Enable configuring minHeight for LinearProgressIndicator and update default to match spec (#55482)

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

@@ -10,7 +10,6 @@ import 'package:flutter/widgets.dart';
 import 'material.dart';
 import 'theme.dart';
 
-const double _kLinearProgressIndicatorHeight = 6.0;
 const double _kMinCircularProgressIndicatorSize = 36.0;
 const int _kIndeterminateLinearDuration = 1800;
 
@@ -221,6 +220,9 @@ class _LinearProgressIndicatorPainter extends CustomPainter {
 /// The indicator line is displayed with [valueColor], an animated value. To
 /// specify a constant color value use: `AlwaysStoppedAnimation<Color>(color)`.
 ///
+/// The minimum height of the indicator can be specified using [minHeight].
+/// The indicator can be made taller by wrapping the widget with a [SizedBox].
+///
 /// See also:
 ///
 ///  * [CircularProgressIndicator], which shows progress along a circular arc.
@@ -236,16 +238,23 @@ class LinearProgressIndicator extends ProgressIndicator {
     double value,
     Color backgroundColor,
     Animation<Color> valueColor,
+    this.minHeight,
     String semanticsLabel,
     String semanticsValue,
-  }) : super(
+  }) : assert(minHeight == null || minHeight > 0),
-         key: key,
+       super(
-         value: value,
+        key: key,
-         backgroundColor: backgroundColor,
+        value: value,
-         valueColor: valueColor,
+        backgroundColor: backgroundColor,
-         semanticsLabel: semanticsLabel,
+        valueColor: valueColor,
-         semanticsValue: semanticsValue,
+        semanticsLabel: semanticsLabel,
-       );
+        semanticsValue: semanticsValue,
+      );
+
+  /// The minimum height of the line used to draw the indicator.
+  ///
+  /// This defaults to 4dp.
+  final double minHeight;
 
   @override
   _LinearProgressIndicatorState createState() => _LinearProgressIndicatorState();
@@ -284,9 +293,9 @@ class _LinearProgressIndicatorState extends State<LinearProgressIndicator> with
     return widget._buildSemanticsWrapper(
       context: context,
       child: Container(
-        constraints: const BoxConstraints(
+        constraints: BoxConstraints(
           minWidth: double.infinity,
-          minHeight: _kLinearProgressIndicatorHeight,
+          minHeight: widget.minHeight ?? 4.0,
         ),
         child: CustomPaint(
           painter: _LinearProgressIndicatorPainter(

packages/flutter/test/material/progress_indicator_test.dart

@@ -49,6 +49,27 @@ void main() {
     handle.dispose();
   });
 
+  testWidgets('LinearProgressIndicator custom minHeight', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const Directionality(
+        textDirection: TextDirection.ltr,
+        child: Center(
+          child: SizedBox(
+            width: 200.0,
+            child: LinearProgressIndicator(value: 0.25, minHeight: 2.0),
+          ),
+        ),
+      ),
+    );
+
+    expect(
+      find.byType(LinearProgressIndicator),
+      paints
+        ..rect(rect: const Rect.fromLTRB(0.0, 0.0, 200.0, 2.0))
+        ..rect(rect: const Rect.fromLTRB(0.0, 0.0, 50.0, 2.0)),
+    );
+  });
+
   testWidgets('LinearProgressIndicator paint (LTR)', (WidgetTester tester) async {
     await tester.pumpWidget(
       const Directionality(
@@ -65,8 +86,8 @@ void main() {
     expect(
       find.byType(LinearProgressIndicator),
       paints
-        ..rect(rect: const Rect.fromLTRB(0.0, 0.0, 200.0, 6.0))
+        ..rect(rect: const Rect.fromLTRB(0.0, 0.0, 200.0, 4.0))
-        ..rect(rect: const Rect.fromLTRB(0.0, 0.0, 50.0, 6.0)),
+        ..rect(rect: const Rect.fromLTRB(0.0, 0.0, 50.0, 4.0)),
     );
 
     expect(tester.binding.transientCallbackCount, 0);
@@ -88,8 +109,8 @@ void main() {
     expect(
       find.byType(LinearProgressIndicator),
       paints
-        ..rect(rect: const Rect.fromLTRB(0.0, 0.0, 200.0, 6.0))
+        ..rect(rect: const Rect.fromLTRB(0.0, 0.0, 200.0, 4.0))
-        ..rect(rect: const Rect.fromLTRB(150.0, 0.0, 200.0, 6.0)),
+        ..rect(rect: const Rect.fromLTRB(150.0, 0.0, 200.0, 4.0)),
     );
 
     expect(tester.binding.transientCallbackCount, 0);
@@ -115,8 +136,8 @@ void main() {
     expect(
       find.byType(LinearProgressIndicator),
       paints
-        ..rect(rect: const Rect.fromLTRB(0.0, 0.0, 200.0, 6.0))
+        ..rect(rect: const Rect.fromLTRB(0.0, 0.0, 200.0, 4.0))
-        ..rect(rect: Rect.fromLTRB(0.0, 0.0, animationValue * 200.0, 6.0)),
+        ..rect(rect: Rect.fromLTRB(0.0, 0.0, animationValue * 200.0, 4.0)),
     );
 
     expect(tester.binding.transientCallbackCount, 1);
@@ -142,8 +163,8 @@ void main() {
     expect(
       find.byType(LinearProgressIndicator),
       paints
-        ..rect(rect: const Rect.fromLTRB(0.0, 0.0, 200.0, 6.0))
+        ..rect(rect: const Rect.fromLTRB(0.0, 0.0, 200.0, 4.0))
-        ..rect(rect: Rect.fromLTRB(200.0 - animationValue * 200.0, 0.0, 200.0, 6.0)),
+        ..rect(rect: Rect.fromLTRB(200.0 - animationValue * 200.0, 0.0, 200.0, 4.0)),
     );
 
     expect(tester.binding.transientCallbackCount, 1);
@@ -169,8 +190,8 @@ void main() {
     expect(
       find.byType(LinearProgressIndicator),
       paints
-        ..rect(rect: const Rect.fromLTRB(0.0, 0.0, 200.0, 6.0))
+        ..rect(rect: const Rect.fromLTRB(0.0, 0.0, 200.0, 4.0))
-        ..rect(rect: const Rect.fromLTRB(0.0, 0.0, 50.0, 6.0), color: Colors.white),
+        ..rect(rect: const Rect.fromLTRB(0.0, 0.0, 50.0, 4.0), color: Colors.white),
     );
   });