Take example screenshots in CI (#8488)

# Objective

- I want to take screenshots of examples in CI to help with validation
of changes

## Solution

- Can override how much time is updated per frame
- Can specify on which frame to take a screenshots
- Save screenshots in CI

I reused the `TimeUpdateStrategy::ManualDuration` to be able to set the
time update strategy to a fixed duration every frame. Its previous
meaning didn't make much sense to me. This change makes it possible to
have screenshots that are exactly the same across runs.

If this gets merged, I'll add visual comparison of screenshots between
runs to ensure nothing gets broken

## Migration Guide

* `TimeUpdateStrategy::ManualDuration` meaning has changed. Instead of
setting time to `Instant::now()` plus the given duration, it sets time
to last update plus the given duration.

.github/example-run/breakout.ron

@@ -1,3 +1,5 @@
 (
-    exit_after: Some(900)
+    exit_after: Some(900),
+    frame_time: Some(0.03),
+    screenshot_frames: [200],
 )

.github/example-run/load_gltf.ron

@@ -1,3 +1,5 @@
 (
-    exit_after: Some(300)
+    exit_after: Some(300),
+    frame_time: Some(0.03),
+    screenshot_frames: [100],
 )

.github/workflows/ci.yml

@@ -199,13 +199,23 @@ jobs:
             echo "running $example_name - "`date`
             time TRACE_CHROME=trace-$example_name.json CI_TESTING_CONFIG=$example xvfb-run cargo run --example $example_name --features "bevy_ci_testing,trace,trace_chrome"
             sleep 10
+            if [ `find ./ -maxdepth 1 -name 'screenshot-*.png' -print -quit` ]; then
+              mkdir screenshots-$example_name
+              mv screenshot-*.png screenshots-$example_name/
+            fi
           done
           zip traces.zip trace*.json
+          zip -r screenshots.zip screenshots-*
       - name: save traces
         uses: actions/upload-artifact@v3
         with:
           name: example-traces.zip
           path: traces.zip
+      - name: save screenshots
+        uses: actions/upload-artifact@v3
+        with:
+          name: screenshots.zip
+          path: screenshots.zip
       - name: Save PR number
         if: ${{ failure() && github.event_name == 'pull_request' }}
         run: |

crates/bevy_app/src/ci_testing.rs

@@ -1,3 +1,5 @@
+//! Utilities for testing in CI environments.
+
 use crate::{app::AppExit, App, Update};
 use serde::Deserialize;
 
@@ -13,6 +15,11 @@ use bevy_utils::tracing::info;
 pub struct CiTestingConfig {
     /// The number of frames after which Bevy should exit.
     pub exit_after: Option<u32>,
+    /// The time in seconds to update for each frame.
+    pub frame_time: Option<f32>,
+    /// Frames at which to capture a screenshot.
+    #[serde(default)]
+    pub screenshot_frames: Vec<u32>,
 }
 
 fn ci_testing_exit_after(

crates/bevy_app/src/lib.rs

@@ -10,7 +10,7 @@ mod plugin_group;
 mod schedule_runner;
 
 #[cfg(feature = "bevy_ci_testing")]
-mod ci_testing;
+pub mod ci_testing;
 
 pub use app::*;
 pub use bevy_derive::DynamicPlugin;

crates/bevy_internal/Cargo.toml

@@ -77,7 +77,7 @@ subpixel_glyph_atlas = ["bevy_text/subpixel_glyph_atlas"]
 webgl = ["bevy_core_pipeline?/webgl", "bevy_pbr?/webgl", "bevy_render?/webgl"]
 
 # enable systems that allow for automated testing on CI
-bevy_ci_testing = ["bevy_app/bevy_ci_testing", "bevy_render?/ci_limits"]
+bevy_ci_testing = ["bevy_app/bevy_ci_testing", "bevy_time/bevy_ci_testing", "bevy_render?/bevy_ci_testing", "bevy_render?/ci_limits"]
 
 # Enable animation support, and glTF animation loading
 animation = ["bevy_animation", "bevy_gltf?/bevy_animation"]

crates/bevy_render/Cargo.toml

@@ -17,6 +17,7 @@ jpeg = ["image/jpeg"]
 bmp = ["image/bmp"]
 webp = ["image/webp"]
 dds = ["ddsfile"]
+bevy_ci_testing = ["bevy_app/bevy_ci_testing"]
 
 shader_format_glsl = ["naga/glsl-in", "naga/wgsl-out"]
 shader_format_spirv = ["wgpu/spirv", "naga/spv-in", "naga/spv-out"]

crates/bevy_render/src/view/window/screenshot.rs

@@ -139,9 +139,37 @@ impl Plugin for ScreenshotPlugin {
         if let Ok(render_app) = app.get_sub_app_mut(RenderApp) {
             render_app.init_resource::<SpecializedRenderPipelines<ScreenshotToScreenPipeline>>();
         }
+
+        #[cfg(feature = "bevy_ci_testing")]
+        if app
+            .world
+            .contains_resource::<bevy_app::ci_testing::CiTestingConfig>()
+        {
+            app.add_systems(bevy_app::Update, ci_testing_screenshot_at);
+        }
     }
 }
 
+#[cfg(feature = "bevy_ci_testing")]
+fn ci_testing_screenshot_at(
+    mut current_frame: bevy_ecs::prelude::Local<u32>,
+    ci_testing_config: bevy_ecs::prelude::Res<bevy_app::ci_testing::CiTestingConfig>,
+    mut screenshot_manager: ResMut<ScreenshotManager>,
+    main_window: Query<Entity, With<bevy_window::PrimaryWindow>>,
+) {
+    if ci_testing_config
+        .screenshot_frames
+        .contains(&*current_frame)
+    {
+        info!("Taking a screenshot at frame {}.", *current_frame);
+        let path = format!("./screenshot-{}.png", *current_frame);
+        screenshot_manager
+            .save_screenshot_to_disk(main_window.single(), path)
+            .unwrap();
+    }
+    *current_frame += 1;
+}
+
 pub(crate) fn align_byte_size(value: u32) -> u32 {
     value + (COPY_BYTES_PER_ROW_ALIGNMENT - (value % COPY_BYTES_PER_ROW_ALIGNMENT))
 }

crates/bevy_time/Cargo.toml

@@ -11,6 +11,7 @@ keywords = ["bevy"]
 [features]
 default = []
 serialize = ["serde"]
+bevy_ci_testing = ["bevy_app/bevy_ci_testing"]
 
 [dependencies]
 # bevy

crates/bevy_time/src/lib.rs

@@ -47,6 +47,19 @@ impl Plugin for TimePlugin {
             .init_resource::<FixedTime>()
             .add_systems(First, time_system.in_set(TimeSystem))
             .add_systems(RunFixedUpdateLoop, run_fixed_update_schedule);
+
+        #[cfg(feature = "bevy_ci_testing")]
+        if let Some(ci_testing_config) = app
+            .world
+            .get_resource::<bevy_app::ci_testing::CiTestingConfig>()
+        {
+            if let Some(frame_time) = ci_testing_config.frame_time {
+                app.world
+                    .insert_resource(TimeUpdateStrategy::ManualDuration(Duration::from_secs_f32(
+                        frame_time,
+                    )));
+            }
+        }
     }
 }
 
@@ -60,7 +73,7 @@ pub enum TimeUpdateStrategy {
     Automatic,
     // Update [`Time`] with an exact `Instant` value
     ManualInstant(Instant),
-    // Update [`Time`] with the current time + a specified `Duration`
+    // Update [`Time`] with the last update time + a specified `Duration`
     ManualDuration(Duration),
 }
 
@@ -107,7 +120,8 @@ fn time_system(
         TimeUpdateStrategy::Automatic => time.update_with_instant(new_time),
         TimeUpdateStrategy::ManualInstant(instant) => time.update_with_instant(*instant),
         TimeUpdateStrategy::ManualDuration(duration) => {
-            time.update_with_instant(Instant::now() + *duration);
+            let last_update = time.last_update().unwrap_or_else(|| time.startup());
+            time.update_with_instant(last_update + *duration);
         }
     }
 }