ui/layout: replace implicit begin/end scopes with explicit function scopes

Before this change, layout objects followed a pattern where a
begin method would set up the layout and return a tweaked set
of constraints, and a end method would take the widget dimensions
and return the tweaked dimensions.

As has been pointed out, this process is error prone, because the
scope of the layout objects are not clear and because it is easy
to swap two begins or two ends.

It turns out that it is possible to implement layout with function
scopes in garbage free way. A typical layout changes from

        al := layout.Align{Alignment: layout.NE}
	cs = al.Begin(ops, cs)
	in := layout.Inset{Top: ui.Dp(16)}
	cs = in.Begin(c, ops, cs)
	txt := fmt.Sprintf("m: %d %s", mallocs, u.profile.Timings)
	dims := text.Label{Material: theme.text, Face: u.face(fonts.mono, 10), Text: txt}.Layout(ops, cs)
	dims = in.End(dims)
	return al.End(dims)

to

        al := layout.Align{Alignment: layout.NE}
	return al.Layout(ops, cs, func(cs layout.Constraints) layout.Dimensions {
	       in := layout.Inset{Top: ui.Dp(16)}
	       return in.Layout(c, ops, cs, func(cs layout.Constraints) layout.Dimensions {
		       txt := fmt.Sprintf("m: %d %s", mallocs, u.profile.Timings)
		       return text.Label{Material: theme.text, Face: u.face(fonts.mono, 10), Text: txt}.Layout(ops, cs)
	       })
	})

Signed-off-by: Elias Naur <mail@eliasnaur.com>

ui/layout/list.go

@@ -56,6 +56,10 @@ type List struct {
 	dir      iterationDir
 }
 
+// ListElement is a function that computes the dimensions of
+// a list element.
+type ListElement func(cs Constraints, index int) Dimensions
+
 type iterationDir uint8
 
 const (
@@ -67,8 +71,8 @@ const (
 const inf = 1e6
 
 // Init prepares the list for iterating through its children with Next.
-func (l *List) Init(cfg ui.Config, q input.Queue, ops *ui.Ops, cs Constraints, len int) {
-	if l.More() {
+func (l *List) init(cfg ui.Config, q input.Queue, ops *ui.Ops, cs Constraints, len int) {
+	if l.more() {
 		panic("unfinished child")
 	}
 	l.config = cfg
@@ -88,7 +92,15 @@ func (l *List) Init(cfg ui.Config, q input.Queue, ops *ui.Ops, cs Constraints, l
 		l.first = len
 	}
 	l.macro.Record(ops)
-	l.Next()
+	l.next()
+}
+
+// Layout the List and return its dimensions.
+func (l *List) Layout(c ui.Config, q input.Queue, ops *ui.Ops, cs Constraints, len int, w ListElement) Dimensions {
+	for l.init(c, q, ops, cs, len); l.more(); l.next() {
+		l.end(w(l.constraints(), l.index()))
+	}
+	return l.layout()
 }
 
 func (l *List) scrollToEnd() bool {
@@ -106,23 +118,23 @@ func (l *List) update() {
 	l.offset += d
 }
 
-// Next advances to the next child.
-func (l *List) Next() {
-	l.dir = l.next()
+// next advances to the next child.
+func (l *List) next() {
+	l.dir = l.nextDir()
 	// The user scroll offset is applied after scrolling to
 	// list end.
-	if l.scrollToEnd() && !l.More() && l.scrollDelta < 0 {
+	if l.scrollToEnd() && !l.more() && l.scrollDelta < 0 {
 		l.beforeEnd = true
 		l.offset += l.scrollDelta
-		l.dir = l.next()
+		l.dir = l.nextDir()
 	}
-	if l.More() {
+	if l.more() {
 		l.child.Record(l.ops)
 	}
 }
 
-// Index is current child's position in the underlying list.
-func (l *List) Index() int {
+// index is current child's position in the underlying list.
+func (l *List) index() int {
 	switch l.dir {
 	case iterateBackward:
 		return l.first - 1
@@ -133,17 +145,17 @@ func (l *List) Index() int {
 	}
 }
 
-// Constraints is the constraints for the current child.
-func (l *List) Constraints() Constraints {
+// constraints is the constraints for the current child.
+func (l *List) constraints() Constraints {
 	return axisConstraints(l.Axis, Constraint{Max: inf}, axisCrossConstraint(l.Axis, l.cs))
 }
 
-// More reports whether more children are needed.
-func (l *List) More() bool {
+// more reports whether more children are needed.
+func (l *List) more() bool {
 	return l.dir != iterateNone
 }
 
-func (l *List) next() iterationDir {
+func (l *List) nextDir() iterationDir {
 	vsize := axisMainConstraint(l.Axis, l.cs).Max
 	last := l.first + len(l.children)
 	// Clamp offset.
@@ -165,7 +177,7 @@ func (l *List) next() iterationDir {
 }
 
 // End the current child by specifying its dimensions.
-func (l *List) End(dims Dimensions) {
+func (l *List) end(dims Dimensions) {
 	l.child.Stop()
 	child := scrollChild{dims.Size, l.child}
 	mainSize := axisMain(l.Axis, child.size)
@@ -184,8 +196,8 @@ func (l *List) End(dims Dimensions) {
 }
 
 // Layout the List and return its dimensions.
-func (l *List) Layout() Dimensions {
-	if l.More() {
+func (l *List) layout() Dimensions {
+	if l.more() {
 		panic("unfinished child")
 	}
 	mainc := axisMainConstraint(l.Axis, l.cs)