Support hairline borders

Previously, border with '0' was ambiguous. Sometimes we treated it as
hairline borders, sometimes as "don't show the border", though even in
the latter case we did some graphics work sometimes. Now we have an
explicit BorderStyle.none flag to not draw the border efficiently.

dev/manual_tests/drag_and_drop.dart

@@ -64,7 +64,7 @@ class DotState extends State<Dot> {
         height: config.size,
         decoration: new BoxDecoration(
           backgroundColor: config.color,
-          border: new Border.all(color: const Color(0xFF000000), width: taps.toDouble()),
+          border: new Border.all(width: taps.toDouble()),
           shape: BoxShape.circle
         ),
         child: config.child

examples/layers/widgets/sectors.dart

@@ -134,7 +134,7 @@ class SectorAppState extends State<SectorApp> {
           child: new Container(
             margin: new EdgeInsets.all(8.0),
             decoration: new BoxDecoration(
-              border: new Border.all(color: new Color(0xFF000000))
+              border: new Border.all()
             ),
             padding: new EdgeInsets.all(8.0),
             child: new WidgetToRenderBoxAdapter(

examples/material_gallery/lib/demo/list_demo.dart

@@ -42,7 +42,7 @@ class ListDemoState extends State<ListDemo> {
     _bottomSheet = scaffoldKey.currentState.showBottomSheet((BuildContext bottomSheetContext) {
       return new Container(
         decoration: new BoxDecoration(
-          border: new Border(top: new BorderSide(color: Colors.black26, width: 1.0))
+          border: new Border(top: new BorderSide(color: Colors.black26))
         ),
         child: new Column(
           mainAxisAlignment: MainAxisAlignment.collapse,

examples/material_gallery/lib/demo/persistent_bottom_sheet_demo.dart

@@ -16,7 +16,7 @@ class PersistentBottomSheetDemo extends StatelessWidget {
     Scaffold.of(context).showBottomSheet((_) {
       return new Container(
         decoration: new BoxDecoration(
-          border: new Border(top: new BorderSide(color: Colors.black26, width: 1.0))
+          border: new Border(top: new BorderSide(color: Colors.black26))
         ),
         child: new Padding(
           padding: const EdgeInsets.all(32.0),

examples/stocks/lib/stock_symbol_viewer.dart

@@ -100,7 +100,7 @@ class StockSymbolBottomSheet extends StatelessWidget {
     return new Container(
       padding: new EdgeInsets.all(10.0),
       decoration: new BoxDecoration(
-        border: new Border(top: new BorderSide(color: Colors.black26, width: 1.0))
+        border: new Border(top: new BorderSide(color: Colors.black26))
       ),
       child: new StockSymbolView(stock: stock)
    );

packages/flutter/lib/src/painting/box_painter.dart

@@ -25,30 +25,52 @@ double _getEffectiveBorderRadius(Rect rect, double borderRadius) {
   return borderRadius > shortestSide ? shortestSide : borderRadius;
 }
 
+/// The style of line to draw for a [BorderSide] in a [Border].
+enum BorderStyle {
+  /// Skip the border.
+  none,
+
+  /// Draw the border as a solid line.
+  solid,
+
+  // if you add more, think about how they will lerp
+}
+
 /// A side of a border of a box.
 class BorderSide {
   const BorderSide({
     this.color: const Color(0xFF000000),
-    this.width: 1.0
+    this.width: 1.0,
+    this.style: BorderStyle.solid
   });
 
   /// The color of this side of the border.
   final Color color;
 
-  /// The width of this side of the border.
+  /// The width of this side of the border, in logical pixels. A
+  /// zero-width border is a hairline border. To omit the border
+  /// entirely, set the [style] to [BorderStyle.none].
   final double width;
 
-  /// A black border side of zero width.
-  static const BorderSide none = const BorderSide(width: 0.0);
+  /// The style of this side of the border.
+  ///
+  /// To omit a side, set [style] to [BorderStyle.none]. This skips
+  /// painting the border, but the border still has a [width].
+  final BorderStyle style;
+
+  /// A hairline black border that is not rendered.
+  static const BorderSide none = const BorderSide(width: 0.0, style: BorderStyle.none);
 
   /// Creates a copy of this border but with the given fields replaced with the new values.
   BorderSide copyWith({
     Color color,
-    double width
+    double width,
+    BorderStyle style
   }) {
     return new BorderSide(
       color: color ?? this.color,
-      width: width ?? this.width
+      width: width ?? this.width,
+      style: style ?? this.style
     );
   }
 
@@ -56,9 +78,38 @@ class BorderSide {
   static BorderSide lerp(BorderSide a, BorderSide b, double t) {
     assert(a != null);
     assert(b != null);
+    if (t == 0.0)
+      return a;
+    if (t == 1.0)
+      return b;
+    if (a.style == b.style) {
       return new BorderSide(
         color: Color.lerp(a.color, b.color, t),
-      width: ui.lerpDouble(a.width, b.width, t)
+        width: ui.lerpDouble(a.width, b.width, t),
+        style: a.style // == b.style
+      );
+    }
+    Color colorA, colorB;
+    switch (a.style) {
+      case BorderStyle.solid:
+        colorA = a.color;
+        break;
+      case BorderStyle.none:
+        colorA = a.color.withAlpha(0x00);
+        break;
+    }
+    switch (b.style) {
+      case BorderStyle.solid:
+        colorB = b.color;
+        break;
+      case BorderStyle.none:
+        colorB = b.color.withAlpha(0x00);
+        break;
+    }
+    return new BorderSide(
+      color: Color.lerp(colorA, colorB, t),
+      width: ui.lerpDouble(a.width, b.width, t),
+      style: BorderStyle.solid
     );
   }
 
@@ -70,14 +121,15 @@ class BorderSide {
       return false;
     final BorderSide typedOther = other;
     return color == typedOther.color &&
-           width == typedOther.width;
+           width == typedOther.width &&
+           style == typedOther.style;
   }
 
   @override
-  int get hashCode => hashValues(color, width);
+  int get hashCode => hashValues(color, width, style);
 
   @override
-  String toString() => 'BorderSide($color, $width)';
+  String toString() => 'BorderSide($color, $width, $style)';
 }
 
 /// A border of a box, comprised of four sides.
@@ -92,9 +144,10 @@ class Border {
   /// A uniform border with all sides the same color and width.
   factory Border.all({
     Color color: const Color(0xFF000000),
-    double width: 1.0
+    double width: 1.0,
+    BorderStyle style: BorderStyle.solid
   }) {
-    final BorderSide side = new BorderSide(color: color, width: width);
+    final BorderSide side = new BorderSide(color: color, width: width, style: style);
     return new Border(top: side, right: side, bottom: side, left: side);
   }
 
@@ -134,6 +187,12 @@ class Border {
         left.width != topWidth)
       return false;
 
+    final BorderStyle topStyle = top.style;
+    if (right.style != topStyle ||
+        bottom.style != topStyle ||
+        left.style != topStyle)
+      return false;
+
     return true;
   }
 
@@ -186,57 +245,99 @@ class Border {
     assert(bottom != null);
     assert(left != null);
 
-    Paint paint = new Paint();
+    Paint paint = new Paint()
+      ..strokeWidth = 0.0; // used for hairline borders
     Path path;
 
-    // TODO(ianh): Handle hairline border by drawing a single line instead of a wedge
-
+    switch (top.style) {
+      case BorderStyle.solid:
         paint.color = top.color;
         path = new Path();
         path.moveTo(rect.left, rect.top);
-    path.lineTo(rect.left + left.width, rect.top + top.width);
-    path.lineTo(rect.right - right.width, rect.top + top.width);
         path.lineTo(rect.right, rect.top);
-    path.close();
+        if (top.width == 0.0) {
+          paint.style = PaintingStyle.stroke;
+        } else {
+          paint.style = PaintingStyle.fill;
+          path.lineTo(rect.right - right.width, rect.top + top.width);
+          path.lineTo(rect.left + left.width, rect.top + top.width);
+        }
         canvas.drawPath(path, paint);
+        break;
+      case BorderStyle.none: ;
+    }
 
+    switch (right.style) {
+      case BorderStyle.solid:
         paint.color = right.color;
         path = new Path();
         path.moveTo(rect.right, rect.top);
-    path.lineTo(rect.right - right.width, rect.top + top.width);
-    path.lineTo(rect.right - right.width, rect.bottom - bottom.width);
         path.lineTo(rect.right, rect.bottom);
-    path.close();
+        if (right.width == 0.0) {
+          paint.style = PaintingStyle.stroke;
+        } else {
+          paint.style = PaintingStyle.fill;
+          path.lineTo(rect.right - right.width, rect.bottom - bottom.width);
+          path.lineTo(rect.right - right.width, rect.top + top.width);
+        }
         canvas.drawPath(path, paint);
+        break;
+      case BorderStyle.none: ;
+    }
 
+    switch (bottom.style) {
+      case BorderStyle.solid:
         paint.color = bottom.color;
         path = new Path();
         path.moveTo(rect.right, rect.bottom);
-    path.lineTo(rect.right - right.width, rect.bottom - bottom.width);
-    path.lineTo(rect.left + left.width, rect.bottom - bottom.width);
         path.lineTo(rect.left, rect.bottom);
-    path.close();
+        if (bottom.width == 0.0) {
+          paint.style = PaintingStyle.stroke;
+        } else {
+          paint.style = PaintingStyle.fill;
+          path.lineTo(rect.left + left.width, rect.bottom - bottom.width);
+          path.lineTo(rect.right - right.width, rect.bottom - bottom.width);
+        }
         canvas.drawPath(path, paint);
+        break;
+      case BorderStyle.none: ;
+    }
 
+    switch (left.style) {
+      case BorderStyle.solid:
         paint.color = left.color;
         path = new Path();
         path.moveTo(rect.left, rect.bottom);
-    path.lineTo(rect.left + left.width, rect.bottom - bottom.width);
-    path.lineTo(rect.left + left.width, rect.top + top.width);
         path.lineTo(rect.left, rect.top);
-    path.close();
+        if (right.width == 0.0) {
+          paint.style = PaintingStyle.stroke;
+        } else {
+          paint.style = PaintingStyle.fill;
+          path.lineTo(rect.left + left.width, rect.top + top.width);
+          path.lineTo(rect.left + left.width, rect.bottom - bottom.width);
+        }
         canvas.drawPath(path, paint);
+        break;
+      case BorderStyle.none: ;
+    }
   }
 
   void _paintBorderWithRadius(Canvas canvas, Rect rect, double borderRadius) {
     assert(isUniform);
-    Color color = top.color;
-    double width = top.width;
+    Paint paint = new Paint()
+      ..color = top.color;
     double radius = _getEffectiveBorderRadius(rect, borderRadius);
-    // TODO(ianh): Handle hairline borders by just drawing an RRect instead
     RRect outer = new RRect.fromRectXY(rect, radius, radius);
+    double width = top.width;
+    if (width == 0.0) {
+      paint
+        ..style = PaintingStyle.stroke
+        ..strokeWidth = 0.0;
+      canvas.drawRRect(outer, paint);
+    } else {
       RRect inner = new RRect.fromRectXY(rect.deflate(width), radius - width, radius - width);
-    canvas.drawDRRect(outer, inner, new Paint()..color = color);
+      canvas.drawDRRect(outer, inner, paint);
+    }
   }
 
   void _paintBorderWithCircle(Canvas canvas, Rect rect) {