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Support scroll padding when scrolling up
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Philipp Reiter committed Sep 15, 2024
1 parent 6e508f1 commit 8de918d
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Showing 2 changed files with 164 additions and 90 deletions.
248 changes: 161 additions & 87 deletions src/utils.rs
View file
Original file line number Diff line number Diff line change
Expand Up @@ -38,15 +38,7 @@ pub(crate) fn layout_on_viewport<T>(
// If none is selected, the first item should be show on top of the viewport.
let selected = state.selected.unwrap_or(0);

// If the selected value is smaller than the offset, we roll
// the offset so that the selected value is at the top
if selected < state.view_state.offset
|| (selected == state.view_state.offset && state.view_state.first_truncated > 0)
{
state.view_state.offset = selected;
state.view_state.first_truncated = 0;
}

// Calculate the effective scroll padding for each widget
let effective_scroll_padding_by_index = calculate_effective_scroll_padding(
state,
builder,
Expand All @@ -56,6 +48,39 @@ pub(crate) fn layout_on_viewport<T>(
scroll_padding,
);

// If the selected value is smaller than the offset, we roll
// the offset so that the selected value is at the top. The complicated
// part is that we also need to account for scroll padding.
let scroll_padding_top = *effective_scroll_padding_by_index
.get(&selected)
.unwrap_or(&0);
let mut first_element = selected;
let mut first_element_truncated = 0;
let mut available_size = scroll_padding_top;
for index in (0..=selected).rev() {
first_element = index;
if available_size == 0 {
break;
}
let context = ListBuildContext {
index,
is_selected: state.selected.map_or(false, |j| index == j),
scroll_axis,
cross_axis_size,
};
let (_, main_axis_size) = builder.call_closure(&context);
available_size = available_size.saturating_sub(main_axis_size);
if available_size > 0 {
first_element_truncated = main_axis_size.saturating_sub(available_size);
}
}
if first_element < state.view_state.offset
|| (first_element == state.view_state.offset && state.view_state.first_truncated > 0)
{
state.view_state.offset = first_element;
state.view_state.first_truncated = first_element_truncated;
}

// Begin a forward pass, starting from `view_state.offset`.
let found_selected = forward_pass(
&mut viewport,
Expand Down Expand Up @@ -91,88 +116,13 @@ pub(crate) fn layout_on_viewport<T>(
&effective_scroll_padding_by_index,
);

// if scroll_padding > 0 {
// let _ = forward_pass(
// &mut viewport,
// state,
// builder,
// selected + 1,
// item_count,
// scroll_padding,
// selected,
// cross_axis_size,
// scroll_axis,
// 0,
// );
// }

viewport
}

/// Calculate the effective scroll padding at the end of the list.
/// Padding is applied until the scroll padding limit is reached,
/// after which elements at the end of the list do not receive padding.
///
/// Returns:
/// A `HashMap` where the keys are the indices of the list items and the values are
/// the corresponding padding applied. If the item is not on the list, `scroll_padding`
/// is unaltered.
fn calculate_effective_scroll_padding<T>(
state: &mut ListState,
builder: &ListBuilder<T>,
item_count: usize,
cross_axis_size: u16,
scroll_axis: ScrollAxis,
scroll_padding: u16,
) -> HashMap<usize, u16> {
let mut padding_by_element = HashMap::new();
let mut total_main_axis_size = 0 as u16;

for index in 0..item_count {
if total_main_axis_size >= scroll_padding {
padding_by_element.insert(index, scroll_padding);
continue;
} else {
padding_by_element.insert(index, total_main_axis_size);
}
padding_by_element.insert(index, total_main_axis_size);

let context = ListBuildContext {
index,
is_selected: state.selected.map_or(false, |j| index == j),
scroll_axis,
cross_axis_size,
};

let (_, item_main_axis_size) = builder.call_closure(&context);
total_main_axis_size += item_main_axis_size;
}

total_main_axis_size = 0;
for index in (0..item_count).rev() {
// Stop applying padding once the scroll padding limit is reached
if total_main_axis_size >= scroll_padding {
break;
}
padding_by_element.insert(index, total_main_axis_size);

let context = ListBuildContext {
index,
is_selected: state.selected.map_or(false, |j| index == j),
scroll_axis,
cross_axis_size,
};

let (_, item_main_axis_size) = builder.call_closure(&context);
total_main_axis_size += item_main_axis_size;
}

return padding_by_element;
}

/// Iterate forward through the list of widgets.
///
/// Returns true if the selected widget is inside the viewport.
#[allow(clippy::too_many_arguments)]
fn forward_pass<T>(
viewport: &mut HashMap<usize, ViewportElement<T>>,
state: &mut ListState,
Expand Down Expand Up @@ -262,11 +212,12 @@ fn forward_pass<T>(
available_size -= main_axis_size;
}

return found_selected;
found_selected
}

// The selected item is out of bounds. We iterate backwards from the selected
// item and determine the first widget that still fits on the screen.
#[allow(clippy::too_many_arguments)]
fn backward_pass<T>(
viewport: &mut HashMap<usize, ViewportElement<T>>,
state: &mut ListState,
Expand Down Expand Up @@ -358,6 +309,66 @@ fn backward_pass<T>(
}
}

/// Calculate the effective scroll padding.
/// Padding is applied until the scroll padding limit is reached,
/// after which elements at the beginning or end of the list do
/// not receive padding.
///
/// Returns:
/// A `HashMap` where the keys are the indices of the list items and the values are
/// the corresponding padding applied. If the item is not on the list, `scroll_padding`
/// is unaltered.
fn calculate_effective_scroll_padding<T>(
state: &mut ListState,
builder: &ListBuilder<T>,
item_count: usize,
cross_axis_size: u16,
scroll_axis: ScrollAxis,
scroll_padding: u16,
) -> HashMap<usize, u16> {
let mut padding_by_element = HashMap::new();
let mut total_main_axis_size = 0;

for index in 0..item_count {
if total_main_axis_size >= scroll_padding {
padding_by_element.insert(index, scroll_padding);
continue;
}
padding_by_element.insert(index, total_main_axis_size);

let context = ListBuildContext {
index,
is_selected: state.selected.map_or(false, |j| index == j),
scroll_axis,
cross_axis_size,
};

let (_, item_main_axis_size) = builder.call_closure(&context);
total_main_axis_size += item_main_axis_size;
}

total_main_axis_size = 0;
for index in (0..item_count).rev() {
// Stop applying padding once the scroll padding limit is reached
if total_main_axis_size >= scroll_padding {
break;
}
padding_by_element.insert(index, total_main_axis_size);

let context = ListBuildContext {
index,
is_selected: state.selected.map_or(false, |j| index == j),
scroll_axis,
cross_axis_size,
};

let (_, item_main_axis_size) = builder.call_closure(&context);
total_main_axis_size += item_main_axis_size;
}

padding_by_element
}

#[allow(dead_code)]
pub fn log_to_file<T: Debug>(data: T) {
let mut file = OpenOptions::new()
Expand Down Expand Up @@ -601,11 +612,74 @@ mod tests {
// -----
// | |
#[test]
fn scroll_padding() {
fn scroll_padding_bottom() {
// given
let mut state = ListState {
num_elements: 3,
selected: Some(1),
..ListState::default()
};
let given_sizes = vec![2, 2, 2];
let given_item_count = given_sizes.len();
let given_total_size = 4;

let expected_view_state = ViewState {
offset: 0,
first_truncated: 1,
};
let expected_viewport = HashMap::from([
(0, ViewportElement::new(TestItem {}, 2, Truncation::Top(1))),
(1, ViewportElement::new(TestItem {}, 2, Truncation::None)),
(2, ViewportElement::new(TestItem {}, 2, Truncation::Bot(1))),
]);

// when
let viewport = layout_on_viewport(
&mut state,
&ListBuilder::new(move |context| {
return (TestItem {}, given_sizes[context.index]);
}),
given_item_count,
given_total_size,
1,
ScrollAxis::Vertical,
1,
);

// then
assert_eq!(viewport, expected_viewport);
assert_eq!(state.view_state, expected_view_state);
}

// From:
//
// -----
// | | 1
// | |
// -----
// | | 2 <-
// | |
// -----
//
// To:
//
// | |
// -----
// | | 1 <-
// | |
// -----
// | |
#[test]
fn scroll_padding_top() {
// given
let view_state = ViewState {
offset: 2,
first_truncated: 0,
};
let mut state = ListState {
num_elements: 3,
selected: Some(1),
view_state,
..ListState::default()
};
let given_sizes = vec![2, 2, 2];
Expand Down
6 changes: 3 additions & 3 deletions src/view.rs
View file
Original file line number Diff line number Diff line change
Expand Up @@ -27,7 +27,7 @@ pub struct ListView<'a, T> {
pub block: Option<Block<'a>>,

/// The scroll padding.
pub(crate) scroll_padding: usize,
pub(crate) scroll_padding: u16,
}

impl<'a, T> ListView<'a, T> {
Expand Down Expand Up @@ -79,7 +79,7 @@ impl<'a, T> ListView<'a, T> {

/// Set the scroll padding of the list.
#[must_use]
pub fn scroll_padding(mut self, scroll_padding: usize) -> Self {
pub fn scroll_padding(mut self, scroll_padding: u16) -> Self {
self.scroll_padding = scroll_padding;
self
}
Expand Down Expand Up @@ -198,7 +198,7 @@ impl<T: Widget> StatefulWidget for ListView<'_, T> {
main_axis_size,
cross_axis_size,
self.scroll_axis,
self.scroll_padding as u16,
self.scroll_padding,
);

let (start, end) = (
Expand Down

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