镖。您需要了解的有关常数的所有信息

常量不仅仅是final变量的怪异版本，它会在梦中困扰着您所有与之相关的错误。Compile-time常量是提高应用程序性能的好方法，而无需多次创建同一对象，也可以说是在编译时“预创建”对象。

const还是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);
}

结论

希望本指南能够阐明const构造函数的含义，以及通常在Dart中的常量的含义。尝试尽可能使用const它，您将逐行在应用程序中进行较小的性能改进。而且，您知道通过少量改进它是如何发生的-它们给出了总体结果。