Screenshots in wasm (#8455)

# Objective

- Enable taking a screenshot in wasm
- Followup on #7163 

## Solution

- Create a blob from the image data, generate a url to that blob, add an
`a` element to the document linking to that url, click on that element,
then revoke the url
- This will automatically trigger a download of the screenshot file in
the browser

Cargo.toml

@@ -1968,7 +1968,7 @@ path = "examples/window/screenshot.rs"
 name = "Screenshot"
 description = "Shows how to save screenshots to disk"
 category = "Window"
-wasm = false
+wasm = true
 
 [[example]]
 name = "transparent_window"

crates/bevy_render/Cargo.toml

@@ -82,3 +82,16 @@ encase = { version = "0.5", features = ["glam"] }
 # For wgpu profiling using tracing. Use `RUST_LOG=info` to also capture the wgpu spans.
 profiling = { version = "1", features = ["profile-with-tracing"], optional = true }
 async-channel = "1.8"
+
+[target.'cfg(target_arch = "wasm32")'.dependencies]
+js-sys = "0.3"
+web-sys = { version = "0.3", features = [
+    'Blob',
+    'Document',
+    'Element',
+    'HtmlElement',
+    'Node',
+    'Url',
+    'Window',
+] }
+wasm-bindgen = "0.2"

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

@@ -71,10 +71,47 @@ impl ScreenshotManager {
                     // discard the alpha channel which stores brightness values when HDR is enabled to make sure
                     // the screenshot looks right
                     let img = dyn_img.to_rgb8();
+                    #[cfg(not(target_arch = "wasm32"))]
                     match img.save_with_format(&path, format) {
                         Ok(_) => info!("Screenshot saved to {}", path.display()),
                         Err(e) => error!("Cannot save screenshot, IO error: {e}"),
                     }
+
+                    #[cfg(target_arch = "wasm32")]
+                    {
+                        match (|| {
+                            use image::EncodableLayout;
+                            use wasm_bindgen::{JsCast, JsValue};
+
+                            let mut image_buffer = std::io::Cursor::new(Vec::new());
+                            img.write_to(&mut image_buffer, format)
+                                .map_err(|e| JsValue::from_str(&format!("{e}")))?;
+                            // SAFETY: `image_buffer` only exist in this closure, and is not used after this line
+                            let parts = js_sys::Array::of1(&unsafe {
+                                js_sys::Uint8Array::view(image_buffer.into_inner().as_bytes())
+                                    .into()
+                            });
+                            let blob = web_sys::Blob::new_with_u8_array_sequence(&parts)?;
+                            let url = web_sys::Url::create_object_url_with_blob(&blob)?;
+                            let window = web_sys::window().unwrap();
+                            let document = window.document().unwrap();
+                            let link = document.create_element("a")?;
+                            link.set_attribute("href", &url)?;
+                            link.set_attribute(
+                                "download",
+                                path.file_name()
+                                    .and_then(|filename| filename.to_str())
+                                    .ok_or_else(|| JsValue::from_str("Invalid filename"))?,
+                            )?;
+                            let html_element = link.dyn_into::<web_sys::HtmlElement>()?;
+                            html_element.click();
+                            web_sys::Url::revoke_object_url(&url)?;
+                            Ok::<(), JsValue>(())
+                        })() {
+                            Ok(_) => info!("Screenshot saved to {}", path.display()),
+                            Err(e) => error!("Cannot save screenshot, error: {e:?}"),
+                        };
+                    }
                 }
                 Err(e) => error!("Cannot save screenshot, requested format not recognized: {e}"),
             },

examples/window/screenshot.rs

@@ -8,17 +8,17 @@ fn main() {
     App::new()
         .add_plugins(DefaultPlugins)
         .add_systems(Startup, setup)
-        .add_systems(Update, screenshot_on_f12)
+        .add_systems(Update, screenshot_on_spacebar)
         .run();
 }
 
-fn screenshot_on_f12(
+fn screenshot_on_spacebar(
     input: Res<Input<KeyCode>>,
     main_window: Query<Entity, With<PrimaryWindow>>,
     mut screenshot_manager: ResMut<ScreenshotManager>,
     mut counter: Local<u32>,
 ) {
-    if input.just_pressed(KeyCode::F12) {
+    if input.just_pressed(KeyCode::Space) {
         let path = format!("./screenshot-{}.png", *counter);
         *counter += 1;
         screenshot_manager
@@ -61,4 +61,21 @@ fn setup(
         transform: Transform::from_xyz(-2.0, 2.5, 5.0).looking_at(Vec3::ZERO, Vec3::Y),
         ..default()
     });
+
+    commands.spawn(
+        TextBundle::from_section(
+            "Press <spacebar> to save a screenshot to disk",
+            TextStyle {
+                font_size: 25.0,
+                color: Color::WHITE,
+                ..default()
+            },
+        )
+        .with_style(Style {
+            position_type: PositionType::Absolute,
+            top: Val::Px(10.0),
+            left: Val::Px(10.0),
+            ..default()
+        }),
+    );
 }