Move IMAP operations to background thread for responsive UI

Spawn a worker thread that owns the IMAP session and processes
fetch commands via channels. The UI thread polls for results
non-blockingly, keeping the app responsive during network operations.
Shows loading indicator while fetching.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/inbox.rs

@@ -7,19 +7,10 @@ const BATCH_SIZE: u32 = 50;
 
 pub(crate) struct Inbox {
     pub emails: Vec<Email>,
-    /// The lowest IMAP sequence number we've fetched so far (1-based).
+    pub oldest_seq: Option<u32>,
-    /// None means we haven't fetched yet or inbox was empty.
-    oldest_seq: Option<u32>,
 }
 
 impl Inbox {
-    pub fn new() -> Self {
-        Self {
-            emails: Vec::new(),
-            oldest_seq: None,
-        }
-    }
-
     pub fn has_older(&self) -> bool {
         self.oldest_seq.map_or(false, |s| s > 1)
     }
@@ -30,14 +21,12 @@ pub(crate) fn refresh(
     session: &mut Option<ImapSession>,
     config: &Config,
 ) -> Result<Inbox, String> {
-    // If we have a session, try NOOP to keep alive / detect changes
     if let Some(s) = session.as_mut() {
         if s.noop().is_ok() {
             return fetch_latest(s);
         }
     }
 
-    // Session is dead or missing — reconnect
     *session = None;
     let mut new_session = connect::connect(config)?;
     let result = fetch_latest(&mut new_session);
@@ -45,42 +34,39 @@ pub(crate) fn refresh(
     result
 }
 
-/// Fetch older emails and append them to the existing inbox.
+/// Fetch a batch of older emails. Returns (emails, new_oldest_seq).
-pub(crate) fn fetch_more(
+pub(crate) fn fetch_older_batch(
     session: &mut Option<ImapSession>,
-    inbox: &mut Inbox,
+    oldest_seq: u32,
     config: &Config,
-) -> Result<(), String> {
+) -> Result<(Vec<Email>, u32), String> {
-    let oldest = match inbox.oldest_seq {
+    if oldest_seq <= 1 {
-        Some(s) if s > 1 => s,
+        return Ok((Vec::new(), 1));
-        _ => return Ok(()),
+    }
-    };
 
-    let ensure_session = |session: &mut Option<ImapSession>| -> Result<(), String> {
+    // Ensure session is alive
-        if session.as_mut().map_or(true, |s| s.noop().is_err()) {
+    if session.as_mut().map_or(true, |s| s.noop().is_err()) {
-            *session = None;
+        *session = None;
-            *session = Some(connect::connect(config)?);
+        *session = Some(connect::connect(config)?);
-        }
+    }
-        Ok(())
-    };
-    ensure_session(session)?;
 
     let s = session.as_mut().unwrap();
-    let end = oldest - 1;
+    let end = oldest_seq - 1;
     let start = end.saturating_sub(BATCH_SIZE - 1).max(1);
     let range = format!("{}:{}", start, end);
 
-    let mut older = fetch_range_emails(s, &range)?;
+    let mut emails = fetch_range_emails(s, &range)?;
-    older.reverse();
+    emails.reverse();
-    inbox.oldest_seq = Some(start);
+    Ok((emails, start))
-    inbox.emails.append(&mut older);
-    Ok(())
 }
 
 fn fetch_latest(session: &mut ImapSession) -> Result<Inbox, String> {
     let exists = select_inbox(session)?;
     if exists == 0 {
-        return Ok(Inbox::new());
+        return Ok(Inbox {
+            emails: Vec::new(),
+            oldest_seq: None,
+        });
     }
     let start = exists.saturating_sub(BATCH_SIZE - 1).max(1);
     let range = format!("{}:{}", start, exists);
@@ -149,4 +135,4 @@ fn parse_emails(fetches: &[imap::types::Fetch]) -> Vec<Email> {
         }
     }
     emails
-}
+}

src/lib.rs

@@ -1,4 +1,6 @@
 use std::io::{Error, Stdout};
+use std::sync::mpsc;
+use std::thread;
 use std::time::{Duration, Instant};
 use crossterm::event;
 use crossterm::event::{Event, KeyCode};
@@ -15,27 +17,107 @@ mod inbox;
 
 const POLL_INTERVAL: Duration = Duration::from_secs(30);
 
-struct Email {
+pub(crate) struct Email {
-    subject: String,
+    pub subject: String,
-    from: String,
+    pub from: String,
-    date: String,
+    pub date: String,
 }
 
-pub fn main(config: &Config, terminal: &mut Terminal<CrosstermBackend<Stdout>>) -> Result<(), Error> {
+enum WorkerCmd {
-    let mut session = connect::connect(config).ok();
+    Refresh,
-    let mut inbox = inbox::refresh(&mut session, config);
+    FetchMore { oldest_seq: u32 },
-    let mut last_fetch = Instant::now();
+    Quit,
-    let mut list_state = ListState::default();
+}
-    if let Ok(ib) = &inbox {
+
-        if !ib.emails.is_empty() {
+enum WorkerResult {
-            list_state.select(Some(0));
+    Refreshed(Result<inbox::Inbox, String>),
+    FetchedMore(Result<(Vec<Email>, u32), String>),
+}
+
+fn worker_loop(
+    config: Config,
+    cmd_rx: mpsc::Receiver<WorkerCmd>,
+    result_tx: mpsc::Sender<WorkerResult>,
+) {
+    let mut session = connect::connect(&config).ok();
+
+    for cmd in &cmd_rx {
+        match cmd {
+            WorkerCmd::Refresh => {
+                let result = inbox::refresh(&mut session, &config);
+                let _ = result_tx.send(WorkerResult::Refreshed(result));
+            }
+            WorkerCmd::FetchMore { oldest_seq } => {
+                let result = inbox::fetch_older_batch(&mut session, oldest_seq, &config);
+                let _ = result_tx.send(WorkerResult::FetchedMore(result));
+            }
+            WorkerCmd::Quit => break,
         }
     }
 
+    if let Some(mut s) = session.take() {
+        let _ = s.logout();
+    }
+}
+
+pub fn main(config: &Config, terminal: &mut Terminal<CrosstermBackend<Stdout>>) -> Result<(), Error> {
+    let (cmd_tx, cmd_rx) = mpsc::channel();
+    let (result_tx, result_rx) = mpsc::channel();
+
+    let worker_config = config.clone();
+    let worker = thread::spawn(move || {
+        worker_loop(worker_config, cmd_rx, result_tx);
+    });
+
+    // Send initial refresh
+    let _ = cmd_tx.send(WorkerCmd::Refresh);
+
+    let mut emails: Vec<Email> = Vec::new();
+    let mut oldest_seq: Option<u32> = None;
+    let mut has_older = false;
+    let mut error: Option<String> = None;
+    let mut loading = true;
+    let mut list_state = ListState::default();
+    let mut last_fetch = Instant::now();
+
     // --- Main loop ---
     loop {
-        if last_fetch.elapsed() >= POLL_INTERVAL {
+        // Check for worker results (non-blocking)
-            inbox = inbox::refresh(&mut session, config);
+        while let Ok(result) = result_rx.try_recv() {
+            match result {
+                WorkerResult::Refreshed(Ok(inbox)) => {
+                    has_older = inbox.has_older();
+                    oldest_seq = inbox.oldest_seq;
+                    emails = inbox.emails;
+                    error = None;
+                    loading = false;
+                    if !emails.is_empty() {
+                        list_state.select(Some(0));
+                    } else {
+                        list_state.select(None);
+                    }
+                }
+                WorkerResult::Refreshed(Err(e)) => {
+                    error = Some(e);
+                    loading = false;
+                }
+                WorkerResult::FetchedMore(Ok((mut new_emails, new_oldest))) => {
+                    has_older = new_oldest > 1;
+                    oldest_seq = Some(new_oldest);
+                    emails.append(&mut new_emails);
+                    loading = false;
+                }
+                WorkerResult::FetchedMore(Err(e)) => {
+                    error = Some(e);
+                    loading = false;
+                }
+            }
+        }
+
+        // Periodic refresh
+        if !loading && last_fetch.elapsed() >= POLL_INTERVAL {
+            let _ = cmd_tx.send(WorkerCmd::Refresh);
+            loading = true;
             last_fetch = Instant::now();
         }
 
@@ -46,43 +128,50 @@ pub fn main(config: &Config, terminal: &mut Terminal<CrosstermBackend<Stdout>>)
                 .constraints([Constraint::Min(3), Constraint::Length(1)])
                 .split(area);
 
-            match &inbox {
+            if let Some(e) = &error {
-                Ok(ib) if !ib.emails.is_empty() => {
+                let p = Paragraph::new(format!("IMAP error: {}", e))
-                    let items: Vec<ListItem> = ib.emails
+                    .block(Block::default().title("Inbox").borders(Borders::ALL))
-                        .iter()
+                    .style(Style::default().fg(Color::Red));
-                        .map(|e| {
+                frame.render_widget(p, layout[0]);
-                            ListItem::new(format!("{}  |  {}  |  {}", e.date, e.from, e.subject))
+            } else if emails.is_empty() && loading {
-                        })
+                let p = Paragraph::new("Loading...")
-                        .collect();
+                    .block(Block::default().title("Inbox").borders(Borders::ALL))
+                    .style(Style::default().fg(Color::Yellow));
+                frame.render_widget(p, layout[0]);
+            } else if emails.is_empty() {
+                let p = Paragraph::new("No messages in inbox.")
+                    .block(Block::default().title("Inbox").borders(Borders::ALL))
+                    .style(Style::default().fg(Color::Yellow));
+                frame.render_widget(p, layout[0]);
+            } else {
+                let items: Vec<ListItem> = emails
+                    .iter()
+                    .map(|e| {
+                        ListItem::new(format!("{}  |  {}  |  {}", e.date, e.from, e.subject))
+                    })
+                    .collect();
 
-                    let list = List::new(items)
+                let title = if loading {
-                        .block(
+                    format!("Inbox ({} messages) [loading...]", emails.len())
-                            Block::default()
+                } else {
-                                .title(format!("Inbox ({} messages)", ib.emails.len()))
+                    format!("Inbox ({} messages)", emails.len())
-                                .borders(Borders::ALL),
+                };
-                        )
-                        .style(Style::default().fg(Color::White))
-                        .highlight_style(
-                            Style::default()
-                                .bg(Color::DarkGray)
-                                .add_modifier(Modifier::BOLD),
-                        )
-                        .highlight_symbol(">> ");
 
-                    frame.render_stateful_widget(list, layout[0], &mut list_state);
+                let list = List::new(items)
-                }
+                    .block(
-                Ok(_) => {
+                        Block::default()
-                    let p = Paragraph::new("No messages in inbox.")
+                            .title(title)
-                        .block(Block::default().title("Inbox").borders(Borders::ALL))
+                            .borders(Borders::ALL),
-                        .style(Style::default().fg(Color::Yellow));
+                    )
-                    frame.render_widget(p, layout[0]);
+                    .style(Style::default().fg(Color::White))
-                }
+                    .highlight_style(
-                Err(e) => {
+                        Style::default()
-                    let p = Paragraph::new(format!("IMAP error: {}", e))
+                            .bg(Color::DarkGray)
-                        .block(Block::default().title("Inbox").borders(Borders::ALL))
+                            .add_modifier(Modifier::BOLD),
-                        .style(Style::default().fg(Color::Red));
+                    )
-                    frame.render_widget(p, layout[0]);
+                    .highlight_symbol(">> ");
-                }
+
+                frame.render_stateful_widget(list, layout[0], &mut list_state);
             }
 
             let status = Paragraph::new(" 'q' quit | 'r' refresh | ↑/↓ navigate")
@@ -96,32 +185,28 @@ pub fn main(config: &Config, terminal: &mut Terminal<CrosstermBackend<Stdout>>)
                 match key.code {
                     KeyCode::Char('q') | KeyCode::Esc => break,
                     KeyCode::Char('r') => {
-                        inbox = inbox::refresh(&mut session, config);
+                        if !loading {
-                        last_fetch = Instant::now();
+                            let _ = cmd_tx.send(WorkerCmd::Refresh);
-                        if let Ok(ib) = &inbox {
+                            loading = true;
-                            if !ib.emails.is_empty() {
+                            last_fetch = Instant::now();
-                                list_state.select(Some(0));
-                            } else {
-                                list_state.select(None);
-                            }
                         }
                     }
                     KeyCode::Down | KeyCode::Char('j') => {
-                        if let Ok(ib) = &mut inbox {
+                        let len = emails.len();
-                            let len = ib.emails.len();
+                        if len > 0 {
                             let i = list_state.selected().map_or(0, |i| i + 1);
-                            if i >= len && ib.has_older() {
+                            if i >= len && has_older && !loading {
-                                let _ = inbox::fetch_more(&mut session, ib, config);
+                                if let Some(seq) = oldest_seq {
+                                    let _ = cmd_tx.send(WorkerCmd::FetchMore { oldest_seq: seq });
+                                    loading = true;
+                                }
                             }
-                            let new_len = ib.emails.len();
+                            list_state.select(Some(i.min(len.saturating_sub(1))));
-                            list_state.select(Some(i.min(new_len.saturating_sub(1))));
                         }
                     }
                     KeyCode::Up | KeyCode::Char('k') => {
-                        if let Ok(_) = &inbox {
+                        let i = list_state.selected().map_or(0, |i| i.saturating_sub(1));
-                            let i = list_state.selected().map_or(0, |i| i.saturating_sub(1));
+                        list_state.select(Some(i));
-                            list_state.select(Some(i));
-                        }
                     }
                     _ => {}
                 }
@@ -129,10 +214,9 @@ pub fn main(config: &Config, terminal: &mut Terminal<CrosstermBackend<Stdout>>)
         }
     }
 
-    // Clean up the session
+    // Clean up worker
-    if let Some(mut s) = session.take() {
+    let _ = cmd_tx.send(WorkerCmd::Quit);
-        let _ = s.logout();
+    let _ = worker.join();
-    }
 
     Ok(())
 }