Struct BackbufferCache 

pub struct BackbufferCache {
    pub pixels: Option<Arc<Vec<u8>>>,
    pub lcd_alpha: Option<Arc<Vec<u8>>>,
    pub width: u32,
    pub height: u32,
    pub dirty: bool,
    pub theme_epoch: u64,
    pub typography_epoch: u64,
    pub async_state_epoch: u64,
    pub lcd_buffer: Option<LcdBuffer>,
    pub partial_allowed: bool,
    pub content_version: u64,
}

A CPU bitmap owned by a widget that opts into backbuffer caching.

The framework re-rasterises when the cache’s explicit dirty flag is set or when global styling epochs change.

Fields

pixels: Option<Arc<Vec<u8>>>

In Rgba mode: top-row-first RGBA8 pixels, straight alpha. Blitted via [DrawCtx::draw_image_rgba_arc].

In LcdCoverage mode: top-row-first colour plane — 3 bytes/pixel (R_premult, G_premult, B_premult) matching the convention of crate::lcd_coverage::LcdBuffer::color_plane flipped to top-down. The companion alpha plane lives in Self::lcd_alpha.

lcd_alpha: Option<Arc<Vec<u8>>>

LcdCoverage-mode companion to pixels: top-row-first per-channel alpha plane (3 bytes/pixel, (R_alpha, G_alpha, B_alpha)). None means this is a plain Rgba cache. When Some, the blit step uses [DrawCtx::draw_lcd_backbuffer_arc] to preserve the per-channel subpixel information through to the destination — required for LCD chroma to survive the cache round-trip.

width: u32

height: u32

dirty: bool

When true, the next paint will re-rasterise rather than reusing pixels. Widgets set this from their mutation paths (set_text, set_color, focus/hover changes, etc.) and the framework clears it after a successful re-raster.

theme_epoch: u64

Visuals epoch (see crate::theme::current_visuals_epoch) recorded the last time this cache was populated. paint_subtree_backbuffered compares it against the live epoch and forces a re-raster on mismatch, so widgets whose text/fill colours come from ctx.visuals() refresh automatically on a dark/light theme flip without needing every widget to subscribe to theme-change events.

typography_epoch: u64

Typography epoch (see crate::font_settings::current_typography_epoch) — same pattern as theme_epoch but for font / size scale / LCD / hinting / gamma / width / interval / faux-* globals. Lets a slider drag in the System window’s typography controls invalidate every cached Label bitmap without bespoke hooks per widget.

async_state_epoch: u64

Async-state epoch (see crate::animation::async_state_epoch) — bumped when an off-thread / async source (e.g. an image fetch + decode) finishes outside the normal event-dispatch path that would otherwise mark widgets dirty. Mismatch forces a re-raster so freshly-loaded data lands in newly-laid-out bounds.

lcd_buffer: Option<LcdBuffer>

Retained LcdCoverage planes (Y-up) kept alive between paints so an over-scan-band widget can partially re-raster only the edited line strip into the existing pixels instead of rebuilding the whole (up to 2×-viewport) buffer on every keystroke. Only populated on the band path (see crate::widget::Widget::backbuffer_band); None for every other widget, so the extra memory and reuse machinery never touch them.

partial_allowed: bool

Set by the framework immediately before a band widget’s paint: true means the retained Self::lcd_buffer was reused unchanged (same size, mode and styling epochs), so the widget may repaint only its dirty line strip and leave the rest of the buffer intact. false forces a full repaint (first raster, resize, re-anchor, theme flip). Transient — the widget reads it during paint and must not rely on it afterwards.

content_version: u64

Stamped with a globally-unique value (see [next_content_version]) on every raster that changes the published pixels / lcd_alpha content. It lets backends that cache GPU textures by buffer identity detect an in-place content change: the dirty-strip path updates the retained planes through Arc::make_mut, which keeps the byte-buffer address stable, so a pointer-keyed texture cache would otherwise miss the edit. Pairing the buffer pointer with this version disambiguates. 0 means “never rastered”.

Implementations

impl BackbufferCache

pub fn new() -> Self

pub fn invalidate(&mut self)

Mark the cache dirty so the next paint re-rasterises.

Trait Implementations

impl Default for BackbufferCache

fn default() -> Self

Returns the “default value” for a type.