refactor(example): add scroll to flex example (#811)

This commit adds `scroll` to the flex example. It also adds more examples to showcase how constraints interact. It improves the UI to make it easier to understand and short terminal friendly.

<img width="380" alt="image" src="https://github.com/ratatui-org/ratatui/assets/1813121/30541efc-ecbe-4e28-b4ef-4d5f1dc63fec"/>

---------

Co-authored-by: Dheepak Krishnamurthy <me@kdheepak.com>

Cargo.toml

@@ -220,7 +220,7 @@ doc-scrape-examples = false
 [[example]]
 name = "flex"
 required-features = ["crossterm"]
-doc-scrape-examples = false
+doc-scrape-examples = true
 
 [[example]]
 name = "list"

examples/flex.rs

@@ -5,13 +5,15 @@ use crossterm::{
     execute,
     terminal::{disable_raw_mode, enable_raw_mode, EnterAlternateScreen, LeaveAlternateScreen},
 };
-use itertools::Itertools;
 use ratatui::{
     layout::{Constraint::*, Flex},
     prelude::*,
-    widgets::*,
+    widgets::{block::Title, *},
 };
 
+const EXAMPLE_HEIGHT: u16 = 5;
+const N_EXAMPLES_PER_TAB: u16 = 11;
+
 fn main() -> Result<(), Box<dyn Error>> {
     // setup terminal
     enable_raw_mode()?;
@@ -36,28 +38,116 @@ fn main() -> Result<(), Box<dyn Error>> {
 }
 
 fn run_app<B: Backend>(terminal: &mut Terminal<B>) -> io::Result<()> {
-    let mut selection = ExampleSelection::Stretch;
+    // we always want to show the last example when scrolling
+    let mut app = App::default().max_scroll_offset((N_EXAMPLES_PER_TAB - 1) * EXAMPLE_HEIGHT);
+
     loop {
-        terminal.draw(|f| f.render_widget(selection, f.size()))?;
+        terminal.draw(|f| f.render_widget(app, f.size()))?;
 
         if let Event::Key(key) = event::read()? {
             use KeyCode::*;
             match key.code {
                 Char('q') => break Ok(()),
-                Char('j') | Char('l') | Down | Right => {
-                    selection = selection.next();
-                }
-                Char('k') | Char('h') | Up | Left => {
-                    selection = selection.previous();
-                }
+                Char('l') | Right => app.next(),
+                Char('h') | Left => app.previous(),
+                Char('j') | Down => app.down(),
+                Char('k') | Up => app.up(),
                 _ => (),
             }
         }
     }
 }
 
-#[derive(Debug, Copy, Clone)]
+#[derive(Default, Clone, Copy)]
+struct App {
+    selected_example: ExampleSelection,
+    scroll_offset: u16,
+    max_scroll_offset: u16,
+}
+
+impl App {
+    fn max_scroll_offset(mut self, max_scroll_offset: u16) -> Self {
+        self.max_scroll_offset = max_scroll_offset;
+        self
+    }
+    fn next(&mut self) {
+        self.selected_example = self.selected_example.next();
+    }
+    fn previous(&mut self) {
+        self.selected_example = self.selected_example.previous();
+    }
+    fn up(&mut self) {
+        self.scroll_offset = self.scroll_offset.saturating_sub(1)
+    }
+    fn down(&mut self) {
+        self.scroll_offset = self
+            .scroll_offset
+            .saturating_add(1)
+            .min(self.max_scroll_offset)
+    }
+
+    fn render_tabs(&self, area: Rect, buf: &mut Buffer) {
+        Tabs::new(
+            [
+                ExampleSelection::Stretch,
+                ExampleSelection::StretchLast,
+                ExampleSelection::Start,
+                ExampleSelection::Center,
+                ExampleSelection::End,
+                ExampleSelection::SpaceAround,
+                ExampleSelection::SpaceBetween,
+            ]
+            .iter()
+            .map(|e| format!("{:?}", e)),
+        )
+        .block(
+            Block::bordered()
+                .title(Title::from("Flex Layouts ".bold()))
+                .title(" Use h l or ◄ ► to change tab and j k or ▲ ▼  to scroll".bold()),
+        )
+        .highlight_style(Style::default().yellow().bold())
+        .select(self.selected_example.selected())
+        .padding(" ", " ")
+        .render(area, buf);
+    }
+}
+
+impl Widget for App {
+    fn render(self, area: Rect, buf: &mut Buffer) {
+        let [tabs_area, demo_area] = area.split(&Layout::vertical([Fixed(3), Proportional(0)]));
+
+        // render demo content into a separate buffer so all examples fit
+        let mut demo_buf = Buffer::empty(Rect::new(
+            0,
+            0,
+            buf.area.width,
+            N_EXAMPLES_PER_TAB * EXAMPLE_HEIGHT,
+        ));
+
+        self.selected_example.render(demo_buf.area, &mut demo_buf);
+
+        // render tabs into a separate buffer
+        let mut tabs_buf = Buffer::empty(tabs_area);
+        self.render_tabs(tabs_area, &mut tabs_buf);
+
+        // Assemble both buffers
+        // NOTE: You shouldn't do this in a production app
+        buf.content = tabs_buf.content;
+        buf.content.append(
+            &mut demo_buf
+                .content
+                .into_iter()
+                .skip((buf.area.width * self.scroll_offset) as usize)
+                .take(demo_area.area() as usize)
+                .collect(),
+        );
+        buf.resize(buf.area);
+    }
+}
+
+#[derive(Default, Debug, Copy, Clone)]
 enum ExampleSelection {
+    #[default]
     Stretch,
     StretchLast,
     Start,
@@ -110,10 +200,6 @@ impl ExampleSelection {
 
 impl Widget for ExampleSelection {
     fn render(self, area: Rect, buf: &mut Buffer) {
-        let [tabs, area] = area.split(&Layout::vertical([Fixed(3), Proportional(0)]));
-
-        self.render_tabs(tabs, buf);
-
         match self {
             ExampleSelection::Stretch => self.render_example(area, buf, Flex::Stretch),
             ExampleSelection::StretchLast => self.render_example(area, buf, Flex::StretchLast),
@@ -127,55 +213,34 @@ impl Widget for ExampleSelection {
 }
 
 impl ExampleSelection {
-    fn render_tabs(&self, area: Rect, buf: &mut Buffer) {
-        Tabs::new(
-            [
-                ExampleSelection::Stretch,
-                ExampleSelection::StretchLast,
-                ExampleSelection::Start,
-                ExampleSelection::Center,
-                ExampleSelection::End,
-                ExampleSelection::SpaceAround,
-                ExampleSelection::SpaceBetween,
-            ]
-            .iter()
-            .map(|e| format!("{:?}", e)),
-        )
-        .block(Block::bordered().title("Flex Layouts"))
-        .highlight_style(Style::default().yellow())
-        .select(self.selected())
-        .padding("  ", "  ")
-        .render(area, buf);
-    }
-
     fn render_example(&self, area: Rect, buf: &mut Buffer, flex: Flex) {
-        let [example1, example2, example3, example4, example5, example6, _] =
-            area.split(&Layout::vertical([Fixed(8); 7]));
+        let areas = &Layout::vertical([Fixed(EXAMPLE_HEIGHT); N_EXAMPLES_PER_TAB as usize])
+            .flex(Flex::Start)
+            .split(area);
 
-        Example::new([Length(20), Length(10)])
-            .flex(flex)
-            .render(example1, buf);
-        Example::new([Length(20), Fixed(10)])
-            .flex(flex)
-            .render(example2, buf);
-        Example::new([Proportional(1), Proportional(1), Length(40), Fixed(20)])
-            .flex(flex)
-            .render(example3, buf);
-        Example::new([Min(20), Length(40), Fixed(20)])
-            .flex(flex)
-            .render(example4, buf);
-        Example::new([Min(20), Proportional(0), Length(40), Fixed(20)])
-            .flex(flex)
-            .render(example5, buf);
-        Example::new([
-            Min(20),
-            Proportional(0),
-            Percentage(10),
-            Length(40),
-            Fixed(20),
-        ])
-        .flex(flex)
-        .render(example6, buf);
+        let examples = [
+            vec![Length(20), Fixed(20), Percentage(20)],
+            vec![Fixed(20), Percentage(20), Length(20)],
+            vec![Percentage(20), Length(20), Fixed(20)],
+            vec![Length(20), Length(15)],
+            vec![Length(20), Fixed(20)],
+            vec![Min(20), Max(20)],
+            vec![Max(20)],
+            vec![Min(20), Max(20), Length(20), Fixed(20)],
+            vec![Proportional(0), Proportional(0)],
+            vec![Proportional(1), Proportional(1), Length(20), Fixed(20)],
+            vec![
+                Min(10),
+                Proportional(3),
+                Proportional(2),
+                Length(15),
+                Fixed(15),
+            ],
+        ];
+
+        for (area, constraints) in areas.iter().zip(examples) {
+            Example::new(constraints).flex(flex).render(*area, buf);
+        }
     }
 }
 
@@ -203,37 +268,30 @@ impl Example {
 
 impl Widget for Example {
     fn render(self, area: Rect, buf: &mut Buffer) {
-        let [title, legend, area] = area.split(&Layout::vertical([Ratio(1, 3); 3]));
+        let [legend, area] = area.split(&Layout::vertical([Ratio(1, 3); 2]));
         let blocks = Layout::horizontal(&self.constraints)
             .flex(self.flex)
             .split(area);
 
-        self.heading().render(title, buf);
-
         self.legend(legend.width as usize).render(legend, buf);
 
-        for (i, (block, _constraint)) in blocks.iter().zip(&self.constraints).enumerate() {
+        for (block, constraint) in blocks.iter().zip(&self.constraints) {
             let text = format!("{} px", block.width);
-            let fg = Color::Indexed(i as u8 + 1);
-            self.illustration(text, fg).render(*block, buf);
+            let fg = match constraint {
+                Constraint::Ratio(_, _) => Color::Indexed(1),
+                Constraint::Percentage(_) => Color::Indexed(2),
+                Constraint::Max(_) => Color::Indexed(3),
+                Constraint::Min(_) => Color::Indexed(4),
+                Constraint::Length(_) => Color::Indexed(5),
+                Constraint::Fixed(_) => Color::Indexed(6),
+                Constraint::Proportional(_) => Color::Indexed(7),
+            };
+            self.illustration(*constraint, text, fg).render(*block, buf);
         }
     }
 }
 
 impl Example {
-    fn heading(&self) -> Paragraph {
-        // Renders the following
-        //
-        // Fixed(40), Proportional(0)
-        let spans = self.constraints.iter().enumerate().map(|(i, c)| {
-            let color = Color::Indexed(i as u8 + 1);
-            Span::styled(format!("{:?}", c), color)
-        });
-        let heading =
-            Line::from(Itertools::intersperse(spans, Span::raw(", ")).collect::<Vec<Span>>());
-        Paragraph::new(heading).block(Block::default().padding(Padding::vertical(1)))
-    }
-
     fn legend(&self, width: usize) -> Paragraph {
         // a bar like `<----- 80 px ----->`
         let width_label = format!("{} px", width);
@@ -241,12 +299,23 @@ impl Example {
             "<{width_label:-^width$}>",
             width = width - width_label.len() / 2
         );
-        Paragraph::new(width_bar.dark_gray()).alignment(Alignment::Center)
+        Paragraph::new(width_bar.dark_gray())
+            .alignment(Alignment::Center)
+            .block(Block::default().padding(Padding {
+                left: 0,
+                right: 0,
+                top: 1,
+                bottom: 0,
+            }))
     }
 
-    fn illustration(&self, text: String, fg: Color) -> Paragraph {
-        Paragraph::new(text)
+    fn illustration(&self, constraint: Constraint, text: String, fg: Color) -> Paragraph {
+        Paragraph::new(format!("{:?}", constraint))
             .alignment(Alignment::Center)
-            .block(Block::bordered().style(Style::default().fg(fg)))
+            .block(
+                Block::bordered()
+                    .style(Style::default().fg(fg))
+                    .title(block::Title::from(text).alignment(Alignment::Center)),
+            )
     }
 }

examples/flex.tape

@@ -0,0 +1,17 @@
+# This is a vhs script. See https://github.com/charmbracelet/vhs for more info.
+# To run this script, install vhs and run `vhs ./examples/layout.tape`
+Output "target/flex.gif"
+Set Theme "Aardvark Blue"
+Set Width 1200
+Set Height 1410
+Hide
+Type "cargo run --example=flex --features=crossterm"
+Enter
+Sleep 2s
+Show
+Sleep 2s
+Right @5s 7
+Sleep 2s
+Left 7
+Sleep 2s
+Down @200ms 50