0Pricing
Flutter Mobile Development · レッスン

Canvas API：パス、ペイント、シェーダー

CanvasのプリミティブとPaintの設定を使って、図形、グラデーション、線を描画します。

「Canvas API：パス、ペイント、シェーダー」はCoddyKit上の無料Flutter Mobile Developmentレッスンです。 これはレッスン1/4です。 下記で完全なレッスンを無料で読むことができます。その後、ブラウザ内の組み込みコードエディタと24時間対応のAIチューターでハンズオン演習できます。 これはFlutter Mobile Development学習パスの一部であり、ウェブとCoddyKitアプリ全体で進捗が同期されます。 Flutter Mobile Developmentコースには全4レッスンが含まれています。

このレッスンの一部はまだ翻訳されておらず、英語で表示されています。

Meet the Canvas

When you build a CustomPainter, Flutter hands you two things in paint(): a Canvas and a Size.

  • Canvas is your drawing surface. It exposes primitives like drawRect, drawCircle, drawPath, and drawLine.
  • Size tells you how big the area is, so you can scale your drawing to fit any device.

Every draw call needs a Paint object that describes how to draw: color, stroke vs fill, width, and more.

void paint(Canvas canvas, Size size) {
  final paint = Paint()..color = const Color(0xFF2196F3);
  canvas.drawRect(
    Rect.fromLTWH(0, 0, size.width, size.height),
    paint,
  );
}

The Paint Object

A Paint is a bundle of styling options. The most important fields are:

  • color — the color used to draw.
  • style — PaintingStyle.fill (solid interior) or PaintingStyle.stroke (outline only).
  • strokeWidth — line thickness when stroking.
  • isAntiAlias — smooth edges, on by default.

Flutter idiom uses the cascade operator .. to configure a freshly created Paint in one expression.

final fill = Paint()
  ..color = const Color(0xFFE91E63)
  ..style = PaintingStyle.fill;

final outline = Paint()
  ..color = const Color(0xFF000000)
  ..style = PaintingStyle.stroke
  ..strokeWidth = 4.0;

Drawing Basic Shapes

The Canvas offers ready-made primitives so you do not always need a path:

  • drawRect(Rect, Paint) — axis-aligned rectangle.
  • drawRRect(RRect, Paint) — rounded rectangle.
  • drawCircle(Offset center, double radius, Paint)
  • drawOval(Rect, Paint) — fills the bounding rect with an ellipse.

Coordinates start at (0,0) in the top-left corner; x grows right, y grows down.

void paint(Canvas canvas, Size size) {
  final p = Paint()..color = const Color(0xFF4CAF50);
  final center = Offset(size.width / 2, size.height / 2);

  canvas.drawCircle(center, 40, p);
  canvas.drawRRect(
    RRect.fromRectAndRadius(
      Rect.fromLTWH(10, 10, 80, 50),
      const Radius.circular(12),
    ),
    p,
  );
}

Strokes: Caps, Joins, and Width

When style is PaintingStyle.stroke, several properties shape the line:

  • strokeWidth — thickness in logical pixels.
  • strokeCap — how line ends look: butt, round, or square.
  • strokeJoin — how corners connect: miter, round, or bevel.

Round caps and joins are common for friendly, smooth chart and signature drawing.

final stroke = Paint()
  ..color = const Color(0xFF9C27B0)
  ..style = PaintingStyle.stroke
  ..strokeWidth = 6.0
  ..strokeCap = StrokeCap.round
  ..strokeJoin = StrokeJoin.round;

// canvas.drawLine(Offset(10, 10), Offset(120, 60), stroke);

Building a Path

A Path describes an arbitrary outline made of line and curve segments. The core methods are:

  • moveTo(x, y) — lift the pen and start a new subpath.
  • lineTo(x, y) — draw a straight segment.
  • quadraticBezierTo / cubicTo — curved segments.
  • close() — connect back to the subpath start.

Draw the finished path with canvas.drawPath(path, paint).

void paint(Canvas canvas, Size size) {
  final path = Path()
    ..moveTo(size.width / 2, 0)
    ..lineTo(size.width, size.height)
    ..lineTo(0, size.height)
    ..close();

  final paint = Paint()..color = const Color(0xFFFF5722);
  canvas.drawPath(path, paint);
}

Curves with Bezier Segments

Smooth shapes come from Bezier curves. The most common is the quadratic Bezier, which uses one control point.

  • quadraticBezierTo(cpx, cpy, x, y) bends the line toward the control point (cpx, cpy) on its way to the end point (x, y).
  • cubicTo(c1x, c1y, c2x, c2y, x, y) uses two control points for S-curves.

These power wave dividers, chart lines, and speech-bubble tails.

void paint(Canvas canvas, Size size) {
  final wave = Path()
    ..moveTo(0, size.height * 0.5)
    ..quadraticBezierTo(
      size.width * 0.25, size.height * 0.2,
      size.width * 0.5, size.height * 0.5,
    )
    ..quadraticBezierTo(
      size.width * 0.75, size.height * 0.8,
      size.width, size.height * 0.5,
    );

  final paint = Paint()
    ..color = const Color(0xFF00BCD4)
    ..style = PaintingStyle.stroke
    ..strokeWidth = 3;
  canvas.drawPath(wave, paint);
}

Plain Dart: Computing Points

Canvas math is just geometry. Before you draw, you often compute coordinates with plain Dart. Here we generate evenly spaced points along a sine wave — the same values you would feed into path.lineTo.

This logic is framework-free, so you can unit-test it without rendering anything.

import 'dart:math';

List<Point<double>> wavePoints(double width, double height, int n) {
  final pts = <Point<double>>[];
  for (var i = 0; i <= n; i++) {
    final x = width * i / n;
    final y = height / 2 + sin(i / n * 2 * pi) * height / 4;
    pts.add(Point(x, y));
  }
  return pts;
}

void main() {
  final pts = wavePoints(200, 100, 4);
  for (final p in pts) {
    print('(${p.x.toStringAsFixed(1)}, ${p.y.toStringAsFixed(1)})');
  }
}

Shaders: Linear Gradients

A shader fills a shape with more than a flat color. The most common is a linear gradient. You build it with ui.Gradient.linear and assign it to paint.shader.

  • from / to are the start and end Offsets of the gradient line.
  • colors lists the gradient stops, blended along that line.

Once shader is set, the paint's color is ignored for fills.

import 'dart:ui' as ui;

Paint gradientPaint(Size size) {
  return Paint()
    ..shader = ui.Gradient.linear(
      Offset.zero,
      Offset(size.width, size.height),
      const [Color(0xFF2196F3), Color(0xFF9C27B0)],
    );
}

Radial and Sweep Gradients

Beyond linear, Flutter offers two more gradient shaders:

  • Radial — ui.Gradient.radial(center, radius, colors) blends outward from a center point, great for glows and spotlights.
  • Sweep — ui.Gradient.sweep(center, colors) rotates colors around a center, ideal for circular progress and color wheels.

You can add an optional stops list (0.0 to 1.0) to control exactly where each color sits.

import 'dart:ui' as ui;

Paint glow(Offset center) {
  return Paint()
    ..shader = ui.Gradient.radial(
      center,
      60,
      const [Color(0xFFFFEB3B), Color(0x00FFEB3B)],
      const [0.0, 1.0],
    );
}

Putting It Together in a Painter

A real CustomPainter combines paths, paints, and shaders. Remember two overrides:

  • paint(canvas, size) — do the drawing.
  • shouldRepaint(old) — return true only when inputs changed, so Flutter avoids needless repaints.

This snippet draws a gradient-filled rounded card with a stroked border.

class CardPainter extends CustomPainter {
  @override
  void paint(Canvas canvas, Size size) {
    final rect = Rect.fromLTWH(0, 0, size.width, size.height);
    final rrect = RRect.fromRectAndRadius(rect, const Radius.circular(16));

    final fill = Paint()
      ..shader = ui.Gradient.linear(
        rect.topLeft, rect.bottomRight,
        const [Color(0xFF42A5F5), Color(0xFF7E57C2)],
      );
    canvas.drawRRect(rrect, fill);

    final border = Paint()
      ..style = PaintingStyle.stroke
      ..strokeWidth = 2
      ..color = const Color(0x33000000);
    canvas.drawRRect(rrect, border);
  }

  @override
  bool shouldRepaint(CardPainter oldDelegate) => false;
}

Save, Restore, and Transforms

The Canvas keeps a stack of transforms and clips. Use it to draw rotated or scaled content without permanently changing state:

  • canvas.save() pushes the current matrix.
  • canvas.translate / rotate / scale modify it.
  • canvas.restore() pops back to where you were.

Always pair every save() with a restore(), or later draws will inherit the transform.

void paint(Canvas canvas, Size size) {
  final p = Paint()..color = const Color(0xFF3F51B5);
  canvas.save();
  canvas.translate(size.width / 2, size.height / 2);
  canvas.rotate(0.785398); // 45 degrees in radians
  canvas.drawRect(
    const Rect.fromLTWH(-30, -30, 60, 60),
    p,
  );
  canvas.restore();
}

Quick Check

Test your understanding of the Paint configuration.

Recap

You now command the Canvas API:

  • Paint controls how you draw: color, style (fill vs stroke), strokeWidth, caps, and joins.
  • Primitives like drawRect, drawRRect, and drawCircle handle common shapes; Path with moveTo, lineTo, and Bezier curves handles anything custom.
  • Shaders (Gradient.linear, radial, sweep) fill or stroke with gradients and override the flat color.
  • save/restore isolates transforms, and shouldRepaint keeps rendering efficient.

With these building blocks you can draw charts, custom controls, and rich decorations entirely in Dart.

よくある質問

「Canvas API：パス、ペイント、シェーダー」レッスンは無料ですか？

はい。「Canvas API：パス、ペイント、シェーダー」の完全なテキストはこのウェブで無料で読めます。インタラクティブに演習し（組み込みコードエディタと24時間対応のAIチューター）、Flutter Mobile Developmentコースの残りをアンロックするには、CoddyKit PROにアップグレードしてください。 Flutter Mobile Developmentコースには全4レッスンが含まれています。

「Canvas API：パス、ペイント、シェーダー」で何を学びますか？

CanvasのプリミティブとPaintの設定を使って、図形、グラデーション、線を描画します。 ブラウザで直接実行するハンズオンコードでFlutter Mobile Developmentを演習し、24時間対応のAIチューターがレッスンを進める中での質問に答えます。

Flutter Mobile Developmentを始めるのに経験は必要ですか？

事前経験は必要ありません。CoddyKitのFlutter Mobile Developmentは初級者から上級者向けに構成されているため、ここから始めるか最初から始めて、自分のペースで進むことができます。 これはレッスン1/4です。

「Canvas API：パス、ペイント、シェーダー」レッスンにはどのくらい時間がかかりますか？

ほとんどのCoddyKitレッスンは約5～10分かかります。各レッスンはコンパクトでインタラクティブなので、着実に進歩し、ウェブとアプリ全体で正確に前回の場所から再開できます。

このFlutter Mobile Developmentレッスンでコードを書いて実行できますか？

はい。すべてのFlutter Mobile Developmentレッスンに組み込みコードエディタが含まれているため、ブラウザでリアルコードを書いて実行し、即座のAIフィードバックを取得できます。ローカル設定は不要です。

このコースのすべてのレッスン

  1. Canvas API：パス、ペイント、シェーダー
  2. インタラクティブなカスタムチャートウィジェットの構築
  3. クリッピング、ブレンドモード、レイヤー合成
  4. ヒットテストと再描画の最適化
← Flutter Mobile Developmentに戻る