
  static const int REJECTED = 0;
  static const int PENDING = 20;
  static const int CONFIRMING = 30;
}

class Strings {
  static const String generalButtonsOk = "OK";
  static const String generalButtonsCancel = "Cancel";
  
  static const String generalMessageSuccess = "Successful";
  static const String generalMessageError = "Failed";
}