Fix ChipThemeData lerp for BorderSide (#173160)

In https://github.com/flutter/flutter/pull/171945#discussion_r2245715705
we realised that some `lerp` method for `BorderSide` had a bug.

This PR
- Fixes a wrong interpolation of `ChipThemeData.lerp` for `side` in the
case of `b` being `null`
- Do some cleaning of other `lerp` of `BorderSide?`

Fixes https://github.com/flutter/flutter/issues/173161

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

@@ -776,7 +776,7 @@ class ButtonStyle with Diagnosticable {
       iconColor: MaterialStateProperty.lerp<Color?>(a?.iconColor, b?.iconColor, t, Color.lerp),
       iconSize: MaterialStateProperty.lerp<double?>(a?.iconSize, b?.iconSize, t, lerpDouble),
       iconAlignment: t < 0.5 ? a?.iconAlignment : b?.iconAlignment,
-      side: _lerpSides(a?.side, b?.side, t),
+      side: WidgetStateBorderSide.lerp(a?.side, b?.side, t),
       shape: MaterialStateProperty.lerp<OutlinedBorder?>(
         a?.shape,
         b?.shape,
@@ -794,16 +794,4 @@ class ButtonStyle with Diagnosticable {
       foregroundBuilder: t < 0.5 ? a?.foregroundBuilder : b?.foregroundBuilder,
     );
   }
-
-  // Special case because BorderSide.lerp() doesn't support null arguments
-  static MaterialStateProperty<BorderSide?>? _lerpSides(
-    MaterialStateProperty<BorderSide?>? a,
-    MaterialStateProperty<BorderSide?>? b,
-    double t,
-  ) {
-    if (a == null && b == null) {
-      return null;
-    }
-    return MaterialStateBorderSide.lerp(a, b, t);
-  }
 }

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

@@ -235,19 +235,19 @@ class CheckboxThemeData with Diagnosticable {
 
   // Special case because BorderSide.lerp() doesn't support null arguments
   static BorderSide? _lerpSides(BorderSide? a, BorderSide? b, double t) {
-    if (a == null || b == null) {
+    if (a == null && b == null) {
       return null;
     }
-    if (identical(a, b)) {
-      return a;
+    if (a is WidgetStateBorderSide) {
+      a = a.resolve(const <WidgetState>{});
     }
-    if (a is MaterialStateBorderSide) {
-      a = a.resolve(<WidgetState>{});
+    if (b is WidgetStateBorderSide) {
+      b = b.resolve(const <WidgetState>{});
     }
-    if (b is MaterialStateBorderSide) {
-      b = b.resolve(<WidgetState>{});
-    }
-    return BorderSide.lerp(a!, b!, t);
+    a ??= BorderSide(width: 0, color: b!.color.withAlpha(0));
+    b ??= BorderSide(width: 0, color: a.color.withAlpha(0));
+
+    return BorderSide.lerp(a, b, t);
   }
 }
 

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

@@ -539,7 +539,7 @@ class ChipThemeData with Diagnosticable {
       labelPadding: EdgeInsetsGeometry.lerp(a?.labelPadding, b?.labelPadding, t),
       padding: EdgeInsetsGeometry.lerp(a?.padding, b?.padding, t),
       side: _lerpSides(a?.side, b?.side, t),
-      shape: _lerpShapes(a?.shape, b?.shape, t),
+      shape: OutlinedBorder.lerp(a?.shape, b?.shape, t),
       labelStyle: TextStyle.lerp(a?.labelStyle, b?.labelStyle, t),
       secondaryLabelStyle: TextStyle.lerp(a?.secondaryLabelStyle, b?.secondaryLabelStyle, t),
       brightness: t < 0.5 ? a?.brightness ?? Brightness.light : b?.brightness ?? Brightness.light,
@@ -566,27 +566,16 @@ class ChipThemeData with Diagnosticable {
     if (a == null && b == null) {
       return null;
     }
-    if (a is MaterialStateBorderSide) {
-      a = a.resolve(<WidgetState>{});
+    if (a is WidgetStateBorderSide) {
+      a = a.resolve(const <WidgetState>{});
     }
-    if (b is MaterialStateBorderSide) {
-      b = b.resolve(<WidgetState>{});
+    if (b is WidgetStateBorderSide) {
+      b = b.resolve(const <WidgetState>{});
     }
-    if (a == null) {
-      return BorderSide.lerp(BorderSide(width: 0, color: b!.color.withAlpha(0)), b, t);
-    }
-    if (b == null) {
-      return BorderSide.lerp(BorderSide(width: 0, color: a.color.withAlpha(0)), a, t);
-    }
-    return BorderSide.lerp(a, b, t);
-  }
+    a ??= BorderSide(width: 0, color: b!.color.withAlpha(0));
+    b ??= BorderSide(width: 0, color: a.color.withAlpha(0));
 
-  // TODO(perclasson): OutlinedBorder needs a lerp method - https://github.com/flutter/flutter/issues/60555.
-  static OutlinedBorder? _lerpShapes(OutlinedBorder? a, OutlinedBorder? b, double t) {
-    if (a == null && b == null) {
-      return null;
-    }
-    return ShapeBorder.lerp(a, b, t) as OutlinedBorder?;
+    return BorderSide.lerp(a, b, t);
   }
 
   @override

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

@@ -155,7 +155,7 @@ class SearchBarThemeData with Diagnosticable {
         t,
         Color.lerp,
       ),
-      side: _lerpSides(a?.side, b?.side, t),
+      side: MaterialStateBorderSide.lerp(a?.side, b?.side, t),
       shape: MaterialStateProperty.lerp<OutlinedBorder?>(
         a?.shape,
         b?.shape,
@@ -304,18 +304,6 @@ class SearchBarThemeData with Diagnosticable {
       ),
     );
   }
-
-  // Special case because BorderSide.lerp() doesn't support null arguments
-  static MaterialStateProperty<BorderSide?>? _lerpSides(
-    MaterialStateProperty<BorderSide?>? a,
-    MaterialStateProperty<BorderSide?>? b,
-    double t,
-  ) {
-    if (identical(a, b)) {
-      return a;
-    }
-    return MaterialStateBorderSide.lerp(a, b, t);
-  }
 }
 
 /// Applies a search bar theme to descendant [SearchBar] widgets.

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

@@ -225,12 +225,18 @@ class SearchViewThemeData with Diagnosticable {
 
   // Special case because BorderSide.lerp() doesn't support null arguments
   static BorderSide? _lerpSides(BorderSide? a, BorderSide? b, double t) {
-    if (a == null || b == null) {
+    if (a == null && b == null) {
       return null;
     }
-    if (identical(a, b)) {
-      return a;
+    if (a is WidgetStateBorderSide) {
+      a = a.resolve(const <WidgetState>{});
     }
+    if (b is WidgetStateBorderSide) {
+      b = b.resolve(const <WidgetState>{});
+    }
+    a ??= BorderSide(width: 0, color: b!.color.withAlpha(0));
+    b ??= BorderSide(width: 0, color: a.color.withAlpha(0));
+
     return BorderSide.lerp(a, b, t);
   }
 }

packages/flutter/lib/src/widgets/widget_state.dart

@@ -582,6 +582,9 @@ abstract class WidgetStateBorderSide extends BorderSide
     if (a == null && b == null) {
       return null;
     }
+    if (identical(a, b)) {
+      return a;
+    }
     return _LerpSides(a, b, t);
   }
 }

packages/flutter/test/material/checkbox_theme_test.dart

@@ -458,8 +458,44 @@ void main() {
       lerped.shape,
       const RoundedRectangleBorder(borderRadius: BorderRadius.all(Radius.circular(2.0))),
     );
-    // Returns null if either lerp value is null.
-    expect(lerped.side, null);
+    expect(lerped.side!.width, 2.0);
+    expect(lerped.side!.color, isSameColorAs(const Color(0x80000000)));
+  });
+
+  test('CheckboxThemeData lerp from null to populated parameters', () {
+    final CheckboxThemeData theme = CheckboxThemeData(
+      fillColor: MaterialStateProperty.all(const Color(0xfffffff0)),
+      checkColor: MaterialStateProperty.all(const Color(0xfffffff1)),
+      overlayColor: MaterialStateProperty.all(const Color(0xfffffff2)),
+      splashRadius: 4.0,
+      materialTapTargetSize: MaterialTapTargetSize.shrinkWrap,
+      visualDensity: const VisualDensity(vertical: 1.0, horizontal: 1.0),
+      shape: const RoundedRectangleBorder(borderRadius: BorderRadius.all(Radius.circular(4.0))),
+      side: const BorderSide(width: 4.0),
+    );
+    final CheckboxThemeData lerped = CheckboxThemeData.lerp(null, theme, 0.25);
+
+    expect(
+      lerped.fillColor!.resolve(const <MaterialState>{}),
+      isSameColorAs(const Color(0x40fffff0)),
+    );
+    expect(
+      lerped.checkColor!.resolve(const <MaterialState>{}),
+      isSameColorAs(const Color(0x40fffff1)),
+    );
+    expect(
+      lerped.overlayColor!.resolve(const <MaterialState>{}),
+      isSameColorAs(const Color(0x40fffff2)),
+    );
+    expect(lerped.splashRadius, 1);
+    expect(lerped.materialTapTargetSize, null);
+    expect(lerped.visualDensity, null);
+    expect(
+      lerped.shape,
+      const RoundedRectangleBorder(borderRadius: BorderRadius.all(Radius.circular(1.0))),
+    );
+    expect(lerped.side!.width, 1.0);
+    expect(lerped.side!.color, isSameColorAs(const Color(0x40000000)));
   });
 
   test('CheckboxThemeData lerp from populated parameters', () {

packages/flutter/test/material/chip_theme_test.dart

@@ -738,7 +738,7 @@ void main() {
     expect(lerpBNull25.showCheckmark, equals(false));
     expect(lerpBNull25.labelPadding, equals(const EdgeInsets.only(left: 6.0, right: 6.0)));
     expect(lerpBNull25.padding, equals(const EdgeInsets.all(3.0)));
-    expect(lerpBNull25.side!.color, isSameColorAs(Colors.black.withAlpha(0x3f)));
+    expect(lerpBNull25.side!.color, isSameColorAs(Colors.black.withAlpha(0xbf)));
     expect(lerpBNull25.shape, isA<StadiumBorder>());
     expect(lerpBNull25.labelStyle?.color, isSameColorAs(Colors.white.withAlpha(0xa7)));
     expect(lerpBNull25.secondaryLabelStyle?.color, isSameColorAs(Colors.black.withAlpha(0xa7)));
@@ -760,7 +760,7 @@ void main() {
     expect(lerpBNull75.showCheckmark, equals(true));
     expect(lerpBNull75.labelPadding, equals(const EdgeInsets.only(left: 2.0, right: 2.0)));
     expect(lerpBNull75.padding, equals(const EdgeInsets.all(1.0)));
-    expect(lerpBNull75.side!.color, isSameColorAs(Colors.black.withAlpha(0xbf)));
+    expect(lerpBNull75.side!.color, isSameColorAs(Colors.black.withAlpha(0x3f)));
     expect(lerpBNull75.shape, isA<StadiumBorder>());
     expect(lerpBNull75.labelStyle?.color, isSameColorAs(Colors.white.withAlpha(0x38)));
     expect(lerpBNull75.secondaryLabelStyle?.color, isSameColorAs(Colors.black.withAlpha(0x38)));