eframe: Add App::transform_primitives and App::post_platform_output hooks

[Le-Syl21]

Summary

Adds two optional methods on the App trait that give integrations a place to inspect or transform the per-frame egui pipeline output before it reaches the platform.

fn transform_primitives(
    &mut self,
    ctx: &egui::Context,
    primitives: &mut Vec<egui::epaint::ClippedPrimitive>,
) {}

fn post_platform_output(
    &mut self,
    ctx: &egui::Context,
    platform_output: &mut egui::PlatformOutput,
) {}

Both default to no-op, so existing apps are unaffected.

Where they fire

Hook	Fires after	Fires before
transform_primitives	Context::tessellate	painter paint_and_update_textures
post_platform_output	Context::run_ui returns	egui_winit.handle_platform_output consumes it

Wired into both glow_integration and wgpu_integration. Web (wasm32) is not touched in this PR — could be a follow-up if there's interest.

Use cases

These are general-purpose hooks, not tied to any specific feature. Examples that motivated them:

• Viewport-level transforms: rotation (e.g. for a 90°-mounted kiosk display), mirroring, custom projections — applied uniformly to the whole frame without touching individual widgets. The shipping companion crate egui-rotate uses these hooks to integrate cleanly with eframe; without them it can only target custom winit/glow integrations.
• Debugging / instrumentation: dump primitive counts, log overdraw, inject debug overlays after the UI has been laid out.
• Custom render passes: anything that needs access to the final primitive list (e.g. injecting a post-processing pass on a subset of layers).
• Software cursor capture: read PlatformOutput::cursor_icon to drive a custom cursor (where the OS cursor is hidden, e.g. cabinet displays where the OS cursor cannot be rotated).
• Output interception: clipboard / IME / cursor-icon overrides for kiosk and embedded scenarios.

Why not a single hook?

The two hooks fire at different points in the pipeline (tessellate→paint vs. run_ui→handle_platform_output) and target different data (primitives vs. platform output). Coalescing them would either add a hook that fires earlier than needed for one of the use cases, or split the work into one hook with two unrelated parameters.

Why not extend Context instead?

These are integration-time concerns (post-tessellation, pre-paint; post-run, pre-OS-dispatch). They don't fit the egui Context API, which is mid-pass. The App trait is the natural place: it's already where raw_input_hook lives for the symmetric input side.

Test plan

• cargo build -p eframe clean
• cargo clippy -p eframe --all-features -- -D warnings clean
• cargo fmt -p eframe --check clean
• cargo test -p eframe --lib passes
• Manual: drive a vendored egui-rotate integration through these hooks on a real eframe app, verify rotation + cursor work — done locally on my pinball cabinet launcher (PinReady), can share details if useful

Background

This is a smaller, more general follow-up to #8113, which tried to integrate viewport rotation directly into egui+eframe and was declined as too niche / too much surface to maintain. That feedback was fair. The follow-up published the rotation logic as a standalone crate (egui-rotate) which works fine with custom integrations (winit + glow / wgpu / SDL3 directly). However it cannot integrate with eframe because eframe owns the tessellate→paint pipeline with no hook in between.

These two hooks fix that — and stay generic. They're not specific to rotation; the doc-comments describe the broader use cases.

If preferred, I'm happy to:

• rename either method (pre_paint / pre_handle_output?)
• gate them behind a feature flag
• only add transform_primitives and drop post_platform_output (the cursor icon use case can be worked around with a fixed icon)
• wire them into the wasm integration too

🤖 Drafted with Claude Code

[github-actions]

Preview is being built...

Preview will be available at https://egui-pr-preview.github.io/pr/8138-eframe-app-hooks

View snapshot changes at kitdiff

[emilk]

Did you try using the egui::Plugin system to achieve this? In particular: Plugin::output_hook

[emilk]

revert: we generate the changelog on release