layout: added number of visible children to List

Also fixed an edge case where the first visible child was off by 1 when it was just fully hidden.

Signed-off-by: pierre <pierre.curto@gmail.com>

layout/list.go

@@ -69,6 +69,8 @@ type Position struct {
 	// Offset is the distance in pixels from the top edge to the child at index
 	// First.
 	Offset int
+	// Count is the number of visible children.
+	Count int
 }
 
 const (
@@ -206,7 +208,8 @@ func (l *List) layout(ops *op.Ops, macro op.MacroOp) Dimensions {
 	for len(children) > 0 {
 		sz := children[0].size
 		mainSize := l.Axis.Convert(sz).X
-		if l.Position.Offset <= mainSize {
+		if l.Position.Offset < mainSize {
+			// First child is partially visible.
 			break
 		}
 		l.Position.First++
@@ -226,6 +229,7 @@ func (l *List) layout(ops *op.Ops, macro op.MacroOp) Dimensions {
 			break
 		}
 	}
+	l.Position.Count = len(children)
 	pos := -l.Position.Offset
 	// ScrollToEnd lists are end aligned.
 	if space := mainMax - size; l.ScrollToEnd && space > 0 {

layout/list_test.go

@@ -0,0 +1,102 @@
+// SPDX-License-Identifier: Unlicense OR MIT
+
+package layout
+
+import (
+	"image"
+	"testing"
+
+	"gioui.org/f32"
+	"gioui.org/io/event"
+	"gioui.org/io/pointer"
+	"gioui.org/io/router"
+	"gioui.org/op"
+)
+
+func TestListPosition(t *testing.T) {
+	_s := func(e ...event.Event) []event.Event { return e }
+	r := new(router.Router)
+	gtx := Context{
+		Ops: new(op.Ops),
+		Constraints: Constraints{
+			Max: image.Pt(20, 10),
+		},
+		Queue: r,
+	}
+	el := func(gtx Context, idx int) Dimensions {
+		return Dimensions{Size: image.Pt(10, 10)}
+	}
+	for _, tc := range []struct {
+		label  string
+		num    int
+		scroll []event.Event
+		first  int
+		count  int
+	}{
+		{label: "no item"},
+		{label: "1 visible 0 hidden", num: 1, count: 1},
+		{label: "2 visible 0 hidden", num: 2, count: 2},
+		{label: "2 visible 1 hidden", num: 3, count: 2},
+		{label: "3 visible 0 hidden small scroll", num: 3, count: 3,
+			scroll: _s(
+				pointer.Event{
+					Source:   pointer.Mouse,
+					Buttons:  pointer.ButtonLeft,
+					Type:     pointer.Press,
+					Position: f32.Pt(0, 0),
+				},
+				pointer.Event{
+					Source: pointer.Mouse,
+					Type:   pointer.Scroll,
+					Scroll: f32.Pt(5, 0),
+				},
+				pointer.Event{
+					Source:   pointer.Mouse,
+					Buttons:  pointer.ButtonLeft,
+					Type:     pointer.Release,
+					Position: f32.Pt(5, 0),
+				},
+			)},
+		{label: "2 visible 1 hidden large scroll", num: 3, count: 2, first: 1,
+			scroll: _s(
+				pointer.Event{
+					Source:   pointer.Mouse,
+					Buttons:  pointer.ButtonLeft,
+					Type:     pointer.Press,
+					Position: f32.Pt(0, 0),
+				},
+				pointer.Event{
+					Source: pointer.Mouse,
+					Type:   pointer.Scroll,
+					Scroll: f32.Pt(10, 0),
+				},
+				pointer.Event{
+					Source:   pointer.Mouse,
+					Buttons:  pointer.ButtonLeft,
+					Type:     pointer.Release,
+					Position: f32.Pt(15, 0),
+				},
+			)},
+	} {
+		t.Run(tc.label, func(t *testing.T) {
+			gtx.Ops.Reset()
+
+			var list List
+			// Initialize the list.
+			list.Layout(gtx, tc.num, el)
+			// Generate the scroll events.
+			r.Frame(gtx.Ops)
+			r.Add(tc.scroll...)
+			// Let the list process the events.
+			list.Layout(gtx, tc.num, el)
+
+			pos := list.Position
+			if got, want := pos.First, tc.first; got != want {
+				t.Errorf("List: invalid first position: got %v; want %v", got, want)
+			}
+			if got, want := pos.Count, tc.count; got != want {
+				t.Errorf("List: invalid number of visible children: got %v; want %v", got, want)
+			}
+		})
+	}
+}