feat(backend): backend provides window_size, add Size struct (#276)

For image (sixel, iTerm2, Kitty...) support that handles graphics in
terms of `Rect` so that the image area can be included in layouts.

For example: an image is loaded with a known pixel-size, and drawn, but
the image protocol has no mechanism of knowing the actual cell/character
area that been drawn on. It is then impossible to skip overdrawing the
area.

Returning the window size in pixel-width / pixel-height, together with
colums / rows, it can be possible to account the pixel size of each cell
/ character, and then known the `Rect` of a given image, and also resize
the image so that it fits exactly in a `Rect`.

Crossterm and termwiz also both return both sizes from one syscall,
while termion does two.

Add a `Size` struct for the cases where a `Rect`'s `x`/`y` is unused
(always zero).

`Size` is not "clipped" for `area < u16::max_value()` like `Rect`. This
is why there are `From` implementations between the two.

src/backend/crossterm.rs

@@ -18,9 +18,10 @@ use crossterm::{
 };
 
 use crate::{
-    backend::{Backend, ClearType},
+    backend::{Backend, ClearType, WindowSize},
     buffer::Cell,
-    layout::Rect,
+    layout::Size,
+    prelude::Rect,
     style::{Color, Modifier},
 };
 
@@ -169,12 +170,26 @@ where
     }
 
     fn size(&self) -> io::Result<Rect> {
-        let (width, height) =
-            terminal::size().map_err(|e| io::Error::new(io::ErrorKind::Other, e.to_string()))?;
-
+        let (width, height) = terminal::size()?;
         Ok(Rect::new(0, 0, width, height))
     }
 
+    fn window_size(&mut self) -> Result<WindowSize, io::Error> {
+        let crossterm::terminal::WindowSize {
+            columns,
+            rows,
+            width,
+            height,
+        } = terminal::window_size()?;
+        Ok(WindowSize {
+            columns_rows: Size {
+                width: columns,
+                height: rows,
+            },
+            pixels: Size { width, height },
+        })
+    }
+
     fn flush(&mut self) -> io::Result<()> {
         self.buffer.flush()
     }

src/backend/mod.rs

@@ -29,7 +29,7 @@ use std::io;
 
 use strum::{Display, EnumString};
 
-use crate::{buffer::Cell, layout::Rect};
+use crate::{buffer::Cell, layout::Size, prelude::Rect};
 
 #[cfg(feature = "termion")]
 mod termion;
@@ -60,6 +60,18 @@ pub enum ClearType {
     UntilNewLine,
 }
 
+/// The window sizes in columns,rows and optionally pixel width,height.
+pub struct WindowSize {
+    /// Size in character/cell columents,rows.
+    pub columns_rows: Size,
+    /// Size in pixel width,height.
+    ///
+    /// The `pixels` fields may not be implemented by all terminals and return `0,0`.
+    /// See https://man7.org/linux/man-pages/man4/tty_ioctl.4.html under section
+    /// "Get and set window size" / TIOCGWINSZ where the fields are commented as "unused".
+    pub pixels: Size,
+}
+
 /// The `Backend` trait provides an abstraction over different terminal libraries.
 /// It defines the methods required to draw content, manipulate the cursor, and
 /// clear the terminal screen.
@@ -111,9 +123,16 @@ pub trait Backend {
         }
     }
 
-    /// Get the size of the terminal screen as a [`Rect`].
+    /// Get the size of the terminal screen in columns/rows as a [`Rect`].
     fn size(&self) -> Result<Rect, io::Error>;
 
+    /// Get the size of the terminal screen in columns/rows and pixels as [`WindowSize`].
+    ///
+    /// The reason for this not returning only the pixel size, given the redundancy with the
+    /// `size()` method, is that the underlying backends most likely get both values with one
+    /// syscall, and the user is also most likely to need columns,rows together with pixel size.
+    fn window_size(&mut self) -> Result<WindowSize, io::Error>;
+
     /// Flush any buffered content to the terminal screen.
     fn flush(&mut self) -> Result<(), io::Error>;
 }

src/backend/termion.rs

@@ -10,9 +10,9 @@ use std::{
 };
 
 use crate::{
-    backend::{Backend, ClearType},
+    backend::{Backend, ClearType, WindowSize},
     buffer::Cell,
-    layout::Rect,
+    prelude::Rect,
     style::{Color, Modifier},
 };
 
@@ -160,6 +160,13 @@ where
         Ok(Rect::new(0, 0, terminal.0, terminal.1))
     }
 
+    fn window_size(&mut self) -> Result<WindowSize, io::Error> {
+        Ok(WindowSize {
+            columns_rows: termion::terminal_size()?.into(),
+            pixels: termion::terminal_size_pixels()?.into(),
+        })
+    }
+
     fn flush(&mut self) -> io::Result<()> {
         self.stdout.flush()
     }

src/backend/termwiz.rs

@@ -11,13 +11,14 @@ use termwiz::{
     cell::{AttributeChange, Blink, Intensity, Underline},
     color::{AnsiColor, ColorAttribute, SrgbaTuple},
     surface::{Change, CursorVisibility, Position},
-    terminal::{buffered::BufferedTerminal, SystemTerminal, Terminal},
+    terminal::{buffered::BufferedTerminal, ScreenSize, SystemTerminal, Terminal},
 };
 
 use crate::{
-    backend::Backend,
+    backend::{Backend, WindowSize},
     buffer::Cell,
-    layout::Rect,
+    layout::Size,
+    prelude::Rect,
     style::{Color, Modifier},
 };
 
@@ -169,22 +170,31 @@ impl Backend for TermwizBackend {
     }
 
     fn size(&self) -> Result<Rect, io::Error> {
-        let (term_width, term_height) = self.buffered_terminal.dimensions();
-        let max = u16::max_value();
-        Ok(Rect::new(
-            0,
-            0,
-            if term_width > usize::from(max) {
-                max
-            } else {
-                term_width as u16
+        let (cols, rows) = self.buffered_terminal.dimensions();
+        Ok(Rect::new(0, 0, u16_max(cols), u16_max(rows)))
+    }
+
+    fn window_size(&mut self) -> Result<WindowSize, io::Error> {
+        let ScreenSize {
+            cols,
+            rows,
+            xpixel,
+            ypixel,
+        } = self
+            .buffered_terminal
+            .terminal()
+            .get_screen_size()
+            .map_err(|e| io::Error::new(io::ErrorKind::Other, e))?;
+        Ok(WindowSize {
+            columns_rows: Size {
+                width: u16_max(cols),
+                height: u16_max(rows),
             },
-            if term_height > usize::from(max) {
-                max
-            } else {
-                term_height as u16
+            pixels: Size {
+                width: u16_max(xpixel),
+                height: u16_max(ypixel),
             },
-        ))
+        })
     }
 
     fn flush(&mut self) -> Result<(), io::Error> {
@@ -221,3 +231,8 @@ impl From<Color> for ColorAttribute {
         }
     }
 }
+
+#[inline]
+fn u16_max(i: usize) -> u16 {
+    u16::try_from(i).unwrap_or(u16::MAX)
+}

src/backend/test.rs

@@ -9,9 +9,9 @@ use std::{
 use unicode_width::UnicodeWidthStr;
 
 use crate::{
-    backend::Backend,
+    backend::{Backend, WindowSize},
     buffer::{Buffer, Cell},
-    layout::Rect,
+    layout::{Rect, Size},
 };
 
 /// A backend used for the integration tests.
@@ -179,6 +179,18 @@ impl Backend for TestBackend {
         Ok(Rect::new(0, 0, self.width, self.height))
     }
 
+    fn window_size(&mut self) -> Result<WindowSize, io::Error> {
+        // Some arbitrary window pixel size, probably doesn't need much testing.
+        static WINDOW_PIXEL_SIZE: Size = Size {
+            width: 640,
+            height: 480,
+        };
+        Ok(WindowSize {
+            columns_rows: (self.width, self.height).into(),
+            pixels: WINDOW_PIXEL_SIZE,
+        })
+    }
+
     fn flush(&mut self) -> Result<(), io::Error> {
         Ok(())
     }

src/layout.rs

@@ -647,6 +647,19 @@ fn try_split(area: Rect, layout: &Layout) -> Result<Rc<[Rect]>, AddConstraintErr
     Ok(results)
 }
 
+/// A simple size struct
+#[derive(Debug, Default, Clone, Copy, Eq, PartialEq, Hash)]
+pub struct Size {
+    pub width: u16,
+    pub height: u16,
+}
+
+impl From<(u16, u16)> for Size {
+    fn from((width, height): (u16, u16)) -> Self {
+        Size { width, height }
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use strum::ParseError;