Optimising rebuilds

Performance conversations in Flutter almost always revolve around rebuilds. The nuance that matters: calling build() is cheap in itself — the problem is what gets rebuilt.

The main tools:

• `const` constructors — a const widget is canonicalised and is not rebuilt at all. The cheapest optimisation there is
• Narrowing the rebuild area — extracting the changing part into its own widget and keeping setState as low in the tree as possible
• `RepaintBoundary` — isolating a frequently changing region into its own layer (animations, video)
• Keys — correct element matching when list items move around

Keys are the most misunderstood topic here. On rebuild Flutter matches old and new widgets by type and position. When a list item is removed or reordered, positional matching produces the wrong result — state such as a checkbox selection "sticks" to the wrong item.

Giving ValueKey(item.id) makes Flutter match by identity, and the problem disappears.

The rule: if you have a list of same-typed StatefulWidgets whose order can change, add keys. A static list that never reorders does not need them.