Introduce WidgetStateBorderSide.lerp (#148122)

fixes [Consolidate `_LerpSides` classes with `WigetStateProperty<BorderSide?>` type  into a new `WidgetStateBorderSide.lerp`](https://github.com/flutter/flutter/issues/148057)

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

@@ -603,30 +603,6 @@ class ButtonStyle with Diagnosticable {
     if (a == null && b == null) {
       return null;
     }
-    return _LerpSides(a, b, t);
+    return MaterialStateBorderSide.lerp(a, b, t);
-  }
-}
-
-class _LerpSides implements MaterialStateProperty<BorderSide?> {
-  const _LerpSides(this.a, this.b, this.t);
-
-  final MaterialStateProperty<BorderSide?>? a;
-  final MaterialStateProperty<BorderSide?>? b;
-  final double t;
-
-  @override
-  BorderSide? resolve(Set<MaterialState> states) {
-    final BorderSide? resolvedA = a?.resolve(states);
-    final BorderSide? resolvedB = b?.resolve(states);
-    if (resolvedA == null && resolvedB == null) {
-      return null;
-    }
-    if (resolvedA == null) {
-      return BorderSide.lerp(BorderSide(width: 0, color: resolvedB!.color.withAlpha(0)), resolvedB, t);
-    }
-    if (resolvedB == null) {
-      return BorderSide.lerp(resolvedA, BorderSide(width: 0, color: resolvedA.color.withAlpha(0)), t);
-    }
-    return BorderSide.lerp(resolvedA, resolvedB, t);
   }
 }

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

@@ -316,7 +316,7 @@ class MenuStyle with Diagnosticable {
       minimumSize: MaterialStateProperty.lerp<Size?>(a?.minimumSize, b?.minimumSize, t, Size.lerp),
       fixedSize: MaterialStateProperty.lerp<Size?>(a?.fixedSize, b?.fixedSize, t, Size.lerp),
       maximumSize: MaterialStateProperty.lerp<Size?>(a?.maximumSize, b?.maximumSize, t, Size.lerp),
-      side: _LerpSides(a?.side, b?.side, t),
+      side: MaterialStateBorderSide.lerp(a?.side, b?.side, t),
       shape: MaterialStateProperty.lerp<OutlinedBorder?>(a?.shape, b?.shape, t, OutlinedBorder.lerp),
       mouseCursor: t < 0.5 ? a?.mouseCursor : b?.mouseCursor,
       visualDensity: t < 0.5 ? a?.visualDensity : b?.visualDensity,
@@ -342,29 +342,3 @@ class MenuStyle with Diagnosticable {
     properties.add(DiagnosticsProperty<AlignmentGeometry>('alignment', alignment, defaultValue: null));
   }
 }
-
-/// A required helper class because [BorderSide.lerp] doesn't support passing or
-/// returning null values.
-class _LerpSides implements MaterialStateProperty<BorderSide?> {
-  const _LerpSides(this.a, this.b, this.t);
-
-  final MaterialStateProperty<BorderSide?>? a;
-  final MaterialStateProperty<BorderSide?>? b;
-  final double t;
-
-  @override
-  BorderSide? resolve(Set<MaterialState> states) {
-    final BorderSide? resolvedA = a?.resolve(states);
-    final BorderSide? resolvedB = b?.resolve(states);
-    if (resolvedA == null && resolvedB == null) {
-      return null;
-    }
-    if (resolvedA == null) {
-      return BorderSide.lerp(BorderSide(width: 0, color: resolvedB!.color.withAlpha(0)), resolvedB, t);
-    }
-    if (resolvedB == null) {
-      return BorderSide.lerp(resolvedA, BorderSide(width: 0, color: resolvedA.color.withAlpha(0)), t);
-    }
-    return BorderSide.lerp(resolvedA, resolvedB, t);
-  }
-}

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

@@ -203,31 +203,7 @@ class SearchBarThemeData with Diagnosticable {
     if (identical(a, b)) {
       return a;
     }
-    return _LerpSides(a, b, t);
+    return MaterialStateBorderSide.lerp(a, b, t);
-  }
-}
-
-class _LerpSides implements MaterialStateProperty<BorderSide?> {
-  const _LerpSides(this.a, this.b, this.t);
-
-  final MaterialStateProperty<BorderSide?>? a;
-  final MaterialStateProperty<BorderSide?>? b;
-  final double t;
-
-  @override
-  BorderSide? resolve(Set<MaterialState> states) {
-    final BorderSide? resolvedA = a?.resolve(states);
-    final BorderSide? resolvedB = b?.resolve(states);
-    if (identical(resolvedA, resolvedB)) {
-      return resolvedA;
-    }
-    if (resolvedA == null) {
-      return BorderSide.lerp(BorderSide(width: 0, color: resolvedB!.color.withAlpha(0)), resolvedB, t);
-    }
-    if (resolvedB == null) {
-      return BorderSide.lerp(resolvedA, BorderSide(width: 0, color: resolvedA.color.withAlpha(0)), t);
-    }
-    return BorderSide.lerp(resolvedA, resolvedB, t);
   }
 }
 

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

@@ -353,6 +353,43 @@ abstract class WidgetStateBorderSide extends BorderSide implements WidgetStatePr
   /// widget or theme.
   @override
   BorderSide? resolve(Set<WidgetState> states);
+
+  /// Linearly interpolate between two [WidgetStateProperty]s of [BorderSide].
+  static WidgetStateProperty<BorderSide?>? lerp(
+    WidgetStateProperty<BorderSide?>? a,
+    WidgetStateProperty<BorderSide?>? b,
+    double t,
+  ) {
+    // Avoid creating a _LerpSides object for a common case.
+    if (a == null && b == null) {
+      return null;
+    }
+    return _LerpSides(a, b, t);
+  }
+}
+
+class _LerpSides implements WidgetStateProperty<BorderSide?> {
+  const _LerpSides(this.a, this.b, this.t);
+
+  final WidgetStateProperty<BorderSide?>? a;
+  final WidgetStateProperty<BorderSide?>? b;
+  final double t;
+
+  @override
+  BorderSide? resolve(Set<WidgetState> states) {
+    final BorderSide? resolvedA = a?.resolve(states);
+    final BorderSide? resolvedB = b?.resolve(states);
+    if (resolvedA == null && resolvedB == null) {
+      return null;
+    }
+    if (resolvedA == null) {
+      return BorderSide.lerp(BorderSide(width: 0, color: resolvedB!.color.withAlpha(0)), resolvedB, t);
+    }
+    if (resolvedB == null) {
+      return BorderSide.lerp(resolvedA, BorderSide(width: 0, color: resolvedA.color.withAlpha(0)), t);
+    }
+    return BorderSide.lerp(resolvedA, resolvedB, t);
+  }
 }
 
 class _WidgetStateBorderSide extends WidgetStateBorderSide {

packages/flutter/test/widgets/widget_state_property_test.dart

@@ -86,6 +86,48 @@ void main() {
     )!.resolve(enabled)!;
     expect(textStyle.fontSize, 20.0);
   });
+
+  test('WidgetStateBorderSide.lerp()', () {
+    const WidgetStateProperty<BorderSide?> borderSide1 =  WidgetStatePropertyAll<BorderSide?>(
+      BorderSide(
+        color: Color(0xffff0000),
+        width: 4.0,
+      ),
+    );
+    const WidgetStateProperty<BorderSide?> borderSide2 = WidgetStatePropertyAll<BorderSide?>(
+      BorderSide(
+        color: Color(0xff0000ff),
+        width: 12.0,
+      ),
+    );
+
+    // Using `0.0` interpolation value.
+    BorderSide borderSide = WidgetStateBorderSide.lerp(
+      borderSide1,
+      borderSide2,
+      0.0,
+    )!.resolve(enabled)!;
+    expect(borderSide.color, const Color(0xffff0000));
+    expect(borderSide.width, 4.0);
+
+    // Using `0.5` interpolation value.
+    borderSide = WidgetStateBorderSide.lerp(
+      borderSide1,
+      borderSide2,
+      0.5,
+    )!.resolve(enabled)!;
+    expect(borderSide.color, const Color(0xff7f007f));
+    expect(borderSide.width, 8.0);
+
+    // Using `1.0` interpolation value.
+    borderSide = WidgetStateBorderSide.lerp(
+      borderSide1,
+      borderSide2,
+      1.0,
+    )!.resolve(enabled)!;
+    expect(borderSide.color, const Color(0xff0000ff));
+    expect(borderSide.width, 12.0);
+  });
 }
 
 Set<WidgetState> enabled = <WidgetState>{};