Grid::num_columns: allow the last column to take up the rest of the space

This allows for resizaeable grids, where the last column will be given the remainder of the width.
To demonstrate, the widget gallery window is now resizeable.

CHANGELOG.md

@@ -6,7 +6,10 @@ NOTE: [`eframe`](eframe/CHANGELOG.md), [`egui_web`](egui_web/CHANGELOG.md) and [
 
 
 ## Unreleased
+
+### Added ⭐
 * [Progress bar](https://github.com/emilk/egui/pull/519)
+* `Grid::num_columns`: allow the last column to take up the rest of the space of the parent `Ui`.
 
 
 ## 0.13.1 - 2021-06-28 - Plot fixes

egui/src/grid.rs

@@ -46,13 +46,16 @@ pub(crate) struct GridLayout {
     prev_state: State,
     /// State accumulated during the current frame.
     curr_state: State,
+    initial_available: Rect,
 
+    // Options:
+    num_columns: Option<usize>,
     spacing: Vec2,
-
-    striped: bool,
-    initial_x: f32,
     min_cell_size: Vec2,
     max_cell_size: Vec2,
+    striped: bool,
+
+    // Cursor:
     col: usize,
     row: usize,
 }
@@ -63,10 +66,9 @@ impl GridLayout {
 
         // TODO: respect current layout
 
-        let available = ui.placer().max_rect().intersect(ui.cursor());
+        let initial_available = ui.placer().max_rect().intersect(ui.cursor());
-        let initial_x = available.min.x;
         assert!(
-            initial_x.is_finite(),
+            initial_available.min.x.is_finite(),
             "Grid not yet available for right-to-left layouts"
         );
 
@@ -76,11 +78,14 @@ impl GridLayout {
             id,
             prev_state,
             curr_state: State::default(),
+            initial_available,
+
+            num_columns: None,
             spacing: ui.spacing().item_spacing,
-            striped: false,
-            initial_x,
             min_cell_size: ui.spacing().interact_size,
             max_cell_size: Vec2::INFINITY,
+            striped: false,
+
             col: 0,
             row: 0,
         }
@@ -109,7 +114,11 @@ impl GridLayout {
     }
 
     pub(crate) fn available_rect_finite(&self, region: &Region) -> Rect {
-        let width = if self.max_cell_size.x.is_finite() {
+        let is_last_column = Some(self.col + 1) == self.num_columns;
+
+        let width = if is_last_column {
+            (self.initial_available.right() - region.cursor.left()).at_most(self.max_cell_size.x)
+        } else if self.max_cell_size.x.is_finite() {
             // TODO: should probably heed `prev_state` here too
             self.max_cell_size.x
         } else {
@@ -183,7 +192,7 @@ impl GridLayout {
     }
 
     pub(crate) fn end_row(&mut self, cursor: &mut Rect, painter: &Painter) {
-        cursor.min.x = self.initial_x;
+        cursor.min.x = self.initial_available.min.x;
         cursor.min.y += self.spacing.y;
         cursor.min.y += self
             .curr_state
@@ -247,6 +256,7 @@ impl GridLayout {
 #[must_use = "You should call .show()"]
 pub struct Grid {
     id_source: Id,
+    num_columns: Option<usize>,
     striped: bool,
     min_col_width: Option<f32>,
     min_row_height: Option<f32>,
@@ -260,6 +270,7 @@ impl Grid {
     pub fn new(id_source: impl std::hash::Hash) -> Self {
         Self {
             id_source: Id::new(id_source),
+            num_columns: None,
             striped: false,
             min_col_width: None,
             min_row_height: None,
@@ -269,6 +280,12 @@ impl Grid {
         }
     }
 
+    /// Setting this will allow the last column to expand to take up the rest of the space of the parent [`Ui`].
+    pub fn num_columns(mut self, num_columns: usize) -> Self {
+        self.num_columns = Some(num_columns);
+        self
+    }
+
     /// If `true`, add a subtle background color to every other row.
     ///
     /// This can make a table easier to read.
@@ -317,6 +334,7 @@ impl Grid {
     pub fn show<R>(self, ui: &mut Ui, add_contents: impl FnOnce(&mut Ui) -> R) -> InnerResponse<R> {
         let Self {
             id_source,
+            num_columns,
             striped,
             min_col_width,
             min_row_height,
@@ -336,10 +354,11 @@ impl Grid {
             ui.horizontal(|ui| {
                 let id = ui.make_persistent_id(id_source);
                 let grid = GridLayout {
+                    num_columns,
                     striped,
-                    spacing,
                     min_cell_size: vec2(min_col_width, min_row_height),
                     max_cell_size,
+                    spacing,
                     row: start_row,
                     ..GridLayout::new(ui, id)
                 };

egui_demo_lib/src/apps/demo/widget_gallery.rs

@@ -42,7 +42,8 @@ impl super::Demo for WidgetGallery {
     fn show(&mut self, ctx: &egui::CtxRef, open: &mut bool) {
         egui::Window::new(self.name())
             .open(open)
-            .resizable(false)
+            .resizable(true)
+            .default_width(300.0)
             .show(ctx, |ui| {
                 use super::View;
                 self.ui(ui);
@@ -57,8 +58,9 @@ impl super::View for WidgetGallery {
             ui.set_enabled(self.enabled);
 
             egui::Grid::new("my_grid")
-                .striped(true)
+                .num_columns(2)
                 .spacing([40.0, 4.0])
+                .striped(true)
                 .show(ui, |ui| {
                     self.gallery_grid_contents(ui);
                 });