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Dart DISCUSSION
My screen's build method is 300 lines, so I split it into private methods such as Widget _buildHeader() and Widget _buildChart(). A reviewer says these should be separate StatelessWidget classes instead, because of performance and because "functions are not widgets". The screen calls setState about ten times per second when new samples arrive.
Is there a real technical difference, or is this only a style preference?
There is a real difference. A helper method is just part of the parent's build: the framework cannot see it. Every setState on the screen re-runs the whole build, including every helper, and recreates all the widgets they return.
A separate widget class gets its own element in the tree, and the framework decides per element whether a rebuild is needed. If the parent rebuilds and hands down a widget instance identical to the previous one, which is what a const constructor gives you, that child's build is skipped entirely.
For your case the more important step is to narrow what setState touches. If only the chart changes ten times a second, move the chart and its state into its own StatefulWidget and call setState there. The header, the buttons and the rest of the screen are then outside the rebuilt subtree regardless of how they were written.
With the state kept at the top of the screen, every sample rebuilds all 300 lines' worth of widgets. At ten updates per second that is usually affordable, but it is wasted work, and it grows with the screen.
A separate widget also gets its own BuildContext, which fixes a family of bugs. Lookups such as Scaffold.of(context) or Theme.of(context) search upward from the context they are given. Inside a helper method you only have the parent's context, which sits above the Scaffold that the same build method creates, so the lookup fails or finds the wrong ancestor. Inside a child widget's build, the context is below it and the lookup works.
It also means that a dependency on an inherited value rebuilds only the small widget that asked for it.
Helper methods are not forbidden. For a small, purely static fragment that is used once and needs no context of its own, a private method is fine and costs little; creating widget objects is cheap, and the framework only updates the render objects whose configuration actually changed.
Measure before and after instead of guessing: run in profile mode, not debug mode, and look at the build times in the performance overlay or DevTools. At 60 Hz a frame has about 16 ms for build, layout and paint together. If build times are far below that, splitting is for readability and correctness; if they are not, the rebuild-scope change above is where the gain is.
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