4 files changed, 149 insertions, 0 deletions
diff --git a/runtime/src/program/state.rs b/runtime/src/program/state.rs
index d83e3f54..35df6078 100644
--- a/runtime/src/program/state.rs
+++ b/runtime/src/program/state.rs
@@ -1,6 +1,7 @@
 use crate::core::event::{self, Event};
 use crate::core::mouse;
 use crate::core::renderer;
+use crate::core::widget::operation::{self, Operation};
 use crate::core::{Clipboard, Size};
 use crate::user_interface::{self, UserInterface};
 use crate::{Command, Debug, Program};
@@ -173,6 +174,43 @@ where
 
         (uncaptured_events, command)
     }
+
+    /// Applies [`widget::Operation`]s to the [`State`]
+    pub fn operate(
+        &mut self,
+        renderer: &mut P::Renderer,
+        operations: impl Iterator<Item = Box<dyn Operation<P::Message>>>,
+        bounds: Size,
+        debug: &mut Debug,
+    ) {
+        let mut user_interface = build_user_interface(
+            &mut self.program,
+            self.cache.take().unwrap(),
+            renderer,
+            bounds,
+            debug,
+        );
+
+        for operation in operations {
+            let mut current_operation = Some(operation);
+
+            while let Some(mut operation) = current_operation.take() {
+                user_interface.operate(renderer, operation.as_mut());
+
+                match operation.finish() {
+                    operation::Outcome::None => {}
+                    operation::Outcome::Some(message) => {
+                        self.queued_messages.push(message)
+                    }
+                    operation::Outcome::Chain(next) => {
+                        current_operation = Some(next);
+                    }
+                };
+            }
+        }
+
+        self.cache = Some(user_interface.into_cache());
+    }
 }
 
 fn build_user_interface<'a, P: Program>(
diff --git a/runtime/src/window.rs b/runtime/src/window.rs
index 9356581a..5219fbfd 100644
--- a/runtime/src/window.rs
+++ b/runtime/src/window.rs
@@ -1,7 +1,10 @@
 //! Build window-based GUI applications.
 mod action;
 
+pub mod screenshot;
+
 pub use action::Action;
+pub use screenshot::Screenshot;
 
 use crate::command::{self, Command};
 use crate::core::time::Instant;
@@ -123,3 +126,10 @@ 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))))
+}
diff --git a/runtime/src/window/action.rs b/runtime/src/window/action.rs
index d0137895..b6964e36 100644
--- a/runtime/src/window/action.rs
+++ b/runtime/src/window/action.rs
@@ -1,6 +1,7 @@
 use crate::core::window::{Icon, Level, Mode, UserAttention};
 use crate::core::Size;
 use crate::futures::MaybeSend;
+use crate::window::Screenshot;
 
 use std::fmt;
 
@@ -87,6 +88,8 @@ 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> {
@@ -117,6 +120,11 @@ 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))
+                }))
+            }
         }
     }
 }
@@ -152,6 +160,7 @@ impl<T> fmt::Debug for Action<T> {
             Self::ChangeIcon(_icon) => {
                 write!(f, "Action::ChangeIcon(icon)")
             }
+            Self::Screenshot(_) => write!(f, "Action::Screenshot"),
         }
     }
 }
diff --git a/runtime/src/window/screenshot.rs b/runtime/src/window/screenshot.rs
new file mode 100644
index 00000000..c84286b6
--- /dev/null
+++ b/runtime/src/window/screenshot.rs
@@ -0,0 +1,92 @@
+//! Take screenshots of a window.
+use crate::core::{Rectangle, Size};
+
+use std::fmt::{Debug, Formatter};
+use std::sync::Arc;
+
+/// 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: Arc<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: Arc::new(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: Arc::new(chopped),
+            size: Size::new(region.width, region.height),
+        })
+    }
+}
+
+impl AsRef<[u8]> for Screenshot {
+    fn as_ref(&self) -> &[u8] {
+        &self.bytes
+    }
+}
+
+#[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,
+}