path_builder_border.dart

import 'dart:math';
import 'dart:ui' as ui;

import 'package:flutter/foundation.dart';
import 'package:flutter/material.dart';

typedef PathBuilder = ui.Path Function(ui.Rect bounds, double phase);
typedef OnPaintFrame = void Function(Canvas canvas, ui.Rect bounds, double phase);

/// Simple [OutlinedBorder] implementation.
/// You can use [PathBuilderBorder] directly in the build tree:
/// ```dart
/// child: Card(
///   shape: PathBuilderBorder(
///     pathBuilder: (r, phase) => roundPolygon(
///       points: [r.topLeft, r.topRight, r.centerRight, r.bottomCenter, r.centerLeft],
///       radii: [8, 8, 8, 32, 8],
///     ),
///   ),
///   ...
/// ```
/// Optional [phase] parameter can be used to 'morph' [PathBuilderBorder] if
/// it is used by widgets that animate their shape (like [AnimatedContainer] or [Material]).
/// In such case it is passed to [pathBuilder] as an interpolation between the old
/// and new value:
/// ```dart
/// int idx = 0;
///
/// @override
/// Widget build(BuildContext context) {
///   return Material(
///     clipBehavior: Clip.antiAlias,
///     shape: PathBuilderBorder(
///       pathBuilder: _phasedPathBuilder,
///       phase: idx.toDouble(),
///     ),
///     color: idx == 0? Colors.teal : Colors.orange,
///     child: InkWell(
///       onTap: () => setState(() => idx = idx ^ 1),
///       child: const Center(child: Text('press me', textScaleFactor: 2)),
///     ),
///   );
/// }
///
/// Path _phasedPathBuilder(Rect bounds, double phase) {
///   print(phase);
///   final radius = phase * rect.shortestSide / 2;
///   return Path()
///     ..addRRect(RRect.fromRectAndRadius(rect, Radius.circular(radius)));
/// }
/// ```
///
/// You can also extend [PathBuilderBorder] if you want to add some
/// customizations, like [dimensions], [paint] etc.
class PathBuilderBorder extends OutlinedBorder {

  const PathBuilderBorder({
    required this.pathBuilder,
    BorderSide side = BorderSide.none,
    this.phase = 0,
    this.painter,
    this.foregroundPainter,
    EdgeInsetsGeometry? dimensions,
  }) : _dimensions = dimensions, super(side: side);

  final PathBuilder pathBuilder;
  final double phase;
  final OnPaintFrame? painter;
  final OnPaintFrame? foregroundPainter;
  final EdgeInsetsGeometry? _dimensions;

  @override
  ShapeBorder? lerpFrom(ShapeBorder? a, double t) {
    if (a is PathBuilderBorder && phase != a.phase) {
      return PathBuilderBorder(
        pathBuilder: pathBuilder,
        side: side == a.side? side : BorderSide.lerp(a.side, side, t),
        phase: ui.lerpDouble(a.phase, phase, t)!,
        painter: painter,
        foregroundPainter: foregroundPainter,
        dimensions: EdgeInsetsGeometry.lerp(a.dimensions, dimensions, t),
      );
    }
    return super.lerpFrom(a, t);
  }

  @override
  EdgeInsetsGeometry get dimensions => _dimensions ?? EdgeInsets.zero;

  @override
  ui.Path getInnerPath(ui.Rect rect, {ui.TextDirection? textDirection}) {
    return getOuterPath(rect, textDirection: textDirection);
  }

  @override
  ui.Path getOuterPath(ui.Rect rect, {ui.TextDirection? textDirection}) {
    return pathBuilder(rect, phase);
  }

  @override
  void paint(ui.Canvas canvas, ui.Rect rect, {ui.TextDirection? textDirection}) {
    painter?.call(canvas, rect, phase);
    if (side != BorderSide.none) {
      canvas.drawPath(pathBuilder(rect, phase), side.toPaint());
    }
    foregroundPainter?.call(canvas, rect, phase);
  }

  @override
  ShapeBorder scale(double t) => this;

  @override
  OutlinedBorder copyWith({BorderSide? side}) {
    return PathBuilderBorder(
      pathBuilder: pathBuilder,
      side: side ?? this.side,
      phase: phase,
      painter: painter,
      foregroundPainter: foregroundPainter,
    );
  }

  @override
  bool operator ==(Object other) {
    if (identical(this, other)) return true;

    return other is PathBuilderBorder &&
      other.phase == phase;
  }

  @override
  int get hashCode => phase.hashCode;
}

// ============================================================================
// ============================================================================
//
// example
//
// ============================================================================
// ============================================================================

main() {
  runApp(MaterialApp(
    home: Theme(
      data: ThemeData(
        cardTheme: CardTheme(
          shape: RoundedRectangleBorder(borderRadius: BorderRadius.circular(6)),
          color: Colors.grey.shade500,
          elevation: 4,
        ),
      ),
      child: Scaffold(
        body: SingleChildScrollView(
          child: Column(
            children: [
              _MorphingButton0(),
              _MorphingButton1(),
              _ChatBubble(),
            ],
          ),
        ),
      ),
    ),
  ));
}

class _MorphingButton0 extends StatefulWidget {
  @override
  State<_MorphingButton0> createState() => _MorphingButton0State();
}

class _MorphingButton0State extends State<_MorphingButton0> {
  int idx = 0;
  final alignments = [
    (const Alignment(-1, -0.25), const Alignment(-0.25, -1)),
    (Alignment.topRight, Alignment.topRight),
    (const Alignment(1, 0), const Alignment(0.25, 1)),
    (const Alignment(-0.25, 1), Alignment.bottomLeft),
  ];
  final colors = [Colors.indigo, Colors.orange];

  @override
  Widget build(BuildContext context) {
    return SizedBox(
      height: 175,
      child: Card(
        child: Padding(
          padding: const EdgeInsets.all(8.0),
          child: Center(
            child: AnimatedContainer(
              duration: Durations.extralong4 * 1.25,
              clipBehavior: Clip.antiAlias,
              curve: Curves.bounceOut,
              decoration: ShapeDecoration(
                shape: PathBuilderBorder(
                  pathBuilder: _phasedPathBuilder,
                  painter: _painter,
                  phase: idx.toDouble(),
                ),
                shadows: const [BoxShadow(blurRadius: 4, offset: Offset(3, 3))],
                color: colors[idx],
              ),
              child: AspectRatio(
                aspectRatio: 1.25,
                child: Material(
                  type: MaterialType.transparency,
                  child: InkWell(
                    highlightColor: Colors.transparent,
                    splashColor: Colors.black26,
                    onTap: () => setState(() => idx = idx ^ 1),
                    child: Center(child: Text('animate', style: Theme.of(context).textTheme.titleLarge)),
                  ),
                ),
              ),
            ),
          ),
        ),
      ),
    );
  }

  void _painter(ui.Canvas canvas, Rect bounds, double phase) {
    final s = Size.square(bounds.shortestSide);
    final r = Alignment.center.inscribe(s, bounds);
    final color = Color.lerp(Colors.cyan, Colors.white60, phase)!;
    const delta = Offset(-32, 32);
    final center = Offset.lerp(r.topRight + delta, r.bottomLeft - delta, phase)!;
    final radius = ui.lerpDouble(r.shortestSide, r.shortestSide / 2, 0.5 - cos(2 * pi * phase) / 2)!;
    final paint = Paint()
      ..blendMode = ui.BlendMode.colorDodge
      ..shader = ui.Gradient.radial(
        center, radius, [Colors.transparent, color, Colors.transparent], [0, 0.5, 1],
      );
    final matrix = _rotatedMatrix(ui.lerpDouble(0.2, 0.7, phase)! * pi, r.center);
    final r2 = Rect.fromCenter(
      center: r.center,
      width: r.shortestSide * 0.5,
      height: r.shortestSide * 2,
    );
    final points = [
      Offset.lerp(r2.topCenter, r2.topLeft, phase)!,
      Offset.lerp(r2.bottomCenter, r2.bottomLeft, phase)!,
      Offset.lerp(r2.topCenter, r2.topRight, phase)!,
      Offset.lerp(r2.bottomCenter, r2.bottomRight, phase)!,
    ];
    final paint2 = Paint()
      ..style = PaintingStyle.stroke
      ..strokeWidth = 16
      ..maskFilter = const MaskFilter.blur(BlurStyle.normal, 6)
      ..color = Colors.white.withOpacity(0.2);
    canvas
      ..save()
      ..clipPath(_phasedPathBuilder(bounds, phase))
      ..transform(matrix.storage)
      ..drawPoints(ui.PointMode.lines, points, paint2)
      ..drawPaint(paint)
      ..restore();
  }

  Path _phasedPathBuilder(Rect bounds, double phase) {
    final points = alignments
      .map((r) => Alignment.lerp(r.$1, r.$2, phase)!.withinRect(bounds))
      .toList();
    return Path()
      ..addPolygon(points, true);
  }

  Matrix4 _rotatedMatrix(double rotation, Offset anchor) => Matrix4.identity()
    ..translate(anchor.dx, anchor.dy)
    ..rotateZ(rotation)
    ..translate(-anchor.dx, -anchor.dy);
}

class _MorphingButton1 extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return ElevatedButtonTheme(
      data: ElevatedButtonThemeData(
        style: ButtonStyle(
          animationDuration: Durations.extralong4,
          // overlayColor: MaterialStateProperty.all(Colors.white30),
          shape: WidgetStateProperty.resolveWith(_shape),
          backgroundColor: WidgetStateProperty.all(Colors.orange),
          padding: WidgetStateProperty.all(const EdgeInsets.all(24)),
          side: WidgetStateProperty.resolveWith(_side),
        ),
      ),
      child: Card(
        child: Padding(
          padding: const EdgeInsets.all(8.0),
          child: Column(
            mainAxisSize: MainAxisSize.min,
            children: [
              Container(
                padding: const EdgeInsets.all(8),
                color: Colors.black12,
                child: const Text('this is a "normal" [ElevatedButton], long-press it to see how it changes'),
              ),
              const SizedBox(height: 16),
              ElevatedButton(
                onPressed: () => debugPrint('pressed'),
                child: Text('long press', style: Theme.of(context).textTheme.titleLarge),
              ),
            ],
          ),
        ),
      ),
    );
  }

  OutlinedBorder? _shape(states) => PathBuilderBorder(
    pathBuilder: _phasedPathBuilder,
    phase: states.contains(WidgetState.pressed) ? 1 : 0,
  );

  BorderSide? _side(states) => states.contains(WidgetState.pressed) ?
    const BorderSide(color: Colors.black, width: 3) :
    const BorderSide(color: Colors.black54, width: 2);

  Path _phasedPathBuilder(Rect bounds, double phase) {
    final points = [
      bounds.topLeft.translate(phase * 24, 0),
      bounds.topRight,
      bounds.bottomRight.translate(phase * -24, 0),
      bounds.bottomLeft,
    ];
    return Path()
      ..addPolygon(points, true);
  }
}


class _ChatBubble extends StatefulWidget {
  @override
  State<_ChatBubble> createState() => _ChatBubbleState();
}

class _ChatBubbleState extends State<_ChatBubble> {
  double phase = 1;

  @override
  Widget build(BuildContext context) {
    const padding = EdgeInsets.only(left: 20);
    return Card(
      child: Padding(
        padding: const EdgeInsets.all(8.0),
        child: Row(
          crossAxisAlignment: CrossAxisAlignment.start,
          children: [
            AnimatedContainer(
              duration: Durations.extralong4,
              constraints: const BoxConstraints(maxWidth: 175),
              clipBehavior: Clip.antiAlias,
              curve: Curves.ease,
              decoration: ShapeDecoration(
                shape: PathBuilderBorder(
                  pathBuilder: (bounds, phase) {
                    const lerp = ui.lerpDouble;
                    // M 6,4 C 5,4 4,4 0,3 C 3,2 4,2 6,0
                    final arrow = Path()
                      ..moveTo(6, 4)
                      ..cubicTo(5, 4, 4, 4, 0, 3)
                      ..cubicTo(3, 2, 4, 2, 6, 0)
                      ..close();
                    final arrowBounds = arrow.getBounds();
                    const maxRadius = 8.0;
                    final radius = lerp(maxRadius, 0, phase)!;
                    final scale = padding.left / arrowBounds.right;
                    final matrix = composeMatrix(
                      translate: Offset(padding.left, bounds.height - 4),
                      scale: lerp(scale, 0, phase)!,
                      anchor: arrowBounds.bottomRight,
                    );
                    final rrect = RRect.fromRectAndCorners(padding.deflateRect(bounds),
                      topLeft: Radius.circular(radius),
                      topRight: Radius.circular(radius),
                      bottomRight: Radius.circular(radius * 2),
                    );
                    return Path()
                      ..addRRect(rrect)
                      ..addPath(arrow, bounds.topLeft, matrix4: matrix.storage);
                  },
                  phase: phase,
                  dimensions: padding,
                ),
                shadows: phase == 0? const [BoxShadow(blurRadius: 3, offset: Offset(1.5, 1.5))] : null,
                gradient: LinearGradient(
                  colors: [
                    Color.lerp(Colors.grey.shade300, Colors.yellow.shade200, phase)!,
                    Color.lerp(Colors.deepPurple.shade200, Colors.amber.shade300, phase)!,
                  ],
                  stops: const [0.7, 0.9],
                  begin: Alignment.topLeft,
                  end: Alignment.bottomRight,
                ),
              ),
              child: Padding(
                padding: const EdgeInsets.symmetric(horizontal: 6),
                child: AnimatedCrossFade(
                  duration: Durations.long4,
                  firstChild: const Text('this rectangular shape will change when you press the button on the right ➜'),
                  secondChild: const Text('now it morphed into a nice chat balloon shape'),
                  crossFadeState: phase == 1? CrossFadeState.showFirst : CrossFadeState.showSecond,
                ),
              ),
            ),
            const SizedBox(width: 8),
            Expanded(
              child: ElevatedButton(
                style: const ButtonStyle(
                  shape: WidgetStatePropertyAll(RoundedRectangleBorder()),
                  padding: WidgetStatePropertyAll(EdgeInsets.all(4)),
                ),
                onPressed: () => setState(() => phase = 1 - phase),
                child: const Text('press to animate the shape'),
              ),
            ),
          ],
        ),
      ),
    );
  }
}

Matrix4 composeMatrix({
  double scale = 1,
  double rotation = 0,
  Offset translate = Offset.zero,
  Offset anchor = Offset.zero,
}) {
  final double c = cos(rotation) * scale;
  final double s = sin(rotation) * scale;
  final double dx = translate.dx - c * anchor.dx + s * anchor.dy;
  final double dy = translate.dy - s * anchor.dx - c * anchor.dy;
  return Matrix4(c, s, 0, 0, -s, c, 0, 0, 0, 0, 1, 0, dx, dy, 0, 1);
}

@pskink , Amazing work such animations and clippings remind me of JavaFX, I'm following this beauty.