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What is the difference between const and final keyword in Dart?
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The only difference between final and const is that the const makes the variable constant from compile-time only. Using const on an object, makes the object's entire deep state strictly fixed at compile-time and that the object with this state will be considered frozen and completely immutable.
A variable with the final keyword will be initialized at runtime and can only be assigned for a single time. In a class and function, you can define a final variable. For Flutter specific, when the state is updated, everything in the build method will be initialized again.
The declaration for cons must be using const . You have to declare it as static const rather than just const . static , final , and const mean entirely distinct things in Dart: static means a member is available on the class itself instead of on instances of the class.
const means its initial value is must be fixed, can not be a dynamic value; final means its initial value is must be fixed but can be a dynamic value, equal to the var with a fixed value.
There is a post on dart's website and it explains it pretty well.
Final:
"final" means single-assignment: a final variable or field must have an initializer. Once assigned a value, a final variable's value cannot be changed. final modifies variables.
Const:
"const" has a meaning that's a bit more complex and subtle in Dart. const modifies values. You can use it when creating collections, like const [1, 2, 3], and when constructing objects (instead of new) like const Point(2, 3). Here, 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.
Const objects have a couple of interesting properties and restrictions:
They must be created from data that can be calculated at compile time. A const object does not have access to anything you would need to calculate at runtime. 1 + 2 is a valid const expression, but new DateTime.now() is not.
They are deeply, transitively immutable. If you have a final field containing a collection, that collection can still be mutable. If you have a const collection, everything in it must also be const, recursively.
They are canonicalized. This is sort of like string interning: for any given const value, a single const object will be created and re-used no matter how many times the const expression(s) are evaluated.
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.
With all of that being said, both const and final cannot be reassigned, but fields in a final object, as long as they aren't const or final themselves, can be reassigned (unlike const).
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Value must be known at compile-time, const birthday = "2008/12/25"
Can't be changed after initialized.
Value must be known at run-time, final birthday = getBirthDateFromDB()
Can't be changed after initialized.
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