With the introduction of NNBD in Dart 2.12, a new keyword was created: late. The primary reason this was created was to allow for non-null fields, that did not have to be immediately initialized.

// Allow this to be null for now, compiler trusts us, that we will assign it.
late final int i; 
MyConstructor(){
   i = getStartIndex(); // Compiler checks at this point, everything is ok!
   i = null; // This will cause an error, compiler is enforcing non-null
}

This was a necessary feature/workaround for Flutter, because of darts requirement to use const initializers, and most Flutter developers are likely familiar with it by now. In this post we’re going to look at some of the other benefits of late!

Occasionally in Flutter, you will need to get the size and/or position of a widget. Some common use cases for this are:

• You need to measure something and make some decision based on that. For example, you might switch from an expanded column, to a scrolling column, at a certain height threshold.
• You may want enable or disable a scrollbar based on the height of some content
• The parent needs to know your position. For example if when you tap a menu button, the parent wants to animate a dot to that position, we need the button to be able to report its position somehow.