Builder

Builder is a convenience widget for the case where you want to build a small piece of user interface inline without creating a named struct.

It receives the same &mut BuildCtx<S> and &View inputs as any other widget and returns a Node. In other words, it is still part of the normal Fission build model. The closure is only an authoring convenience.

Use Builder for short, local composition helpers. Do not use it as a place to hide side effects, and do not let it grow into a large anonymous screen with lots of product logic. When the user interface becomes reusable or conceptually important, move it into a named widget type.

Example

use fission::prelude::*;
use fission::core::ui::Text;
use fission::core::ui::widgets::builder::Builder;

let greeting = Builder::new(|_ctx, view: &View<MyState>| {
    let label = if view.state.is_signed_in {
        "Welcome back"
    } else {
        "Please sign in"
    };

    Text::new(label).into_node()
});

This is a good Builder use case because the closure is small, reads from View, and returns a simple node tree.

Field table

Field	Type	Meaning	Notes / default behavior
Builder::new(...)	Fn(&mut BuildCtx<S>, &View) -> Node	Creates the inline widget from a pure build closure.	This is the main public entry point. The closure must be Send + Sync + 'static.

When to use it and when not to

Builder is helpful when a small fragment would otherwise force a one-off named widget that adds more ceremony than clarity. It keeps local composition readable.

It becomes a code smell when you start putting large branching product logic, async decisions, or many nested layout helpers into one anonymous closure. In those situations, use a named widget or a Selector plus a normal widget implementation so the code stays understandable.

Most importantly, Builder does not change the purity rule of build(). The closure should read its inputs and return a user interface description. It should not fetch data, mutate global state, or perform outside work.

Related

LayoutBuilder, Widget, and Node.