Glint-Runtime/docs/en/modules/07-app-perf.md

App and Perf modules

src/app.rs — GlintApp

Iced application entry point. Holds state and implements update/view.

GlintApp

pub struct GlintApp {
    pub doc:        Document<'static>,
    pub vdom_roots: Vec<Element<'static>>,
    pub rhei:       RheiContext,
}

• doc — original Document (styles, variables, tracker)
• vdom_roots — result of the last evaluate_vdom (_incr)
• rhei — Rhai engine with cached ASTs

update(&mut self, message: Message) -> iced::Task<Message>

Processing messages from Iced:

Message	Action
WindowScrolled(y)	Set __scroll_y, call on_variable_changed
ScrollableScrolled(id, y)	Set __scroll_{id}, call on_variable_changed
EventTriggered(script)	execute_action(), then diff variables to find changed ones
InputChanged(var, val)	Set variable, call on_variable_changed
ToggleChanged(var, val)	Set Bool, call on_variable_changed
SliderChanged(var, val)	Set Float, call on_variable_changed

After processing the message:

1. tracker.take_dirty_set() — get dirty elements
2. evaluate_vdom_incr() — recalculate VDOM
3. Save to self.vdom_roots

Perf: VDOM phase is measured with PerfScope::new("vdom").

view(&self) -> iced::Element

Build Iced widgets from self.vdom_roots:

1. Collect scroll_positions from __scroll_* variables
2. For each root: render_element() → push into column
3. Overlay global fixed/absolute/sticky layers
4. Return container(layout).into()

Perf: Render phase is measured with PerfScope::new("render"). At the end perf::print_frame() — output to stderr.

---

src/perf.rs — Performance Monitoring

Phase timing system (VDOM, Style, Render). Enabled with --perf flag.

PerfReport

pub struct PerfReport {
    pub vdom_eval: f64,  // ms
    pub style: f64,      // ms
    pub render: f64,     // ms
    pub total: f64,      // ms
}

Stored in thread_local! RefCell<PerfReport>.

PerfScope

Drop-guard: measures time from creation to destruction.

pub struct PerfScope {
    name: &'static str,
    start: Instant,
}

On drop(): adds elapsed ms to the corresponding PerfReport field. Names: "vdom", "style", "render".

Functions

Function	Description
set_enabled(bool)	Enable/disable measurements
is_enabled() -> bool	Check state
report_and_reset() -> Option<String>	Collect report, check >16ms threshold
reset()	Reset PerfReport
print_frame()	Call report_and_reset + reset, print to stderr

Example output

[perf] VDOM: 6.63ms | Style: 0.26ms | Render: 7.19ms | Total: 14.08ms
[perf] VDOM: 13.26ms | Style: 0.52ms | Render: 7.18ms | Total: 20.95ms ⚠️ Frame budget exceeded! 20.95ms > 16ms

Instrumentation points

File	Phase	Location
app.rs:update	vdom	Around evaluate_vdom_incr()
app.rs:view	render	Entire view() function
style.rs:compute_cached	style	Inside StyleSheet::compute_cached()