widget: update glyph iteration

This commit updates the textIterator and glyphIndex types to consume
new flag information provided on glyphs. These changes allow widget.Label
and widget.Editor to correctly compute text bounding boxes and to
generate valid cursor positions at the end of text.

Signed-off-by: Chris Waldon <christopher.waldon.dev@gmail.com>

widget/label.go

@@ -35,7 +35,7 @@ func (l Label) Layout(gtx layout.Context, lt *text.Shaper, font text.Font, size
 	}, cs.Min.X, cs.Max.X, gtx.Locale, txt)
 	m := op.Record(gtx.Ops)
 	viewport := image.Rectangle{Max: cs.Max}
-	it := textIterator{viewport: viewport}
+	it := textIterator{viewport: viewport, maxLines: l.MaxLines}
 	semantic.LabelOp(txt).Add(gtx.Ops)
 	var glyphs [32]text.Glyph
 	line := glyphs[:0]
@@ -66,7 +66,11 @@ type textIterator struct {
 	// viewport is the rectangle of document coordinates that the iterator is
 	// trying to fill with text.
 	viewport image.Rectangle
+	// maxLines is the maximum number of text lines that should be displayed.
+	maxLines int
 
+	// linesSeen tracks the quantity of line endings this iterator has seen.
+	linesSeen int
 	// lineOff tracks the origin for the glyphs in the current line.
 	lineOff image.Point
 	// padding is the space needed outside of the bounds of the text to ensure no
@@ -86,9 +90,13 @@ type textIterator struct {
 // processGlyph checks whether the glyph is visible within the iterator's configured
 // viewport and (if so) updates the iterator's text dimensions to include the glyph.
 func (it *textIterator) processGlyph(g text.Glyph, ok bool) (_ text.Glyph, visibleOrBefore bool) {
-	bounds := image.Rectangle{
-		Min: image.Pt(g.Bounds.Min.X.Floor(), g.Bounds.Min.Y.Floor()),
-		Max: image.Pt(g.Bounds.Max.X.Ceil(), g.Bounds.Max.Y.Ceil()),
+	if it.maxLines > 0 {
+		if g.Flags&text.FlagLineBreak != 0 {
+			it.linesSeen++
+		}
+		if it.linesSeen == it.maxLines && g.Flags&text.FlagParagraphBreak != 0 {
+			return g, false
+		}
 	}
 	// Compute the maximum extent to which glyphs overhang on the horizontal
 	// axis.
@@ -98,26 +106,26 @@ func (it *textIterator) processGlyph(g text.Glyph, ok bool) (_ text.Glyph, visib
 	if d := (g.Bounds.Max.X - g.Advance).Ceil(); d > it.padding.Max.X {
 		it.padding.Max.X = d
 	}
-	// Convert the bounds from dot-relative coordinates to document coordinates.
-	bounds = bounds.Add(image.Pt(g.X.Round(), int(g.Y)))
+	logicalBounds := image.Rectangle{
+		Min: image.Pt(g.X.Floor(), int(g.Y)-g.Ascent.Ceil()),
+		Max: image.Pt((g.X + g.Advance).Ceil(), int(g.Y)+g.Descent.Ceil()),
+	}
 	if !it.first {
 		it.first = true
 		it.baseline = int(g.Y)
-		it.bounds = bounds
+		it.bounds = logicalBounds
 	}
 
-	lineTop := int(g.Y) - g.Ascent.Ceil()
-	lineBottom := int(g.Y) + g.Descent.Ceil()
-	above := lineBottom < it.viewport.Min.Y
-	below := lineTop > it.viewport.Max.Y
-	left := bounds.Max.X < it.viewport.Min.X
-	right := bounds.Min.X > it.viewport.Max.X
+	above := logicalBounds.Max.Y < it.viewport.Min.Y
+	below := logicalBounds.Min.Y > it.viewport.Max.Y
+	left := logicalBounds.Max.X < it.viewport.Min.X
+	right := logicalBounds.Min.X > it.viewport.Max.X
 	it.visible = !above && !below && !left && !right
 	if it.visible {
-		it.bounds.Min.X = min(it.bounds.Min.X, bounds.Min.X)
-		it.bounds.Min.Y = min(it.bounds.Min.Y, lineTop)
-		it.bounds.Max.X = max(it.bounds.Max.X, bounds.Max.X)
-		it.bounds.Max.Y = max(it.bounds.Max.Y, lineBottom)
+		it.bounds.Min.X = min(it.bounds.Min.X, logicalBounds.Min.X)
+		it.bounds.Min.Y = min(it.bounds.Min.Y, logicalBounds.Min.Y)
+		it.bounds.Max.X = max(it.bounds.Max.X, logicalBounds.Max.X)
+		it.bounds.Max.Y = max(it.bounds.Max.Y, logicalBounds.Max.Y)
 	}
 	return g, ok && !below
 }
@@ -131,14 +139,13 @@ func (it *textIterator) processGlyph(g text.Glyph, ok bool) (_ text.Glyph, visib
 // to the heap.
 func (it *textIterator) paintGlyph(gtx layout.Context, shaper *text.Shaper, glyph text.Glyph, line []text.Glyph) ([]text.Glyph, bool) {
 	_, visibleOrBefore := it.processGlyph(glyph, true)
-	done := !visibleOrBefore
 	if it.visible {
 		if len(line) == 0 {
 			it.lineOff = image.Point{X: glyph.X.Floor(), Y: int(glyph.Y)}.Sub(it.viewport.Min)
 		}
 		line = append(line, glyph)
 	}
-	if glyph.Flags&text.FlagLineBreak > 0 || cap(line)-len(line) == 0 || done {
+	if glyph.Flags&text.FlagLineBreak > 0 || cap(line)-len(line) == 0 || !visibleOrBefore {
 		t := op.Offset(it.lineOff).Push(gtx.Ops)
 		op := clip.Outline{Path: shaper.Shape(line)}.Op().Push(gtx.Ops)
 		paint.PaintOp{}.Add(gtx.Ops)
@@ -146,5 +153,5 @@ func (it *textIterator) paintGlyph(gtx layout.Context, shaper *text.Shaper, glyp
 		t.Pop()
 		line = line[:0]
 	}
-	return line, !done
+	return line, visibleOrBefore
 }