layout: add List.Gap for spacing out items

Signed-off-by: Egon Elbre <egonelbre@gmail.com>

layout/list.go

@@ -31,6 +31,8 @@ type List struct {
 	Alignment Alignment
 	// ScrollAnyAxis allows any scroll axis to scroll the list, not just the main axis.
 	ScrollAnyAxis bool
+	// Gap is the space in pixels between children.
+	Gap int
 
 	cs          Constraints
 	scroll      gesture.Scroll
@@ -130,7 +132,7 @@ func (l *List) Layout(gtx Context, len int, w ListElement) Dimensions {
 	}
 
 	if numLaidOut > 0 {
-		l.Position.Length = laidOutTotalLength * len / numLaidOut
+		l.Position.Length = laidOutTotalLength*len/numLaidOut + l.Gap*(len-1)
 	} else {
 		l.Position.Length = 0
 	}
@@ -223,11 +225,11 @@ func (l *List) nextDir() iterationDir {
 	if len(l.children) > 0 {
 		if l.Position.First > 0 {
 			firstChild := l.children[0]
-			firstSize = l.Axis.Convert(firstChild.size).X
+			firstSize = l.Axis.Convert(firstChild.size).X + l.Gap
 		}
 		if last < l.len {
 			lastChild := l.children[len(l.children)-1]
-			lastSize = l.Axis.Convert(lastChild.size).X
+			lastSize = l.Axis.Convert(lastChild.size).X + l.Gap
 		}
 	}
 	switch {
@@ -245,6 +247,9 @@ func (l *List) nextDir() iterationDir {
 func (l *List) end(dims Dimensions, call op.CallOp) {
 	child := scrollChild{dims.Size, call}
 	mainSize := l.Axis.Convert(child.size).X
+	if len(l.children) > 0 {
+		l.maxSize += l.Gap
+	}
 	l.maxSize += mainSize
 	switch l.dir {
 	case iterateForward:
@@ -254,7 +259,7 @@ func (l *List) end(dims Dimensions, call op.CallOp) {
 		copy(l.children[1:], l.children)
 		l.children[0] = child
 		l.Position.First--
-		l.Position.Offset += mainSize
+		l.Position.Offset += mainSize + l.Gap
 	default:
 		panic("call Next before End")
 	}
@@ -279,7 +284,7 @@ func (l *List) layout(ops *op.Ops, macro op.MacroOp) Dimensions {
 			break
 		}
 		l.Position.First++
-		l.Position.Offset -= mainSize
+		l.Position.Offset -= mainSize + l.Gap
 		first = child
 		children = children[1:]
 	}
@@ -291,6 +296,9 @@ func (l *List) layout(ops *op.Ops, macro op.MacroOp) Dimensions {
 		if c := sz.Y; c > maxCross {
 			maxCross = c
 		}
+		if i > 0 {
+			size += l.Gap
+		}
 		size += sz.X
 		if size >= mainMax {
 			if i < len(children)-1 {
@@ -326,14 +334,19 @@ func (l *List) layout(ops *op.Ops, macro op.MacroOp) Dimensions {
 	// Lay out leading invisible child.
 	if first != (scrollChild{}) {
 		sz := l.Axis.Convert(first.size)
-		pos -= sz.X
+		pos -= sz.X + l.Gap
 		layout(first)
+		pos += l.Gap
+	}
+	for i, child := range children {
+		if i > 0 {
+			pos += l.Gap
 		}
-	for _, child := range children {
 		layout(child)
 	}
 	// Lay out trailing invisible child.
 	if last != (scrollChild{}) {
+		pos += l.Gap
 		layout(last)
 	}
 	atStart := l.Position.First == 0 && l.Position.Offset <= 0

layout/list_test.go

@@ -184,6 +184,93 @@ func TestListPosition(t *testing.T) {
 	}
 }
 
+func TestListGap(t *testing.T) {
+	gtx := Context{
+		Ops: new(op.Ops),
+		Constraints: Constraints{
+			Max: image.Pt(100, 20),
+		},
+	}
+
+	// Two 10px children with 5px gap: total 25px.
+	l := List{Gap: 5}
+	dims := l.Layout(gtx, 2, func(gtx Context, idx int) Dimensions {
+		return Dimensions{Size: image.Pt(10, 10)}
+	})
+	if got, exp := dims.Size.X, 25; got != exp {
+		t.Errorf("two children with gap: got width %d, expected %d", got, exp)
+	}
+
+	// Three 10px children with 5px gap: total 40px.
+	l = List{Gap: 5}
+	dims = l.Layout(gtx, 3, func(gtx Context, idx int) Dimensions {
+		return Dimensions{Size: image.Pt(10, 10)}
+	})
+	if got, exp := dims.Size.X, 40; got != exp {
+		t.Errorf("three children with gap: got width %d, expected %d", got, exp)
+	}
+
+	// Single child: no gap.
+	l = List{Gap: 5}
+	dims = l.Layout(gtx, 1, func(gtx Context, idx int) Dimensions {
+		return Dimensions{Size: image.Pt(10, 10)}
+	})
+	if got, exp := dims.Size.X, 10; got != exp {
+		t.Errorf("single child with gap: got width %d, expected %d", got, exp)
+	}
+
+	// Zero children: no gap.
+	l = List{Gap: 5}
+	dims = l.Layout(gtx, 0, nil)
+	if got, exp := dims.Size.X, 0; got != exp {
+		t.Errorf("no children with gap: got width %d, expected %d", got, exp)
+	}
+}
+
+func TestListGapVertical(t *testing.T) {
+	gtx := Context{
+		Ops: new(op.Ops),
+		Constraints: Constraints{
+			Max: image.Pt(20, 100),
+		},
+	}
+
+	l := List{Axis: Vertical, Gap: 10}
+	dims := l.Layout(gtx, 3, func(gtx Context, idx int) Dimensions {
+		return Dimensions{Size: image.Pt(10, 15)}
+	})
+	// 3*15 + 2*10 = 65.
+	if got, exp := dims.Size.Y, 65; got != exp {
+		t.Errorf("vertical list with gap: got height %d, expected %d", got, exp)
+	}
+}
+
+func TestListGapPosition(t *testing.T) {
+	gtx := Context{
+		Ops: new(op.Ops),
+		Constraints: Constraints{
+			Max: image.Pt(30, 20),
+		},
+	}
+
+	// Viewport 30px, 5 children of 10px with 5px gap.
+	// Children fill: 10, 10+5+10=25, 25+5+10=40 >= 30, so 3 visible (last partially).
+	l := List{Gap: 5}
+	l.Layout(gtx, 5, func(gtx Context, idx int) Dimensions {
+		return Dimensions{Size: image.Pt(10, 10)}
+	})
+	if got, exp := l.Position.Count, 3; got != exp {
+		t.Errorf("visible count with gap: got %d, expected %d", got, exp)
+	}
+	if got, exp := l.Position.First, 0; got != exp {
+		t.Errorf("first with gap: got %d, expected %d", got, exp)
+	}
+	// OffsetLast = mainMax - size = 30 - 40 = -10.
+	if got, exp := l.Position.OffsetLast, -10; got != exp {
+		t.Errorf("offset last with gap: got %d, expected %d", got, exp)
+	}
+}
+
 func TestExtraChildren(t *testing.T) {
 	var l List
 	l.Position.First = 1