Dart 中的const和final关键字有什么区别

There is a post on dart's website and it explains it pretty well.个

Final:

"final"指单个赋值:final变量或字段must具有初始值设定项.一旦赋值，最终变量的值就不能更改.最终修改为variables.

Const:

"const"在DART中有一个更为复杂和微妙的含义.常量修改为values.您可以在创建集合(如const[1，2，3])和构造对象(而不是新建)(如const Point(2，3))时使用它.在这里，const意味着对象的整个深层状态可以在编译时完全确定，并且对象将被冻结并且完全不可变.

Const对象有几个有趣的属性和限制:

它们必须从可以在编译时计算的数据创建.常量对象无权访问您在运行时需要计算的任何内容.1+2是有效的常量表达式，但new DateTime.now()不是.

它们是深刻的、可传递的不可改变的.如果有包含集合的最后一个字段，则该集合仍然可以是可变的.如果您有一个常量集合，那么其中的所有内容也必须是常量，递归地.

他们是canonicalized岁.这有点像字符串插入:对于任何给定的常量值，无论常量表达式计算多少次，都将创建并重用单个常量对象.

那么，这意味着什么呢？

Const:
If the value you have is computed at runtime (new DateTime.now(), for example), you can not use a const for it. However, if the value is known at compile time (const a = 1;), then you should use const over final. There are 2 other large differences between const and final. Firstly, if you're using const, you have to declare it as static const rather than just const. Secondly, if you have a const collection, everything inside of that is in const. If you have a final collection, everything inside of that is not final.

Final:
final should be used over const if you don't know the value at compile time, and it will be calculated/grabbed at runtime. If you want an HTTP response that can't be changed, if you want to get something from a database, or if you want to read from a local file, use final. Anything that isn't known at compile time should be final over const.

尽管如此，const和final都不能被重新分配，但是final对象中的字段，只要它们本身不是const或final，就可以被重新分配(与const不同).