helix-xml

name: helix-xml description: > HelixScreen XML UI engine — triggers when editing files in ui_xml/, lib/helix-xml/, src/xml_registration.cpp, or include/ headers that register subjects/components/events. Also for creating modal dialogs via ModalStack (src/ui/modals/), defining custom XML widgets/components in ui_xml/components/, writing C++ code that binds lv_subject_t to XML layouts, working with design tokens (#space_md, #card_bg) in XML, or using bind_text/bind_flag/bind_style reactive bindings. Covers the full XML→Subject→C++ pipeline, declarative UI rules, component system, event registration, and gotchas specific to this engine.

HelixScreen XML UI Engine (helix-xml)

The helix-xml library (lib/helix-xml/) provides the declarative XML UI system for HelixScreen. XML layouts live in ui_xml/, components are registered in src/xml_registration.cpp, and C++ subjects are bound via lv_xml_register_subject(). The engine implements a Subject-Observer pipeline:

XML Layout (components) → bind_text/bind_value/bind_flag → Subjects (C++) → UI auto-updates

Core Architecture

Registration Flow (startup order matters)

// 1. Fonts
lv_xml_register_font(NULL, "montserrat_16", &lv_font_montserrat_16);
// 2. Globals FIRST (constants must exist before components reference them)
lv_xml_register_component_from_file("A:ui_xml/globals.xml");
// 3. Responsive tokens
ui_theme_register_responsive_spacing();  // #space_md, etc.
ui_theme_register_responsive_fonts();    // #font_body, etc.
// 4. Components (order doesn't matter after globals)
lv_xml_register_component_from_file("A:ui_xml/home_panel.xml");

Component Structure

<component>
    <api>                          <!-- Properties from parent -->
        <prop name="text" type="string" default="Click me"/>
    </api>
    <consts>                       <!-- Local constants -->
        <px name="size" value="36"/>
    </consts>
    <styles>                       <!-- NO style_ prefix here! -->
        <style name="base" bg_color="0x333" text_color="0xfff"/>
    </styles>
    <view extends="lv_button" width="#size">   <!-- The UI tree -->
        <lv_label text="$text" align="center"/>
        <style name="base"/>
    </view>
</component>

Sigils

For standard widgets, bind_text="subject_name" always treats its value as a subject (no @ needed).

Subject Lifecycle

static lv_subject_t subj;
static char buf[128];  // MUST be static/heap — never stack
lv_subject_init_string(&subj, buf, NULL, sizeof(buf), "Initial");
lv_xml_register_subject(NULL, "status_text", &subj);  // BEFORE creating XML
// Later: UI updates automatically
lv_subject_copy_string(&subj, "New status");

Subject types: lv_subject_init_string(), lv_subject_init_int(), lv_subject_init_pointer(), lv_subject_init_color().

Data Binding (XML side)

<!-- Text binding -->
<lv_label bind_text="status"/>
<lv_label bind_text="temp" bind_text-fmt="%.1f°C"/>

<!-- Conditional visibility -->
<lv_obj>
    <bind_flag_if_eq subject="step" flag="hidden" ref_value="1"/>
</lv_obj>

<!-- Conditional style (inline styles OVERRIDE bind_style!) -->
<styles>
    <style name="active" bg_color="0x00ff00"/>
    <style name="inactive" bg_color="0xff0000"/>
</styles>
<lv_button>
    <bind_style name="inactive" subject="is_on" ref_value="0"/>
    <bind_style name="active" subject="is_on" ref_value="1"/>
</lv_button>

Comparison operators: bind_flag_if_eq, _not_eq, _gt, _ge, _lt, _le (same for bind_state_if_* and bind_style_if_*).

Flags: hidden, clickable, checkable, scrollable, disabled, ignore_layout, floating.

Declarative UI Rules (MANDATORY)

These rules are non-negotiable in this codebase:

1. ALL UI updates via reactive subjects — never lv_label_set_text(), lv_obj_add_flag(HIDDEN), lv_obj_set_style_*(), or lv_obj_add_state()
2. Events declared in XML with <event_cb trigger="clicked" callback="name"/>, registered in C++ via lv_xml_register_event_cb(). NEVER use lv_obj_add_event_cb()
3. Inline styles override bind_style — when using reactive style changes, do NOT set inline style_* attributes for those properties. Use two bind_style entries instead
4. Register subjects BEFORE creating XML that binds to them
5. Subject string buffers MUST be static or heap — stack-allocated buffers cause use-after-free
6. Use design tokens (#card_bg, #space_md) not hardcoded pixels/colors
7. Semantic widgets (<text_heading>, <text_body>, <text_small>) not raw <lv_label> with hardcoded fonts
8. Component names required: <controls_panel name="controls_panel"/> not <controls_panel/>

Exceptions (acceptable direct access): LV_EVENT_DELETE cleanup, widget pool recycling, chart data, animations, one-time setup().

Flex Layout

Three properties needed to center (unlike CSS):

<lv_obj flex_flow="column" height="100%"
        style_flex_main_place="center"
        style_flex_cross_place="center"
        style_flex_track_place="center">

• flex_grow="1" expands to fill; parent MUST have explicit height="100%"
• width="content" not width="LV_SIZE_CONTENT" (which parses as 0)
• style_pad_column / style_pad_row for flex gaps

Events

<lv_button name="btn">
    <event_cb trigger="clicked" callback="on_btn_clicked"/>
</lv_button>

// Register BEFORE XML creation
lv_xml_register_event_cb(nullptr, "on_btn_clicked", [](lv_event_t* e) {
    // handle
});

Triggers: clicked, value_changed, pressed, released, long_pressed, focused, ready.

Gotchas

• Unknown XML attributes are silently ignored — typos produce no error
• style_image_recolor not style_img_recolor (full words only)
• No zoom attribute → use scale_x/scale_y (256 = 100%)
• No flex_align attribute → use style_flex_main_place/style_flex_cross_place/style_flex_track_place
• Dropdown options: <lv_dropdown options="A&#10;B&#10;C"/> (XML entity for newline)
• lv_bar value=0 bug: set to 1 then 0 as workaround
• XML <subjects> block shadows global C++ subjects — don't declare in both places
• Unregistered components are silently empty — no error, no log

Observer Cleanup

Track observers for heap-allocated widget data:

data->text_observer = lv_label_bind_text(label, &subject, "%s");
// In DELETE handler:
lv_observer_remove(data->text_observer);
delete data;

For custom widgets with owned subjects, detach children BEFORE deiniting:

lv_obj_remove_from_subject(label, nullptr);  // removes ALL observers
lv_subject_deinit(&owned_subject);

File Index