feat: Query if ScrollViewState is_at_bottom()

tui-scrollview/src/scroll_view.rs

@@ -194,7 +194,6 @@ impl ScrollView {
     ///
     /// This should not be confused with the `render` method, which renders the visible area of the
     /// ScrollView into the main buffer.
-
     pub fn render_stateful_widget<W: StatefulWidget>(
         &mut self,
         widget: W,
@@ -451,6 +450,57 @@ mod tests {
         )
     }
 
+    #[rstest]
+    fn move_to_bottom(scroll_view: ScrollView) {
+        let mut buf = Buffer::empty(Rect::new(0, 0, 6, 6));
+        let mut state = ScrollViewState::default();
+
+        // Prior rendering, page and buffer size are unkown. We default to `true`.
+        assert!(state.is_at_bottom());
+
+        scroll_view.clone().render(buf.area, &mut buf, &mut state);
+
+        // The vertical view size is five which means the page size is five.
+        // We have not scrolled yet, view is at the top and not the at the bottom.
+        // => We see the top five rows
+        assert!(!state.is_at_bottom());
+
+        // Since the content height is ten,
+        assert_eq!(state.size.unwrap().height, 10);
+        // if we scroll down one page (five rows),
+        state.scroll_down();
+        state.scroll_down();
+        state.scroll_down();
+        state.scroll_down();
+        state.scroll_down();
+
+        // we reach the bottom,
+        assert!(state.is_at_bottom());
+        assert_eq!(state.offset.y, 5);
+
+        // and we see the last five rows of the content.
+        scroll_view.render(buf.area, &mut buf, &mut state);
+        assert_eq!(
+            buf,
+            Buffer::with_lines(vec![
+                "YZABC▲",
+                "IJKLM║",
+                "STUVW█",
+                "CDEFG█",
+                "MNOPQ▼",
+                "◄██═► ",
+            ])
+        );
+
+        // We could also jump directly to the bottom...
+        state.scroll_to_bottom();
+        assert!(state.is_at_bottom());
+
+        // ...which sets the offset to the last row of content,
+        // ensuring to be at the bottom regardless of the page size.
+        assert_eq!(state.offset.y, state.size.unwrap().height - 1);
+    }
+
     #[rstest]
     fn hides_both_scrollbars(scroll_view: ScrollView) {
         let mut buf = Buffer::empty(Rect::new(0, 0, 10, 10));
@@ -790,7 +840,7 @@ mod tests {
         let items: Vec<String> = (1..=10).map(|i| format!("Item {}", i)).collect();
         let list = List::new(items);
         scroll_view.render_stateful_widget(list, scroll_view.area(), &mut list_state);
-        scroll_view.render(buf.area, &mut buf, &mut state);
+        scroll_view.clone().render(buf.area, &mut buf, &mut state);
         assert_eq!(
             buf,
             Buffer::with_lines(vec![

tui-scrollview/src/state.rs

@@ -74,6 +74,9 @@ impl ScrollViewState {
     }
 
     /// Move the scroll view state to the bottom of the buffer
+    ///
+    /// If the buffer size is not yet computed (done during the first rendering), it will not
+    /// be taken into account and the scroll offset will be set to the maximum value: `u16::MAX`
     pub fn scroll_to_bottom(&mut self) {
         // the render call will adjust the offset to ensure that we don't scroll past the end of
         // the buffer, so we can set the offset to the maximum value here
@@ -82,4 +85,18 @@ impl ScrollViewState {
             .map_or(u16::MAX, |size| size.height.saturating_sub(1));
         self.offset.y = bottom;
     }
+
+    /// True if the scroll view state is at the bottom of the buffer
+    ///
+    /// This takes the page size into account. It returns true if the current scroll offset plus
+    /// the page size matches or exceeds the buffer length.
+    ///
+    /// The buffer and the page size are unknown until computed during the first rendering. If the
+    /// page size is not yet known, it won't be taken into account. If the buffer is not yet known,
+    /// this function always returns true.
+    pub fn is_at_bottom(&self) -> bool {
+        let bottom = self.size.map_or(0, |size| size.height.saturating_sub(1));
+        let page_size = self.page_size.map_or(0, |size| size.height);
+        self.offset.y.saturating_add(page_size) >= bottom
+    }
 }