wayland/alpha_modifier: add AlphaModifierSurfaceCachedState::set_multiplier

[gaoyia]

Summary

• Add a public set_multiplier() accessor to AlphaModifierSurfaceCachedState, so compositors can drive the alpha multiplier from sources other than the wp_alpha_modifier protocol.

Problem

AlphaModifierSurfaceCachedState exposes read accessors (multiplier(), multiplier_f32(), multiplier_f64()), but the multiplier field itself is private outside the alpha_modifier module. Only smithay's own wp_alpha_modifier dispatch code can write it.

Compositors that receive per-surface opacity through other channels cannot reuse this state — and with it the built-in rendering support in WaylandSurfaceRenderElement — without patching smithay. A concrete example is bridging Chromium's zcr_alpha_compositing_v1 extension: its set_alpha request carries the same double-buffered per-surface opacity semantics, and mapping it onto AlphaModifierSurfaceCachedState lets the existing render path apply it for free.

Solution

Add a minimal setter:

/// Sets the alpha multiplier for the surface.
///
/// This allows compositors to drive the multiplier from sources other than
/// `wp_alpha_modifier` (e.g. bridging Chromium's `zcr_alpha_compositing_v1`).
/// When called on the pending state, the value takes effect on the next
/// `wl_surface.commit`, just like a client `set_multiplier` request.
pub fn set_multiplier(&mut self, multiplier: Option<u32>) {
    self.multiplier = multiplier;
}

Callers are expected to write the pending state (cached_state.get::<AlphaModifierSurfaceCachedState>().pending()), mirroring what the wp_alpha_modifier dispatch code does; the value is then merged into the current state on wl_surface.commit like any other Cacheable state.

No behavior change for existing users; this only widens the public API surface by one method, symmetric with the existing getters.

Test plan

• cargo check --no-default-features --features wayland_frontend builds.
• cargo doc produces no new warnings.
• Downstream usage: a compositor handler for another opacity protocol calls set_multiplier(Some(...)) / set_multiplier(None) on the pending state and commits; WaylandSurfaceRenderElement renders the surface with the expected opacity.

Related

• wp_alpha_modifier staging protocol — multiplier is double-buffered on wl_surface.commit.
• Chromium zcr_alpha_compositing_v1 — downstream compositors bridge it onto AlphaModifierSurfaceCachedState to reuse the same rendering path.

[Drakulix]

Hmm, that primitive is really not meant to be written into from the outside by to represent protocol state. An out-of-tree implementation of a similar protocol should simply add it's own CachedState.

That said, I don't really see an issue with simply merging an implementation of the chromium-protocol in smithay. That way we could safely re-use the same code.