feat(examples): user_input example cursor movement (#302)

The user_input example now responds to left/right and allows the
character at the cursor position to be deleted / inserted.

Co-authored-by: Leon Sautour <leon1.sautour@epitech.eu>

examples/user_input.rs

@@ -6,18 +6,20 @@ use std::{error::Error, io};
 /// started.
 ///
 /// This is a very simple example:
-///   * A input box always focused. Every character you type is registered
-///   here
-///   * Pressing Backspace erases a character
+///   * An input box always focused. Every character you type is registered
+///   here.
+///   * An entered character is inserted at the cursor position.
+///   * Pressing Backspace erases the left character before the cursor position
 ///   * Pressing Enter pushes the current input in the history of previous
-///   messages
+///   messages.
+/// **Note: ** as this is a relatively simple example unicode characters are unsupported and
+/// their use will result in undefined behaviour.
 use crossterm::{
     event::{self, DisableMouseCapture, EnableMouseCapture, Event, KeyCode, KeyEventKind},
     execute,
     terminal::{disable_raw_mode, enable_raw_mode, EnterAlternateScreen, LeaveAlternateScreen},
 };
 use ratatui::prelude::*;
-use unicode_width::UnicodeWidthStr;
 
 enum InputMode {
     Normal,
@@ -28,6 +30,8 @@ enum InputMode {
 struct App {
     /// Current value of the input box
     input: String,
+    /// Position of cursor in the editor area.
+    cursor_position: usize,
     /// Current input mode
     input_mode: InputMode,
     /// History of recorded messages
@@ -40,10 +44,65 @@ impl Default for App {
             input: String::new(),
             input_mode: InputMode::Normal,
             messages: Vec::new(),
+            cursor_position: 0,
         }
     }
 }
 
+impl App {
+    fn move_cursor_left(&mut self) {
+        let cursor_moved_left = self.cursor_position.saturating_sub(1);
+        self.cursor_position = self.clamp_cursor(cursor_moved_left);
+    }
+
+    fn move_cursor_right(&mut self) {
+        let cursor_moved_right = self.cursor_position.saturating_add(1);
+        self.cursor_position = self.clamp_cursor(cursor_moved_right);
+    }
+
+    fn enter_char(&mut self, new_char: char) {
+        self.input.insert(self.cursor_position, new_char);
+
+        self.move_cursor_right();
+    }
+
+    fn delete_char(&mut self) {
+        let is_not_cursor_leftmost = self.cursor_position != 0;
+        if is_not_cursor_leftmost {
+            // Method "remove" is not used on the saved text for deleting the selected char.
+            // Reason: Using remove on String works on bytes instead of the chars.
+            // Using remove would require special care because of char boundaries.
+
+            let current_index = self.cursor_position;
+            let from_left_to_current_index = current_index - 1;
+
+            // Getting all characters before the selected character.
+            let before_char_to_delete = self.input.chars().take(from_left_to_current_index);
+            // Getting all characters after selected character.
+            let after_char_to_delete = self.input.chars().skip(current_index);
+
+            // Put all characters together except the selected one.
+            // By leaving the selected one out, it is forgotten and therefore deleted.
+            self.input = before_char_to_delete.chain(after_char_to_delete).collect();
+            self.move_cursor_left();
+        }
+    }
+
+    fn clamp_cursor(&self, new_cursor_pos: usize) -> usize {
+        new_cursor_pos.clamp(0, self.input.len())
+    }
+
+    fn reset_cursor(&mut self) {
+        self.cursor_position = 0;
+    }
+
+    fn submit_message(&mut self) {
+        self.messages.push(self.input.clone());
+        self.input.clear();
+        self.reset_cursor();
+    }
+}
+
 fn main() -> Result<(), Box<dyn Error>> {
     // setup terminal
     enable_raw_mode()?;
@@ -88,14 +147,18 @@ fn run_app<B: Backend>(terminal: &mut Terminal<B>, mut app: App) -> io::Result<(
                     _ => {}
                 },
                 InputMode::Editing if key.kind == KeyEventKind::Press => match key.code {
-                    KeyCode::Enter => {
-                        app.messages.push(app.input.drain(..).collect());
-                    }
-                    KeyCode::Char(c) => {
-                        app.input.push(c);
+                    KeyCode::Enter => app.submit_message(),
+                    KeyCode::Char(to_insert) => {
+                        app.enter_char(to_insert);
                     }
                     KeyCode::Backspace => {
-                        app.input.pop();
+                        app.delete_char();
+                    }
+                    KeyCode::Left => {
+                        app.move_cursor_left();
+                    }
+                    KeyCode::Right => {
+                        app.move_cursor_right();
                     }
                     KeyCode::Esc => {
                         app.input_mode = InputMode::Normal;
@@ -165,8 +228,9 @@ fn ui<B: Backend>(f: &mut Frame<B>, app: &App) {
             // Make the cursor visible and ask ratatui to put it at the specified coordinates after
             // rendering
             f.set_cursor(
-                // Put cursor past the end of the input text
-                chunks[1].x + app.input.width() as u16 + 1,
+                // Draw the cursor at the current position in the input field.
+                // This position is can be controlled via the left and right arrow key
+                chunks[1].x + app.cursor_position as u16 + 1,
                 // Move one line down, from the border to the input line
                 chunks[1].y + 1,
             )