Added offscreen rendering support for wgpu & tiny-skia exposed with the window::screenshot command.

runtime/src/lib.rs

@@ -60,6 +60,7 @@ mod debug;
 #[cfg(not(feature = "debug"))]
 #[path = "debug/null.rs"]
 mod debug;
+mod screenshot;
 
 pub use iced_core as core;
 pub use iced_futures as futures;
@@ -68,4 +69,5 @@ pub use command::Command;
 pub use debug::Debug;
 pub use font::Font;
 pub use program::Program;
+pub use screenshot::{CropError, Screenshot};
 pub use user_interface::UserInterface;

runtime/src/screenshot.rs

@@ -0,0 +1,80 @@
+use iced_core::{Rectangle, Size};
+use std::fmt::{Debug, Formatter};
+
+/// Data of a screenshot, captured with `window::screenshot()`.
+///
+/// The `bytes` of this screenshot will always be ordered as `RGBA` in the sRGB color space.
+#[derive(Clone)]
+pub struct Screenshot {
+    /// The bytes of the [`Screenshot`].
+    pub bytes: Vec<u8>,
+    /// The size of the [`Screenshot`].
+    pub size: Size<u32>,
+}
+
+impl Debug for Screenshot {
+    fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result {
+        write!(
+            f,
+            "Screenshot: {{ \n bytes: {}\n size: {:?} }}",
+            self.bytes.len(),
+            self.size
+        )
+    }
+}
+
+impl Screenshot {
+    /// Creates a new [`Screenshot`].
+    pub fn new(bytes: Vec<u8>, size: Size<u32>) -> Self {
+        Self { bytes, size }
+    }
+
+    /// Crops a [`Screenshot`] to the provided `region`. This will always be relative to the
+    /// top-left corner of the [`Screenshot`].
+    pub fn crop(&self, region: Rectangle<u32>) -> Result<Self, CropError> {
+        if region.width == 0 || region.height == 0 {
+            return Err(CropError::Zero);
+        }
+
+        if region.x + region.width > self.size.width
+            || region.y + region.height > self.size.height
+        {
+            return Err(CropError::OutOfBounds);
+        }
+
+        // Image is always RGBA8 = 4 bytes per pixel
+        const PIXEL_SIZE: usize = 4;
+
+        let bytes_per_row = self.size.width as usize * PIXEL_SIZE;
+        let row_range = region.y as usize..(region.y + region.height) as usize;
+        let column_range = region.x as usize * PIXEL_SIZE
+            ..(region.x + region.width) as usize * PIXEL_SIZE;
+
+        let chopped = self.bytes.chunks(bytes_per_row).enumerate().fold(
+            vec![],
+            |mut acc, (row, bytes)| {
+                if row_range.contains(&row) {
+                    acc.extend(&bytes[column_range.clone()]);
+                }
+
+                acc
+            },
+        );
+
+        Ok(Self {
+            bytes: chopped,
+            size: Size::new(region.width, region.height),
+        })
+    }
+}
+
+#[derive(Debug, thiserror::Error)]
+/// Errors that can occur when cropping a [`Screenshot`].
+pub enum CropError {
+    #[error("The cropped region is out of bounds.")]
+    /// The cropped region's size is out of bounds.
+    OutOfBounds,
+    #[error("The cropped region is not visible.")]
+    /// The cropped region's size is zero.
+    Zero,
+}

runtime/src/window.rs

@@ -7,6 +7,7 @@ use crate::command::{self, Command};
 use crate::core::time::Instant;
 use crate::core::window::{Event, Icon, Level, Mode, UserAttention};
 use crate::futures::subscription::{self, Subscription};
+use crate::screenshot::Screenshot;
 
 /// Subscribes to the frames of the window of the running application.
 ///
@@ -115,3 +116,10 @@ pub fn fetch_id<Message>(
 pub fn change_icon<Message>(icon: Icon) -> Command<Message> {
     Command::single(command::Action::Window(Action::ChangeIcon(icon)))
 }
+
+/// Captures a [`Screenshot`] from the window.
+pub fn screenshot<Message>(
+    f: impl FnOnce(Screenshot) -> Message + Send + 'static,
+) -> Command<Message> {
+    Command::single(command::Action::Window(Action::Screenshot(Box::new(f))))
+}

runtime/src/window/action.rs

@@ -1,6 +1,7 @@
 use crate::core::window::{Icon, Level, Mode, UserAttention};
 use crate::futures::MaybeSend;
 
+use crate::screenshot::Screenshot;
 use std::fmt;
 
 /// An operation to be performed on some window.
@@ -89,6 +90,8 @@ pub enum Action<T> {
     /// - **X11:** Has no universal guidelines for icon sizes, so you're at the whims of the WM. That
     ///   said, it's usually in the same ballpark as on Windows.
     ChangeIcon(Icon),
+    /// Screenshot the viewport of the window.
+    Screenshot(Box<dyn FnOnce(Screenshot) -> T + 'static>),
 }
 
 impl<T> Action<T> {
@@ -118,6 +121,11 @@ impl<T> Action<T> {
             Self::ChangeLevel(level) => Action::ChangeLevel(level),
             Self::FetchId(o) => Action::FetchId(Box::new(move |s| f(o(s)))),
             Self::ChangeIcon(icon) => Action::ChangeIcon(icon),
+            Self::Screenshot(tag) => {
+                Action::Screenshot(Box::new(move |screenshot| {
+                    f(tag(screenshot))
+                }))
+            }
         }
     }
 }
@@ -155,6 +163,7 @@ impl<T> fmt::Debug for Action<T> {
             Self::ChangeIcon(_icon) => {
                 write!(f, "Action::ChangeIcon(icon)")
             }
+            Self::Screenshot(_) => write!(f, "Action::Screenshot"),
         }
     }
 }