How to set final/const properties in Dart constructor
From the Dart language tour:
Note: Instance variables can be
finalbut notconst. Final instance variables must be initialized before the constructor body starts — at the variable declaration, by a constructor parameter, or in the constructor’s initializer list.
And the section on initializer lists says:
Besides invoking a superclass constructor, you can also initialize instance variables before the constructor body runs. Separate initializers with commas.
// Initializer list sets instance variables before// the constructor body runs.Point.fromJson(Map<String, num> json) : x = json['x'], y = json['y'] { print('In Point.fromJson(): ($x, $y)');}
So the general way is through initialization lists.
As mentioned above, you also can initialize at variable declaration:
class Foo { final x = 42;}or can initialize them by a constructor parameter:
class Foo { final x; Foo(this.x);}although those other approaches might not always be applicable.