feat: Allow setting custom Layout paddings

Introduce a `LayoutOption` concept, and allow setting custom paddings
for a Layout. The `BaseLayout` struct implements the default paddings
retrieval, to reduce the amount of changes required by project
maintainers to comply with the new interface methods.

This change uses the Option pattern in case other customizations can be
added to Layouts in the future, without changing the public API.

Main motivation for this change has been Supersonic needing to copy the
entire Vbox/Hbox implementation, just to add custom paddings [1]. Also
related to feature request fyne-io#1031.

[1]

cmd/fyne_demo/tutorials/welcome.go

@@ -56,7 +56,7 @@ func welcomeScreen(_ fyne.Window) fyne.CanvasObject {
 	underlay := canvas.NewImageFromResource(data.FyneLogo)
 	bg := canvas.NewRectangle(bgColor)
 	underlayer := underLayout{}
-	slideBG := container.New(underlayer, underlay)
+	slideBG := container.New(&underlayer, underlay)
 	footerBG := canvas.NewRectangle(shadowColor)
 
 	listen := make(chan fyne.Settings)
@@ -81,10 +81,10 @@ func welcomeScreen(_ fyne.Window) fyne.CanvasObject {
 
 	bgClip := container.NewScroll(slideBG)
 	bgClip.Direction = container.ScrollNone
-	return container.NewStack(container.New(unpad{top: true}, bgClip, bg),
+	return container.NewStack(container.New(&unpad{top: true}, bgClip, bg),
 		container.NewBorder(nil,
 			container.NewStack(footerBG, footer), nil, nil,
-			container.New(unpad{top: true, bottom: true}, scroll)))
+			container.New(&unpad{top: true, bottom: true}, scroll)))
 }
 
 func withAlpha(c color.Color, alpha uint8) color.Color {
@@ -93,6 +93,8 @@ func withAlpha(c color.Color, alpha uint8) color.Color {
 }
 
 type underLayout struct {
+	layout.BaseLayout
+
 	offset float32
 }
 
@@ -107,21 +109,23 @@ func (u underLayout) MinSize(_ []fyne.CanvasObject) fyne.Size {
 }
 
 type unpad struct {
+	layout.BaseLayout
+
 	top, bottom bool
 }
 
 func (u unpad) Layout(objs []fyne.CanvasObject, s fyne.Size) {
-	pad := theme.Padding()
+	topPadding, bottomPadding, _, _ := u.GetPaddings()
 	var pos fyne.Position
 	if u.top {
-		pos = fyne.NewPos(0, -pad)
+		pos = fyne.NewPos(0, -topPadding)
 	}
 	size := s
 	if u.top {
-		size = size.AddWidthHeight(0, pad)
+		size = size.AddWidthHeight(0, topPadding)
 	}
 	if u.bottom {
-		size = size.AddWidthHeight(0, pad)
+		size = size.AddWidthHeight(0, bottomPadding)
 	}
 	for _, o := range objs {
 		o.Move(pos)

container/multiplewindows.go

@@ -3,6 +3,7 @@ package container
 import (
 	"fyne.io/fyne/v2"
 	intWidget "fyne.io/fyne/v2/internal/widget"
+	"fyne.io/fyne/v2/layout"
 	"fyne.io/fyne/v2/widget"
 )
 
@@ -92,6 +93,7 @@ func (m *MultipleWindows) setupChild(w *InnerWindow) {
 }
 
 type multiWinLayout struct {
+	layout.BaseLayout
 }
 
 func (m *multiWinLayout) Layout(objects []fyne.CanvasObject, _ fyne.Size) {

container_test.go

@@ -181,6 +181,12 @@ func (c *customLayout) MinSize(objs []CanvasObject) Size {
 	return NewSize(10, float32(10*len(objs)))
 }
 
+func (c *customLayout) GetPaddings() (float32, float32, float32, float32) {
+	return 0, 0, 0, 0
+}
+
+func (c *customLayout) SetPaddingFn(paddingFn func() (float32, float32, float32, float32)) {}
+
 type dummyObject struct {
 	size   Size
 	pos    Position

dialog/base.go

@@ -197,6 +197,8 @@ func (renderer *themedBackgroundRenderer) Refresh() {
 // ===============================================================
 
 type dialogLayout struct {
+	layout.BaseLayout
+
 	d *dialog
 }
 

dialog/file.go

@@ -12,6 +12,7 @@ import (
 	"fyne.io/fyne/v2"
 	"fyne.io/fyne/v2/container"
 	"fyne.io/fyne/v2/lang"
+	"fyne.io/fyne/v2/layout"
 	"fyne.io/fyne/v2/storage"
 	"fyne.io/fyne/v2/storage/repository"
 	"fyne.io/fyne/v2/theme"
@@ -911,10 +912,12 @@ func storageURI(a fyne.App) fyne.URI {
 // iconPaddingLayout adds padding to the left of a widget.Icon().
 // NOTE: It assumes that the slice only contains one item.
 type iconPaddingLayout struct {
+	layout.BaseLayout
 }
 
 func (i *iconPaddingLayout) Layout(objects []fyne.CanvasObject, size fyne.Size) {
-	padding := theme.Padding() * 2
+	_, _, leftPadding, _ := i.GetPaddings()
+	padding := leftPadding * 2
 	objects[0].Move(fyne.NewPos(padding, 0))
 	objects[0].Resize(size.SubtractWidthHeight(padding, 0))
 }

internal/driver/glfw/canvas_test.go

@@ -567,6 +567,12 @@ func (l *recordingLayout) MinSize([]fyne.CanvasObject) fyne.Size {
 	return fyne.NewSize(6, 9)
 }
 
+func (l *recordingLayout) GetPaddings() (float32, float32, float32, float32) {
+	return 0, 0, 0, 0
+}
+
+func (l *recordingLayout) SetPaddingFn(paddingFn func() (float32, float32, float32, float32)) {}
+
 func (l *recordingLayout) popLayoutEvent() (e any) {
 	e, l.layoutEvents = pop(l.layoutEvents)
 	return

layout.go

@@ -8,4 +8,10 @@ type Layout interface {
 	// MinSize calculates the smallest size that will fit the listed
 	// CanvasObjects using this Layout algorithm.
 	MinSize(objects []CanvasObject) Size
+	// GetPaddings returns the top, bottom, left and right paddings used
+	// by this layout.
+	GetPaddings() (float32, float32, float32, float32)
+	// SetPaddingFn sets the function that will be called to get the
+	// padding values for this layout.
+	SetPaddingFn(fn func() (float32, float32, float32, float32))
 }

layout/baselayout.go

@@ -0,0 +1,19 @@
+package layout
+
+import "fyne.io/fyne/v2/theme"
+
+type BaseLayout struct {
+	paddingFn func() (float32, float32, float32, float32)
+}
+
+func (b *BaseLayout) SetPaddingFn(fn func() (float32, float32, float32, float32)) {
+	b.paddingFn = fn
+}
+
+func (b *BaseLayout) GetPaddings() (float32, float32, float32, float32) {
+	if b.paddingFn == nil {
+		padding := theme.Padding()
+		return padding, padding, padding, padding
+	}
+	return b.paddingFn()
+}

layout/borderlayout.go

@@ -2,52 +2,58 @@ package layout
 
 import (
 	"fyne.io/fyne/v2"
-	"fyne.io/fyne/v2/theme"
 )
 
 // Declare conformity with Layout interface
 var _ fyne.Layout = (*borderLayout)(nil)
 
 type borderLayout struct {
+	BaseLayout
+
 	top, bottom, left, right fyne.CanvasObject
 }
 
 // NewBorderLayout creates a new BorderLayout instance with top, bottom, left
 // and right objects set. All other items in the container will fill the centre
 // space
-func NewBorderLayout(top, bottom, left, right fyne.CanvasObject) fyne.Layout {
-	return &borderLayout{top, bottom, left, right}
+func NewBorderLayout(top, bottom, left, right fyne.CanvasObject, options ...LayoutOption) fyne.Layout {
+	l := &borderLayout{top: top, bottom: bottom, left: left, right: right}
+	for _, option := range options {
+		option(l)
+	}
+	return l
 }
 
 // Layout is called to pack all child objects into a specified size.
 // For BorderLayout this arranges the top, bottom, left and right widgets at
 // the sides and any remaining widgets are maximised in the middle space.
 func (b *borderLayout) Layout(objects []fyne.CanvasObject, size fyne.Size) {
-	padding := theme.Padding()
+	topPadding, bottomPadding, leftPadding, rightPadding := b.GetPaddings()
+
 	var topSize, bottomSize, leftSize, rightSize fyne.Size
 	if b.top != nil && b.top.Visible() {
 		topHeight := b.top.MinSize().Height
 		b.top.Resize(fyne.NewSize(size.Width, topHeight))
 		b.top.Move(fyne.NewPos(0, 0))
-		topSize = fyne.NewSize(size.Width, topHeight+padding)
+		topSize = fyne.NewSize(size.Width, topHeight+topPadding)
 	}
 	if b.bottom != nil && b.bottom.Visible() {
 		bottomHeight := b.bottom.MinSize().Height
 		b.bottom.Resize(fyne.NewSize(size.Width, bottomHeight))
 		b.bottom.Move(fyne.NewPos(0, size.Height-bottomHeight))
-		bottomSize = fyne.NewSize(size.Width, bottomHeight+padding)
+		bottomSize = fyne.NewSize(size.Width, bottomHeight+bottomPadding)
 	}
 	if b.left != nil && b.left.Visible() {
 		leftWidth := b.left.MinSize().Width
 		b.left.Resize(fyne.NewSize(leftWidth, size.Height-topSize.Height-bottomSize.Height))
 		b.left.Move(fyne.NewPos(0, topSize.Height))
-		leftSize = fyne.NewSize(leftWidth+padding, size.Height-topSize.Height-bottomSize.Height)
+		leftSize = fyne.NewSize(leftWidth+leftPadding, size.Height-topSize.Height-bottomSize.Height)
 	}
 	if b.right != nil && b.right.Visible() {
 		rightWidth := b.right.MinSize().Width
 		b.right.Resize(fyne.NewSize(rightWidth, size.Height-topSize.Height-bottomSize.Height))
 		b.right.Move(fyne.NewPos(size.Width-rightWidth, topSize.Height))
-		rightSize = fyne.NewSize(rightWidth+padding, size.Height-topSize.Height-bottomSize.Height)
+		rightSize = fyne.NewSize(rightWidth+rightPadding, size.Height-topSize.Height-bottomSize.Height)
 	}
 
 	middleSize := fyne.NewSize(size.Width-leftSize.Width-rightSize.Width, size.Height-topSize.Height-bottomSize.Height)
@@ -80,28 +86,28 @@ func (b *borderLayout) MinSize(objects []fyne.CanvasObject) fyne.Size {
 		}
 	}
 
-	padding := theme.Padding()
+	topPadding, bottomPadding, leftPadding, rightPadding := b.GetPaddings()
 
 	if b.left != nil && b.left.Visible() {
 		leftMin := b.left.MinSize()
 		minHeight := fyne.Max(minSize.Height, leftMin.Height)
-		minSize = fyne.NewSize(minSize.Width+leftMin.Width+padding, minHeight)
+		minSize = fyne.NewSize(minSize.Width+leftMin.Width+leftPadding, minHeight)
 	}
 	if b.right != nil && b.right.Visible() {
 		rightMin := b.right.MinSize()
 		minHeight := fyne.Max(minSize.Height, rightMin.Height)
-		minSize = fyne.NewSize(minSize.Width+rightMin.Width+padding, minHeight)
+		minSize = fyne.NewSize(minSize.Width+rightMin.Width+rightPadding, minHeight)
 	}
 
 	if b.top != nil && b.top.Visible() {
 		topMin := b.top.MinSize()
 		minWidth := fyne.Max(minSize.Width, topMin.Width)
-		minSize = fyne.NewSize(minWidth, minSize.Height+topMin.Height+padding)
+		minSize = fyne.NewSize(minWidth, minSize.Height+topMin.Height+topPadding)
 	}
 	if b.bottom != nil && b.bottom.Visible() {
 		bottomMin := b.bottom.MinSize()
 		minWidth := fyne.Max(minSize.Width, bottomMin.Width)
-		minSize = fyne.NewSize(minWidth, minSize.Height+bottomMin.Height+padding)
+		minSize = fyne.NewSize(minWidth, minSize.Height+bottomMin.Height+bottomPadding)
 	}
 
 	return minSize

layout/boxlayout.go

@@ -2,29 +2,38 @@ package layout
 
 import (
 	"fyne.io/fyne/v2"
-	"fyne.io/fyne/v2/theme"
 )
 
 // NewVBoxLayout returns a vertical box layout for stacking a number of child
 // canvas objects or widgets top to bottom. The objects are always displayed
 // at their vertical MinSize. Use a different layout if the objects are intended
 // to be larger then their vertical MinSize.
-func NewVBoxLayout() fyne.Layout {
-	return vBoxLayout{}
+func NewVBoxLayout(options ...LayoutOption) fyne.Layout {
+	l := &vBoxLayout{}
+	for _, option := range options {
+		option(l)
+	}
+	return l
 }
 
 // NewHBoxLayout returns a horizontal box layout for stacking a number of child
 // canvas objects or widgets left to right. The objects are always displayed
 // at their horizontal MinSize. Use a different layout if the objects are intended
 // to be larger then their horizontal MinSize.
-func NewHBoxLayout() fyne.Layout {
-	return hBoxLayout{}
+func NewHBoxLayout(options ...LayoutOption) fyne.Layout {
+	l := &hBoxLayout{}
+	for _, option := range options {
+		option(l)
+	}
+	return l
 }
 
 // Declare conformity with Layout interface
 var _ fyne.Layout = (*vBoxLayout)(nil)
 
-type vBoxLayout struct{}
+type vBoxLayout struct {
+	BaseLayout
+}
 
 // Layout is called to pack all child objects into a specified size.
 // This will pack objects into a single column where each item
@@ -50,10 +59,10 @@ func (v vBoxLayout) Layout(objects []fyne.CanvasObject, size fyne.Size) {
 		total += child.MinSize().Height
 	}
 
-	padding := theme.Padding()
+	topPadding, _, _, _ := v.GetPaddings()
 
 	// Amount of space not taken up by visible objects and inter-object padding
-	extra := size.Height - total - (padding * float32(visibleObjects-1))
+	extra := size.Height - total - (topPadding * float32(visibleObjects-1))
 
 	// Spacers split extra space equally
 	spacerSize := float32(0)
@@ -74,7 +83,7 @@ func (v vBoxLayout) Layout(objects []fyne.CanvasObject, size fyne.Size) {
 		child.Move(fyne.NewPos(x, y))
 
 		height := child.MinSize().Height
-		y += padding + height
+		y += topPadding + height
 		child.Resize(fyne.NewSize(size.Width, height))
 	}
 }
@@ -85,7 +94,7 @@ func (v vBoxLayout) Layout(objects []fyne.CanvasObject, size fyne.Size) {
 func (v vBoxLayout) MinSize(objects []fyne.CanvasObject) fyne.Size {
 	minSize := fyne.NewSize(0, 0)
 	addPadding := false
-	padding := theme.Padding()
+	topPadding, _, _, _ := v.GetPaddings()
 	for _, child := range objects {
 		if !child.Visible() || isVerticalSpacer(child) {
 			continue
@@ -95,7 +104,7 @@ func (v vBoxLayout) MinSize(objects []fyne.CanvasObject) fyne.Size {
 		minSize.Width = fyne.Max(childMin.Width, minSize.Width)
 		minSize.Height += childMin.Height
 		if addPadding {
-			minSize.Height += padding
+			minSize.Height += topPadding
 		}
 		addPadding = true
 	}
@@ -105,7 +114,9 @@ func (v vBoxLayout) MinSize(objects []fyne.CanvasObject) fyne.Size {
 // Declare conformity with Layout interface
 var _ fyne.Layout = (*hBoxLayout)(nil)
 
-type hBoxLayout struct{}
+type hBoxLayout struct {
+	BaseLayout
+}
 
 // Layout is called to pack all child objects into a specified size.
 // For a VBoxLayout this will pack objects into a single column where each item
@@ -131,10 +142,10 @@ func (g hBoxLayout) Layout(objects []fyne.CanvasObject, size fyne.Size) {
 		total += child.MinSize().Width
 	}
 
-	padding := theme.Padding()
+	_, _, leftPadding, _ := g.GetPaddings()
 
 	// Amount of space not taken up by visible objects and inter-object padding
-	extra := size.Width - total - (padding * float32(visibleObjects-1))
+	extra := size.Width - total - (leftPadding * float32(visibleObjects-1))
 
 	// Spacers split extra space equally
 	spacerSize := float32(0)
@@ -155,7 +166,7 @@ func (g hBoxLayout) Layout(objects []fyne.CanvasObject, size fyne.Size) {
 		child.Move(fyne.NewPos(x, y))
 
 		width := child.MinSize().Width
-		x += padding + width
+		x += leftPadding + width
 		child.Resize(fyne.NewSize(width, size.Height))
 	}
 }
@@ -166,7 +177,7 @@ func (g hBoxLayout) Layout(objects []fyne.CanvasObject, size fyne.Size) {
 func (g hBoxLayout) MinSize(objects []fyne.CanvasObject) fyne.Size {
 	minSize := fyne.NewSize(0, 0)
 	addPadding := false
-	padding := theme.Padding()
+	_, _, leftPadding, _ := g.GetPaddings()
 	for _, child := range objects {
 		if !child.Visible() || isHorizontalSpacer(child) {
 			continue
@@ -176,7 +187,7 @@ func (g hBoxLayout) MinSize(objects []fyne.CanvasObject) fyne.Size {
 		minSize.Height = fyne.Max(childMin.Height, minSize.Height)
 		minSize.Width += childMin.Width
 		if addPadding {
-			minSize.Width += padding
+			minSize.Width += leftPadding
 		}
 		addPadding = true
 	}