Does using const in the widget tree improve performance?

Question

When creating a widget tree, will inserting const before static widgets improve performance?

ie

child: const Text('This is some text'); 

vs

child: Text('This is some text'); 

I know that, with Dart 2, const is optional and will be inserted automatically is some places. Is this one of those situations? If it isn't, will using const reduce memory usage/improve performance?

Thanks for your answers!

Answer 1

It is a small performance improvement, but it can add up in larger apps or apps where the view is rebuilt often for example because of animations.
const reduces the required work for the Garbage Collector.

You can enable some linter rules in analysis_options.yaml that tell you when you should add const because it's not inferred but would be possible like

• http://dart-lang.github.io/linter/lints/prefer_const_constructors.html
• http://dart-lang.github.io/linter/lints/prefer_const_declarations.html
• http://dart-lang.github.io/linter/lints/prefer_const_literals_to_create_immutables.html

or that reminds you when you use const but it is inferred anyway

• http://dart-lang.github.io/linter/lints/unnecessary_const.html

See also https://www.dartlang.org/guides/language/analysis-options

Answer 2

In the case of Flutter, the real gain with const is not having less instantiation. Flutter has a special treatment for when the instance of a widget doesn't change: it doesn't rebuild them.

Consider the following:

Foo(   child: const Bar(     child: Baz()    ), ) 

In the case of build method being called again (setState, parent rebuild, Inheritedwidget...), then due to the const for Bar subtree, only Foo will see its build method called.

Bar will never get rebuilt because of its parent, because Flutter knows that since the widget instance didn't change, there's nothing to update.