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//! Helpers for drag and drop support for reordering hierarchical lists.
//!
//! Works well in combination with [`crate::list_item::ListItem`].
pub enum ItemKind<ItemId: Copy> {
/// Root container item.
///
/// Root container don't have a parent and are restricted when hovered as drop target (the dragged item can only go
/// _in_ the root container, not before or after).
RootContainer,
/// Container item.
Container {
parent_id: ItemId,
position_index_in_parent: usize,
},
/// Leaf item.
Leaf {
parent_id: ItemId,
position_index_in_parent: usize,
},
}
/// Context information about the hovered item.
///
/// This is used by [`find_drop_target`] to compute the [`DropTarget`], if any.
pub struct ItemContext<ItemId: Copy> {
/// ID of the item being hovered during drag
pub id: ItemId,
/// What kind of item is this?
pub item_kind: ItemKind<ItemId>,
/// ID of the container just before this item within the parent, if such a container exists.
pub previous_container_id: Option<ItemId>,
}
/// Drop target information, including where to draw the drop indicator and where to insert the dragged item.
#[derive(Clone, Debug)]
pub struct DropTarget<ItemId: Copy> {
/// Range of X coordinates for the drag target indicator
pub indicator_span_x: egui::Rangef,
/// Y coordinate for drag target indicator
pub indicator_position_y: f32,
/// Destination container ID
pub target_parent_id: ItemId,
/// Destination position within the container
pub target_position_index: usize,
}
impl<ItemId: Copy> DropTarget<ItemId> {
pub fn new(
indicator_span_x: egui::Rangef,
indicator_position_y: f32,
target_parent_id: ItemId,
target_position_index: usize,
) -> Self {
Self {
indicator_span_x,
indicator_position_y,
target_parent_id,
target_position_index,
}
}
}
/// Compute the geometry of the drag cursor and where the dragged item should be inserted.
///
/// This function performs the following tasks:
/// - based on `item_rect` and `body_rect`, establish the geometry of actual drop zones (see below)
/// - test the mouse cursor against these zones
/// - if one is a match:
/// - compute the geometry of a drop insertion indicator
/// - use the context provided in `item_context` to return the "logical" drop target (ie. the target container and
/// position within it)
///
/// This function implements the following logic:
/// ```text
///
/// insert insert last in container before me
/// before me (if any) or insert before me
/// │ │
/// ╔═══▼═════════════════════════════▼══════════════════╗
/// ║ │ ║
/// leaf item ║ ─────┴──────────────────────────────────────────── ║
/// ║ ║
/// ╚═════════════════════▲══════════════════════════════╝
/// │
/// insert after me
///
///
/// insert insert last in container before me
/// before me (if any) or insert before me
/// │ │
/// ╔═══▼═════════════════════════════▼══════════════════╗
/// leaf item ║ │ ║
/// with body ║ ─────┴──────────────────────────────────────────── ║
/// ║ ║
/// ╚══════╦══════════════════════════════════════▲══════╣ ─┐
/// │ ║ │ ║ │
/// │ ║ insert ║ │
/// │ ║ after me ║ │
/// │ ╠══ ══╣ │
/// │ ║ no insertion possible ║ │
/// │ ║ here by definition of ║ │ body
/// │ ║ parent being a leaf ║ │
/// │ ╠══ ══╣ │
/// │ ║ ║ │
/// │ ║ ║ │
/// │ ║ ║ │
/// └──▲── ╚══════════════════════════▲══════════════════╝ ─┘
/// │ │
/// insert insert
/// after me after me
///
///
/// insert insert last in container before me
/// before me (if any) or insert before me
/// │ │
/// ╔═══▼═════════════════════════════▼══════════════════╗
/// container item ║ │ ║
/// (empty/collapsed ║ ─────┼──────────────────────────────────────────── ║
/// body) ║ │ ║
/// ╚═══▲═════════════════════════════▲══════════════════╝
/// │ │
/// insert insert inside me
/// after me at pos = 0
///
///
/// insert insert last in container before me
/// before me (if any) or insert before me
/// │ │
/// ╔═══▼═════════════════════════════▼══════════════════╗
/// container item ║ │ ║
/// with body ║ ─────┴──────────────────────────────────────────── ║
/// ║ ║
/// ╚═▲════╦═════════════════════════════════════════════╣ ─┐
/// │ ║ ║ │
/// insert ║ ║ │
/// inside me ║ ║ │
/// at pos = 0 ╠══ ══╣ │
/// ║ same logic ║ │
/// ║ recursively ║ │ body
/// insert ║ applied here ║ │
/// after me ╠══ ══╣ │
/// │ ║ ║ │
/// ┌─▼─── ║ ║ │
/// │ ║ ║ │
/// └───── ╚═════════════════════════════════════════════╝ ─┘
///
/// not a valid drop zone
/// │
/// ╔═════════════════════════════════▼══════════════════╗
/// root container ║ ║
/// item ║ ────────────────────────────────────────────────── ║
/// ║ ║
/// ╚════════════════════════▲═══════════════════════════╝
/// │
/// insert inside me
/// at pos = 0
/// ```
///
/// Here are a few observations of the above that help navigate the "if-statement-of-death"
/// in the implementation:
/// - The top parts of the item are treated the same in most cases (root container is an
/// exception).
/// - Handling of the body can be simplified by making the sensitive area either a small
/// corner (container case), or the entire body (leaf case). Then, that area always maps
/// to "insert after me".
/// - The bottom parts have the most difference between cases and need case-by-case handling.
/// In both leaf item cases, the entire bottom part maps to "insert after me", though.
///
/// **Note**: in debug builds, press `Alt` to visualize the drag zones while dragging.
pub fn find_drop_target<ItemId: Copy>(
ui: &egui::Ui,
item_context: &ItemContext<ItemId>,
item_rect: egui::Rect,
body_rect: Option<egui::Rect>,
item_height: f32,
) -> Option<DropTarget<ItemId>> {
let indent = ui.spacing().indent;
let item_id = item_context.id;
let item_kind = &item_context.item_kind;
let is_non_root_container = matches!(item_kind, ItemKind::Container { .. });
// For both leaf and containers we have two drag zones on the upper half of the item.
let (mut top, mut bottom) = item_rect.split_top_bottom_at_fraction(0.5);
let mut left_top = egui::Rect::NOTHING;
if matches!(item_kind, ItemKind::RootContainer) {
top = egui::Rect::NOTHING;
} else {
(left_top, top) = top.split_left_right_at_x(top.left() + indent);
}
// For the lower part of the item, the story is more complicated:
// - for leaf and root container items, we have a single drag zone on the entire lower half
// - for container item, we must distinguish between the indent part and the rest, plus check some area in the
// body
let mut left_bottom = egui::Rect::NOTHING;
if is_non_root_container {
(left_bottom, bottom) = bottom.split_left_right_at_x(bottom.left() + indent);
}
// For the body area we have two cases:
// - container item: it's handled recursively by the nested items, so we only need to check a small area down
// left, which maps to "insert after me"
// - leaf item: the entire body area, if any, cannot receive a drag (by definition) and thus homogeneously maps
// to "insert after me"
let body_insert_after_me_area = if let Some(body_rect) = body_rect {
if is_non_root_container {
egui::Rect::from_two_pos(
body_rect.left_bottom() + egui::vec2(indent, -item_height / 2.0),
body_rect.left_bottom(),
)
} else {
body_rect
}
} else {
egui::Rect::NOTHING
};
// body rect, if any AND it actually contains something
let non_empty_body_rect = body_rect.filter(|r| r.height() > 0.0);
// visualize the drag zones in debug builds, when the `Alt` key is pressed during drag
#[cfg(debug_assertions)]
{
// Visualize the drag zones
if ui.input(|i| i.modifiers.alt) {
ui.ctx()
.debug_painter()
.debug_rect(top, egui::Color32::RED, "t");
ui.ctx()
.debug_painter()
.debug_rect(bottom, egui::Color32::GREEN, "b");
ui.ctx().debug_painter().debug_rect(
left_top,
egui::Color32::RED.gamma_multiply(0.5),
"lt",
);
ui.ctx().debug_painter().debug_rect(
left_bottom,
egui::Color32::GREEN.gamma_multiply(0.5),
"lb",
);
ui.ctx().debug_painter().debug_rect(
body_insert_after_me_area,
egui::Color32::YELLOW,
"bdy",
);
}
}
match *item_kind {
ItemKind::RootContainer => {
// we just need to test the bottom section in this case.
if ui.rect_contains_pointer(bottom) {
Some(DropTarget::new(
item_rect.x_range(),
bottom.bottom(),
item_id,
0,
))
} else {
None
}
}
ItemKind::Container {
parent_id,
position_index_in_parent,
}
| ItemKind::Leaf {
parent_id,
position_index_in_parent,
} => {
/* ===== TOP SECTIONS (same leaf/container items) ==== */
if ui.rect_contains_pointer(left_top) {
// insert before me
Some(DropTarget::new(
item_rect.x_range(),
top.top(),
parent_id,
position_index_in_parent,
))
} else if ui.rect_contains_pointer(top) {
// insert last in the previous container if any, else insert before me
if let Some(previous_container_id) = item_context.previous_container_id {
Some(DropTarget::new(
(item_rect.left() + indent..=item_rect.right()).into(),
top.top(),
previous_container_id,
usize::MAX,
))
} else {
Some(DropTarget::new(
item_rect.x_range(),
top.top(),
parent_id,
position_index_in_parent,
))
}
}
/* ==== BODY SENSE AREA ==== */
else if ui.rect_contains_pointer(body_insert_after_me_area) {
// insert after me in my parent
Some(DropTarget::new(
item_rect.x_range(),
body_insert_after_me_area.bottom(),
parent_id,
position_index_in_parent + 1,
))
}
/* ==== BOTTOM SECTIONS (leaf item) ==== */
else if !is_non_root_container {
if ui.rect_contains_pointer(bottom) {
let position_y = if let Some(body_rect) = non_empty_body_rect {
body_rect.bottom()
} else {
bottom.bottom()
};
// insert after me
Some(DropTarget::new(
item_rect.x_range(),
position_y,
parent_id,
position_index_in_parent + 1,
))
} else {
None
}
}
/* ==== BOTTOM SECTIONS (container item) ==== */
else if let Some(body_rect) = non_empty_body_rect {
if ui.rect_contains_pointer(left_bottom) || ui.rect_contains_pointer(bottom) {
// insert at pos = 0 inside me
Some(DropTarget::new(
(body_rect.left() + indent..=body_rect.right()).into(),
left_bottom.bottom(),
item_id,
0,
))
} else {
None
}
} else if ui.rect_contains_pointer(left_bottom) {
// insert after me in my parent
Some(DropTarget::new(
item_rect.x_range(),
left_bottom.bottom(),
parent_id,
position_index_in_parent + 1,
))
} else if ui.rect_contains_pointer(bottom) {
// insert at pos = 0 inside me
Some(DropTarget::new(
(item_rect.left() + indent..=item_rect.right()).into(),
bottom.bottom(),
item_id,
0,
))
}
/* ==== Who knows where else the mouse cursor might wander… ¯\_(ツ)_/¯ ==== */
else {
None
}
}
}
}