Dart. Everything you need to know about constants

Constants are not just a strange version of finalvariables that will haunt you in a dream with all the errors associated with them. Compile-timeconstants are a good way to improve the performance of your application, without creating the same object several times, and so to speak, “pre-creating” objects at compile time.

const or final?

, , . , , const final . .

void run() {
  const myConstNumber = 5;
  final myFinalNumber = 5;

  myConstNumber = 42; // 
  myFinalNumber = 42; // 
}

final, const , . , const .

class MyClass {
  final int myFinalField;
  const int myConstField; // 

  MyClass(this.myFinalField, this.myConstField);
}

, , final, const. , , (. ). ...

Dart . , "hello", , 3.14, , .

, .

void run() {
  const myList = [1, 2, 3];
  const myMap = {'one': 1};
  const mySet = {1, 2, 3};
}

: , if, spread , .

, const final , , . Dart , , . .

, , , . , .

void run() {
  final finalVariable = 123456;
  const constVariable = 123456;

  const notWorking = finalVariable; // 
  const working = constVariable;
}

, – , , , ...

Flutter const , , EdgeInsets, . - , .

const EdgeInsets.all(8), . , const , .

, const . : final .

class MyConstClass {
  final int field1;
  final List<String> field2;
  const MyConstClass(this.field1, this.field2);
}

void run() {
  //      
  const constVariable = 123456;
  const x = MyConstClass(constVariable, ['hello', 'there']);
}

, , const .

class MyWannabeConstClass {
  //  Future  const   
  final Future<int> field;
  // Dart  ,  ,  :
  const MyWannabeConstClass(this.field);
}

void run() {
  // Dart      const:
  const x = MyWannabeConstClass(Future.value(123)); // 
}

Dart , , , ""? , . const , . .

void run() {
  //  Future        
  final implicitNew = MyWannabeConstClass(Future.value(123));
  final explicitNew = new MyWannabeConstClass(Future.value(123));
}

const , , , Flutter .

, const final .

, new , const .

void run() {
  const implicitConst = MyConstClass(1);
  const explicitConst = const MyConstClass(1);
}

const , , , .

void run() {
  final regularVariable = const MyConstClass(1);
}

Conclusion

Hopefully this guide has been able to clarify the meaning of constconstructors and, in general, constants in Dart. Try to use constit where possible, and you will make small performance improvements in your applications line by line. And you know how it happens with small improvements - they give an overall result.