[Material] Simple API for skipping over certain Slider shapes (#27613)

* Provide a simmplified API for skipping over slider thumb, overlay, and tick mark painting

* doc fixes

* comments

* comments

* comments

* comments

* comments

* analyzer

* comments

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

@@ -154,6 +154,16 @@ enum ShowValueIndicator {
 /// [RoundSliderTickMarkShape], [PaddleSliderValueIndicatorShape], and
 /// [RoundSliderOverlayShape] for examples.
 ///
+/// The track painting can be skipped by specifying 0 for [trackHeight].
+/// The thumb painting can be skipped by specifying
+/// [SliderComponentShape.noThumb] for [SliderThemeData.thumbShape].
+/// The overlay painting can be skipped by specifying
+/// [SliderComponentShape.noOverlay] for [SliderThemeData.overlayShape].
+/// The tick mark painting can be skipped by specifying
+/// [SliderTickMarkShape.noTickMark] for [SliderThemeData.tickMarkShape].
+/// The value indicator painting can be skipped by specifying the
+/// appropriate [ShowValueIndicator] for [SliderThemeData.showValueIndicator].
+///
 /// See also:
 ///
 ///  * [SliderTheme] widget, which can override the slider theme of its
@@ -706,6 +716,9 @@ abstract class SliderTrackShape {
 /// This is a simplified version of [SliderComponentShape] with a
 /// [SliderThemeData] passed when getting the preferred size.
 ///
+/// The tick mark painting can be skipped by specifying [noTickMark] for
+/// [SliderThemeData.tickMarkShape].
+///
 /// See also:
 ///
 ///  * [RoundSliderTickMarkShape] for a simple example of a tick mark shape.
@@ -759,6 +772,45 @@ abstract class SliderTickMarkShape {
     bool isEnabled,
     TextDirection textDirection,
   });
+
+  /// Special instance of [SliderTickMarkShape] to skip the tick mark painting.
+  ///
+  /// See also:
+  ///
+  /// * [SliderThemeData.tickMarkShape], which is the shape that the [Slider]
+  /// uses when painting tick marks.
+  static final SliderTickMarkShape noTickMark = _EmptySliderTickMarkShape();
+}
+
+/// A special version of [SliderTickMarkShape] that has a zero size and paints
+/// nothing.
+///
+/// This class is used to create a special instance of a [SliderTickMarkShape]
+/// that will not paint any tick mark shape. A static reference is stored in
+/// [SliderTickMarkShape.noTickMark]. When this value  is specified for
+/// [SliderThemeData.tickMarkShape], the tick mark painting is skipped.
+class _EmptySliderTickMarkShape extends SliderTickMarkShape {
+  @override
+  Size getPreferredSize({
+    SliderThemeData sliderTheme,
+    bool isEnabled,
+  }) {
+    return Size.zero;
+  }
+
+  @override
+  void paint(
+    PaintingContext context,
+    Offset center, {
+    RenderBox parentBox,
+    SliderThemeData sliderTheme,
+    Animation<double> enableAnimation,
+    Offset thumbCenter,
+    bool isEnabled,
+    TextDirection textDirection,
+  }) {
+    // no-op.
+  }
 }
 
 /// Base class for slider thumb, thumb overlay, and value indicator shapes.
@@ -768,6 +820,12 @@ abstract class SliderTickMarkShape {
 /// All shapes are painted to the same canvas and ordering is important.
 /// The overlay is painted first, then the value indicator, then the thumb.
 ///
+/// The thumb painting can be skipped by specifying [noThumb] for
+/// [SliderThemeData.thumbShape].
+///
+/// The overlay painting can be skipped by specifying [noOverlay] for
+/// [SliderThemeData.overlayShape].
+///
 /// See also:
 ///
 ///  * [RoundSliderThumbShape], which is the the default thumb shape.
@@ -821,6 +879,52 @@ abstract class SliderComponentShape {
     TextDirection textDirection,
     double value,
   });
+
+  /// Special instance of [SliderComponentShape] to skip the thumb drawing.
+  ///
+  /// See also:
+  ///
+  /// * [SliderThemeData.thumbShape], which is the shape that the [Slider]
+  /// uses when painting the thumb.
+  static final SliderComponentShape noThumb = _EmptySliderComponentShape();
+
+  /// Special instance of [SliderComponentShape] to skip the overlay drawing.
+  ///
+  /// See also:
+  ///
+  /// * [SliderThemeData.overlayShape], which is the shape that the [Slider]
+  /// uses when painting the overlay.
+  static final SliderComponentShape noOverlay = _EmptySliderComponentShape();
+}
+
+/// A special version of [SliderComponentShape] that has a zero size and paints
+/// nothing.
+///
+/// This class is used to create a special instance of a [SliderComponentShape]
+/// that will not paint any component shape. A static reference is stored in
+/// [SliderTickMarkShape.noThumb] and [SliderTickMarkShape.noOverlay]. When this value
+/// is specified for [SliderThemeData.thumbShape], the thumb painting is
+/// skipped.  When this value is specified for [SliderThemeData.overlaySHape],
+/// the overlay painting is skipped.
+class _EmptySliderComponentShape extends SliderComponentShape {
+  @override
+  Size getPreferredSize(bool isEnabled, bool isDiscrete) => Size.zero;
+
+  @override
+  void paint(
+    PaintingContext context,
+    Offset center, {
+    Animation<double> activationAnimation,
+    Animation<double> enableAnimation,
+    bool isDiscrete,
+    TextPainter labelPainter,
+    RenderBox parentBox,
+    SliderThemeData sliderTheme,
+    TextDirection textDirection,
+    double value,
+  }) {
+    // no-op.
+  }
 }
 
 // The following shapes are the material defaults.
@@ -894,6 +998,12 @@ class RectangularSliderTrackShape extends SliderTrackShape {
     bool isDiscrete,
     bool isEnabled,
   }) {
+    // If the slider track height is 0, then it makes no difference whether the
+    // track is painted or not, therefore the painting can be a no-op.
+    if (sliderTheme.trackHeight == 0) {
+      return;
+    }
+
     // Assign the track segment paints, which are left: active, right: inactive,
     // but reversed for right to left text.
     final ColorTween activeTrackColorTween = ColorTween(begin: sliderTheme.disabledActiveTrackColor , end: sliderTheme.activeTrackColor);

packages/flutter/test/material/slider_theme_test.dart

@@ -590,6 +590,152 @@ void main() {
       )
     );
   });
+
+  // Only the thumb, overlay, and tick mark have special shortcuts to provide
+  // no-op or empty shapes.
+  //
+  // The track can also be skipped by providing 0 height.
+  //
+  // The value indicator can be skipped by passing the appropriate
+  // [ShowValueIndicator].
+  testWidgets('The slider can skip all of its comoponent painting', (WidgetTester tester) async {
+    // Pump a slider with all shapes skipped.
+    await tester.pumpWidget(_buildApp(
+      ThemeData().sliderTheme.copyWith(
+        trackHeight: 0,
+        overlayShape: SliderComponentShape.noOverlay,
+        thumbShape: SliderComponentShape.noThumb,
+        tickMarkShape: SliderTickMarkShape.noTickMark,
+        showValueIndicator: ShowValueIndicator.never,
+      ),
+      value: 0.5,
+      divisions: 4
+    ));
+
+    final RenderBox sliderBox = tester.firstRenderObject<RenderBox>(find.byType(Slider));
+
+    expect(sliderBox, paintsNothing);
+  });
+
+  testWidgets('The slider can skip all component painting except the track', (WidgetTester tester) async {
+    // Pump a slider with just a track.
+    await tester.pumpWidget(_buildApp(
+      ThemeData().sliderTheme.copyWith(
+        overlayShape: SliderComponentShape.noOverlay,
+        thumbShape: SliderComponentShape.noThumb,
+        tickMarkShape: SliderTickMarkShape.noTickMark,
+        showValueIndicator: ShowValueIndicator.never,
+      ),
+      value: 0.5,
+      divisions: 4
+    ));
+
+    final RenderBox sliderBox = tester.firstRenderObject<RenderBox>(find.byType(Slider));
+
+    // Only 2 track segments.
+    expect(sliderBox, paintsExactlyCountTimes(#drawRect, 2));
+    expect(sliderBox, paintsExactlyCountTimes(#drawCircle, 0));
+    expect(sliderBox, paintsExactlyCountTimes(#drawPath, 0));
+  });
+
+  testWidgets('The slider can skip all component painting except the tick marks', (WidgetTester tester) async {
+    // Pump a slider with just tick marks.
+    await tester.pumpWidget(_buildApp(
+      ThemeData().sliderTheme.copyWith(
+        trackHeight: 0,
+        overlayShape: SliderComponentShape.noOverlay,
+        thumbShape: SliderComponentShape.noThumb,
+        showValueIndicator: ShowValueIndicator.never,
+      ),
+      value: 0.5,
+      divisions: 4
+    ));
+
+    final RenderBox sliderBox = tester.firstRenderObject<RenderBox>(find.byType(Slider));
+
+    // Only 5 tick marks.
+    expect(sliderBox, paintsExactlyCountTimes(#drawRect, 0));
+    expect(sliderBox, paintsExactlyCountTimes(#drawCircle, 5));
+    expect(sliderBox, paintsExactlyCountTimes(#drawPath, 0));
+  });
+
+  testWidgets('The slider can skip all component painting except the thumb', (WidgetTester tester) async {
+    // Pump a slider with just a thumb.
+    await tester.pumpWidget(_buildApp(
+      ThemeData().sliderTheme.copyWith(
+        trackHeight: 0,
+        overlayShape: SliderComponentShape.noOverlay,
+        tickMarkShape: SliderTickMarkShape.noTickMark,
+        showValueIndicator: ShowValueIndicator.never,
+      ),
+      value: 0.5,
+      divisions: 4
+    ));
+
+    final RenderBox sliderBox = tester.firstRenderObject<RenderBox>(find.byType(Slider));
+
+    // Only 1 thumb.
+    expect(sliderBox, paintsExactlyCountTimes(#drawRect, 0));
+    expect(sliderBox, paintsExactlyCountTimes(#drawCircle, 1));
+    expect(sliderBox, paintsExactlyCountTimes(#drawPath, 0));
+  });
+
+  testWidgets('The slider can skip all component painting except the overlay', (WidgetTester tester) async {
+    // Pump a slider with just an overlay.
+    await tester.pumpWidget(_buildApp(
+      ThemeData().sliderTheme.copyWith(
+        trackHeight: 0,
+        thumbShape: SliderComponentShape.noThumb,
+        tickMarkShape: SliderTickMarkShape.noTickMark,
+        showValueIndicator: ShowValueIndicator.never,
+      ),
+      value: 0.5,
+      divisions: 4
+    ));
+
+    final RenderBox sliderBox = tester.firstRenderObject<RenderBox>(find.byType(Slider));
+
+    // Tap the center of the track and wait for animations to finish.
+    final Offset center = tester.getCenter(find.byType(Slider));
+    final TestGesture gesture = await tester.startGesture(center);
+    await tester.pumpAndSettle();
+
+    // Only 1 overlay.
+    expect(sliderBox, paintsExactlyCountTimes(#drawRect, 0));
+    expect(sliderBox, paintsExactlyCountTimes(#drawCircle, 1));
+    expect(sliderBox, paintsExactlyCountTimes(#drawPath, 0));
+
+    await gesture.up();
+  });
+
+  testWidgets('The slider can skip all component painting except the value indicator', (WidgetTester tester) async {
+    // Pump a slider with just a value indicator.
+    await tester.pumpWidget(_buildApp(
+      ThemeData().sliderTheme.copyWith(
+        trackHeight: 0,
+        overlayShape: SliderComponentShape.noOverlay,
+        thumbShape: SliderComponentShape.noThumb,
+        tickMarkShape: SliderTickMarkShape.noTickMark,
+        showValueIndicator: ShowValueIndicator.always,
+      ),
+      value: 0.5,
+      divisions: 4
+    ));
+
+    final RenderBox sliderBox = tester.firstRenderObject<RenderBox>(find.byType(Slider));
+
+    // Tap the center of the track and wait for animations to finish.
+    final Offset center = tester.getCenter(find.byType(Slider));
+    final TestGesture gesture = await tester.startGesture(center);
+    await tester.pumpAndSettle();
+
+    // Only 1 value indicator.
+    expect(sliderBox, paintsExactlyCountTimes(#drawRect, 0));
+    expect(sliderBox, paintsExactlyCountTimes(#drawCircle, 0));
+    expect(sliderBox, paintsExactlyCountTimes(#drawPath, 1));
+
+    await gesture.up();
+  });
 }
 
 Widget _buildApp(