Generalized TabBar selected tab indicator (#14741)

packages/flutter/lib/material.dart

@@ -81,6 +81,7 @@ export 'src/material/stepper.dart';
 export 'src/material/switch.dart';
 export 'src/material/switch_list_tile.dart';
 export 'src/material/tab_controller.dart';
+export 'src/material/tab_indicator.dart';
 export 'src/material/tabs.dart';
 export 'src/material/text_field.dart';
 export 'src/material/text_form_field.dart';

packages/flutter/lib/src/material/tab_indicator.dart

+// Copyright 2018 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter/widgets.dart';
+
+import 'colors.dart';
+
+/// Used with [TabBar.indicator] to draw a horizontal line below the
+/// selected tab.
+///
+/// The selected tab underline is inset from the tab's boundary by [insets].
+/// The [borderSide] defines the line's color and weight.
+///
+/// The [TabBar.indicatorSize] property can be used to define the indicator's
+/// bounds in terms of its (centered) widget with [TabIndicatorSize.label],
+/// or the entire tab with [TabIndicatorSize.tab].
+class UnderlineTabIndicator extends Decoration {
+  /// Create an underline style selected tab indicator.
+  ///
+  /// The [borderSide] and [insets] arguments must not be null.
+  const UnderlineTabIndicator({
+    this.borderSide: const BorderSide(width: 2.0, color: Colors.white),
+    this.insets: EdgeInsets.zero,
+  }) : assert(borderSide != null), assert(insets != null);
+
+  /// The color and weight of the horizontal line drawn below the selected tab.
+  final BorderSide borderSide;
+
+  /// Locates the selected tab's underline relative to the tab's boundary.
+  ///
+  /// The [TabBar.indicatorSize] property can be used to define the
+  /// tab indicator's bounds in terms of its (centered) tab widget with
+  /// [TabIndicatorSize.label], or the entire tab with [TabIndicatorSize.tab].
+  final EdgeInsetsGeometry insets;
+
+  @override
+  Decoration lerpFrom(Decoration a, double t) {
+    if (a is UnderlineTabIndicator) {
+      return new UnderlineTabIndicator(
+        borderSide: BorderSide.lerp(a.borderSide, borderSide, t),
+        insets: EdgeInsetsGeometry.lerp(a.insets, insets, t),
+      );
+    }
+    return super.lerpFrom(a, t);
+  }
+
+  @override
+  Decoration lerpTo(Decoration b, double t) {
+    if (b is UnderlineTabIndicator) {
+      return new UnderlineTabIndicator(
+        borderSide: BorderSide.lerp(borderSide, b.borderSide, t),
+        insets: EdgeInsetsGeometry.lerp(insets, b.insets, t),
+      );
+    }
+    return super.lerpTo(b, t);
+  }
+
+  @override
+  _UnderlinePainter createBoxPainter([VoidCallback onChanged]) {
+    return new _UnderlinePainter(this, onChanged);
+  }
+}
+
+class _UnderlinePainter extends BoxPainter {
+  _UnderlinePainter(this.decoration, VoidCallback onChanged)
+    : assert(decoration != null), super(onChanged);
+
+  final UnderlineTabIndicator decoration;
+
+  BorderSide get borderSide => decoration.borderSide;
+  EdgeInsetsGeometry get insets => decoration.insets;
+
+  Rect _indicatorRectFor(Rect rect, TextDirection textDirection) {
+    assert(rect != null);
+    assert(textDirection != null);
+    final Rect indicator = insets.resolve(textDirection).deflateRect(rect);
+    return new Rect.fromLTWH(
+      indicator.left,
+      indicator.bottom - borderSide.width,
+      indicator.width,
+      borderSide.width,
+    );
+  }
+
+  @override
+  void paint(Canvas canvas, Offset offset, ImageConfiguration configuration) {
+    assert(configuration != null);
+    assert(configuration.size != null);
+    final Rect rect = offset & configuration.size;
+    final TextDirection textDirection = configuration.textDirection;
+    final Rect indicator = _indicatorRectFor(rect, textDirection).deflate(borderSide.width / 2.0);
+    canvas.drawLine(indicator.bottomLeft, indicator.bottomRight, borderSide.toPaint());
+  }
+}

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

@@ -188,7 +188,7 @@ abstract class Decoration extends Diagnosticable {
 abstract class BoxPainter {
   /// Abstract const constructor. This constructor enables subclasses to provide
   /// const constructors so that they can be used in const expressions.
-  BoxPainter([this.onChanged]);
+  const BoxPainter([this.onChanged]);
 
   /// Paints the [Decoration] for which this object was created on the
   /// given canvas using the given configuration.

packages/flutter/test/rendering/mock_canvas.dart

@@ -270,8 +270,8 @@ abstract class PaintPattern {
   /// Indicates that a line is expected next.
   ///
   /// The next line is examined. Any arguments that are passed to this method
-  /// are compared to the actual [Canvas.drawLine] call's `paint` argument, and
-  /// any mismatches result in failure.
+  /// are compared to the actual [Canvas.drawLine] call's `p1`, `p2`, and
+  /// `paint` arguments, and any mismatches result in failure.
   ///
   /// If no call to [Canvas.drawLine] was made, then this results in failure.
   ///
@@ -283,7 +283,7 @@ abstract class PaintPattern {
   /// painting has completed, not at the time of the call. If the same [Paint]
   /// object is reused multiple times, then this may not match the actual
   /// arguments as they were seen by the method.
-  void line({ Color color, double strokeWidth, bool hasMaskFilter, PaintingStyle style });
+  void line({ Offset p1, Offset p2, Color color, double strokeWidth, bool hasMaskFilter, PaintingStyle style });
 
   /// Indicates that an arc is expected next.
   ///
@@ -690,8 +690,8 @@ class _TestRecordingCanvasPatternMatcher extends _TestRecordingCanvasMatcher imp
   }
 
   @override
-  void line({ Color color, double strokeWidth, bool hasMaskFilter, PaintingStyle style }) {
-    _predicates.add(new _LinePaintPredicate(color: color, strokeWidth: strokeWidth, hasMaskFilter: hasMaskFilter, style: style));
+  void line({ Offset p1, Offset p2, Color color, double strokeWidth, bool hasMaskFilter, PaintingStyle style }) {
+    _predicates.add(new _LinePaintPredicate(p1: p1, p2: p2, color: color, strokeWidth: strokeWidth, hasMaskFilter: hasMaskFilter, style: style));
   }
 
   @override
@@ -1073,11 +1073,38 @@ class _PathPaintPredicate extends _DrawCommandPaintPredicate {
   }
 }
 
-// TODO(ianh): add arguments to test the points, length, angle, that kind of thing
+// TODO(ianh): add arguments to test the length, angle, that kind of thing
 class _LinePaintPredicate extends _DrawCommandPaintPredicate {
-  _LinePaintPredicate({ Color color, double strokeWidth, bool hasMaskFilter, PaintingStyle style }) : super(
+  _LinePaintPredicate({ this.p1, this.p2, Color color, double strokeWidth, bool hasMaskFilter, PaintingStyle style }) : super(
     #drawLine, 'a line', 3, 2, color: color, strokeWidth: strokeWidth, hasMaskFilter: hasMaskFilter, style: style
   );
+
+  final Offset p1;
+  final Offset p2;
+
+  @override
+  void verifyArguments(List<dynamic> arguments) {
+    super.verifyArguments(arguments); // Checks the 3rd argument, a Paint
+    if (arguments.length != 3)
+      throw 'It called $methodName with ${arguments.length} arguments; expected 3.';
+    final Offset p1Argument = arguments[0];
+    final Offset p2Argument = arguments[1];
+    if (p1 != null && p1Argument != p1) {
+        throw 'It called $methodName with p1 endpoint, $p1Argument, which was not exactly the expected endpoint ($p1).';
+    }
+    if (p2 != null && p2Argument != p2) {
+        throw 'It called $methodName with p2 endpoint, $p2Argument, which was not exactly the expected endpoint ($p2).';
+    }
+  }
+
+  @override
+  void debugFillDescription(List<String> description) {
+    super.debugFillDescription(description);
+    if (p1 != null)
+      description.add('end point p1: $p1');
+    if (p2 != null)
+      description.add('end point p2: $p2');
+  }
 }
 
 class _ArcPaintPredicate extends _DrawCommandPaintPredicate {