Make buffers return their items lazily from the end instead of materializing them

src/buffers/log.rs

@@ -24,7 +24,7 @@ use tokio::sync::mpsc::UnboundedReceiver;
 use tracing_error::ErrorLayer;
 use tracing_subscriber::prelude::*;
 
-use super::Buffer;
+use super::{Buffer, BufferItem};
 
 /// Maximum number of log lines to be stored in memory
 const MAX_MEM_LOG_LINES: usize = 100;
@@ -61,19 +61,21 @@ impl Buffer for LogBuffer {
     self.lines.push_back(line);
   }
 
-  fn content(&self) -> Vec<Text> {
+  fn content<'a>(&'a self) -> Box<dyn Iterator<Item = BufferItem<'a>> + 'a> {
     use ansi_to_tui::IntoText;
     let (slice1, slice2) = self.lines.as_slices();
-    slice1
-      .into_iter()
-      .chain(slice2.into_iter())
-      .cloned()
-      .map(|line| {
-        line.into_text().unwrap_or_else(|e| {
-          tracing::error!("Could not convert line from ANSI codes to ratatui: {}", e);
-          Text::raw(line)
-        })
-      })
-      .collect()
+    Box::new(
+      slice1
+        .into_iter()
+        .chain(slice2.into_iter())
+        .rev()
+        .cloned()
+        .map(|line| BufferItem {
+          text: line.into_text().unwrap_or_else(|e| {
+            tracing::error!("Could not convert line from ANSI codes to ratatui: {}", e);
+            Text::raw(line)
+          }),
+        }),
+    )
   }
 }

src/buffers/mod.rs

@@ -18,12 +18,17 @@ use futures::stream::FuturesUnordered;
 use futures::StreamExt;
 use matrix_sdk::async_trait;
 use nonempty::NonEmpty;
+use ratatui::text::Text;
 
 mod log;
 pub use log::LogBuffer;
 mod room;
 pub use room::RoomBuffer;
 
+pub struct BufferItem<'a> {
+  pub text: Text<'a>,
+}
+
 #[async_trait]
 pub trait Buffer: Send + Sync {
   /// A short human-readable name for the room, eg. to show in compact buflist
@@ -34,7 +39,7 @@ pub trait Buffer: Send + Sync {
   }
   /// Returns if there are any updates to apply.
   async fn poll_updates(&mut self);
-  fn content(&self) -> Vec<ratatui::text::Text>; // TODO: make this lazy, only the last few are used
+  fn content<'a>(&'a self) -> Box<dyn Iterator<Item = BufferItem<'a>> + 'a>;
   /// Called when the user is being showned the oldest items this buffer returned.
   ///
   /// This should return immediately, not waiting for anything to be loaded.

src/buffers/room.rs

@@ -32,7 +32,7 @@ use matrix_sdk_ui::timeline::{
 use ratatui::text::Text;
 use smallvec::SmallVec;
 
-use super::Buffer;
+use super::{Buffer, BufferItem};
 
 pub struct SingleClientRoomBuffer {
   room_id: OwnedRoomId,
@@ -304,16 +304,20 @@ impl Buffer for RoomBuffer {
     .await;
   }
 
-  fn content(&self) -> Vec<Text> {
+  fn content<'a>(&'a self) -> Box<dyn Iterator<Item = BufferItem<'a>> + 'a> {
     // TODO: merge buffers, etc.
-    self
-      .buffers
-      .first()
-      .unwrap_or_else(|| panic!("No sub-buffer for {}", self.room_id))
-      .items
-      .iter()
-      .map(|line| Text::raw(line))
-      .collect()
+    Box::new(
+      self
+        .buffers
+        .first()
+        .unwrap_or_else(|| panic!("No sub-buffer for {}", self.room_id))
+        .items
+        .iter()
+        .rev()
+        .map(|line| BufferItem {
+          text: Text::raw(line),
+        }),
+    )
   }
 
   fn request_back_pagination(&self, num: u16) {

src/components/backlog.rs

@@ -64,8 +64,6 @@ impl Component for Backlog {
 
     let active_buffer = buffers.active_buffer();
     let mut items = active_buffer.content();
-    items.reverse();
-    let mut items = items.into_iter();
     let mut scroll = self.scroll;
 
     // Skip widgets at the bottom (if scrolled up), and render the first visible one
@@ -73,7 +71,7 @@ impl Component for Backlog {
       let Some(item) = items.next() else {
         break;
       };
-      let widget = BottomAlignedParagraph::new(item);
+      let widget = BottomAlignedParagraph::new(item.text);
       let expected_height = widget.height(text_area.width);
 
       if scroll.saturating_sub(expected_height) > text_area.height.into() {
@@ -95,7 +93,7 @@ impl Component for Backlog {
 
     // Render other widgets
     for item in items {
-      let widget = BottomAlignedParagraph::new(item);
+      let widget = BottomAlignedParagraph::new(item.text);
       let height = widget.render_overlap(text_area, frame.buffer_mut());
       assert!(area.height >= height, "{:?} {}", area, height);
       text_area.height = text_area.height.saturating_sub(height); // Remove lines at the bottom used by this paragraph