[makspll]

Draft: the API below is a first cut, see the open choices.

ui.list_template(items, function(item) .. end): a list whose function runs once for the whole list. item stands for the item of each row, the nodes and texts the function returns are a template, and every row is made from it in Rust: no script code runs per row.

ui.list_template(visible, function(item)            -- `visible` holds a list of entities
    local data = ui.watch_item(item, Item)          -- the Item component of each row's entity
    local name = ui.path(data, "data.name")
    return ui.node({
        name = ui.format("slot_{}", { name }),
        background_color = ui.when(ui.selected(is_selected, name), "#ffcc00", "#333333"),
        on_click = function(entity) select(entity) end,   -- one function for all rows, given the row's item
    }, {
        ui.text({ text = ui.format("x{}", { ui.path(data, "data.count") }) }),
    })
end)

How a row is made (bind_template):

• values computed in Rust are bound to the row (Expr::bind): item becomes the item, path(watch_item(..)) reads a watch of the row's entity (owned by the row, removed with it), selected(selector, value) reads its key from the row once. A bound value which reads nothing is a plain property, spawned with the entity.
• functions (handlers and bound properties) are curried with the item, so they are shared by all rows.
• the result is an ordinary element, mounted, patched (on reload) and unmounted like any other.

Works with closures restricted to capturing constants (Rune), as nothing has to capture the item. Lua only tests and benches, as the rest of the crate.

Open choices:

• rows are node and text only: components and regions inside a template are an error (they run script code per instance)
• watch_item needs the items to be entities. Items which are plain values work with item itself (format("{}", { item }), selected(selector, item))
• no branching on the item in script code, only through when
• a separate list_template function, rather than a flag or a wrapper around list's function

Stacked on #724. Same binary, 3 rounds (new Template bench variant, the Paths ui with a templated list):

bench	Closures	Paths	Template
inventory mount or unmount 200	1.78 ms	2.02 ms	0.90 ms
inventory search	1.49 ms	1.68 ms	0.79 ms
inventory add or remove one	125 µs	127 µs	173 µs
inventory select	22.8 µs	18.7 µs	24.4 µs

The two slower rows are the bench's own choices, not the template: its list holds 200 entity references instead of 200 strings (costlier to pass to Rust and back), and its click handler reads the item's name from the component.

[semanticdiff-com]

Review changes with  

Changed Files

File	Status
crates/bevy_mod_scripting_reactive_ui/src/element.rs	38% smaller
crates/bevy_mod_scripting_reactive_ui/src/expr.rs	17% smaller
assets/tests/reactive_ui/inventory/inventory.lua	13% smaller
crates/bevy_mod_scripting_reactive_ui/src/runtime.rs	11% smaller
assets/macro_benchmarks/reactive_ui/template.lua	0% smaller
benches/benchmarks.rs	0% smaller
crates/bevy_mod_scripting_reactive_ui/src/lib.rs	0% smaller

[github-actions]

Bencher Report

Project	Bevy Scripting
Branch	agent/reactive-ui-templates
Testbed	linux-gha

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