Conclusion

This document describes the inner workings of the Flutter toolkit that make
Flutter’s API possible. Because Flutter widgets are built using aggressive
composition, user interfaces built with Flutter have a large number of
widgets.  To support this workload, Flutter uses sublinear algorithms for
layout and building widgets as well as data structures that make tree
surgery efficient and that have a number of constant-factor optimizations.
With some additional details, this design also makes it easy for developers
to create infinite scrolling lists using callbacks that build exactly those
widgets that are visible to the user.

Aggressive composability

One of the most distinctive aspects of Flutter is its aggressive
composability. Widgets are built by composing other widgets,
which are themselves built out of progressively more basic widgets.
For example, Padding is a widget rather than a property of other widgets.
As a result, user interfaces built with Flutter consist of many,
many widgets.