Flutter Mobile Development · レッスン

CustomPainterとCanvas

`CustomPainter`と`Canvas` APIを使ったカスタム描画を学び、独自の図形、グラフ、視覚効果を作成します。

「CustomPainterとCanvas」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。

Intro to Custom Drawing

Flutter's widgets are powerful, but sometimes you need to draw something truly unique. Think custom graphs, unique shapes, or special visual effects!

This is where custom drawing comes in. It gives you pixel-level control over what appears on the screen, letting you create anything imaginable.

Meet CustomPainter

At the heart of custom drawing in Flutter is the CustomPainter class. You extend this class to define what and how to draw.

It provides a canvas to draw on.
It requires you to implement two key methods: paint and shouldRepaint.

The Canvas: Your Drawing Board

The paint method receives a Canvas object. Think of the Canvas as your blank drawing board, ready for your artistic commands.

It uses a 2D Cartesian coordinate system: the top-left corner is (0,0). X-values increase to the right, and Y-values increase downwards.

Styling with Paint

Before you draw, you need to tell Flutter how to draw. This is done using a Paint object. The Paint object defines properties like:

Color: What color should the drawing be?
StrokeWidth: How thick should lines be?
Style: Should it be filled (PaintingStyle.fill) or just an outline (PaintingStyle.stroke)?

Drawing Basic Shapes: Line

Let's draw our first shape: a simple line! We use canvas.drawLine(), providing start and end points (Offset) and our Paint style.

Try running this example:

import 'package:flutter/material.dart';

void main() {
  runApp(const MyApp());
}

class MyApp extends StatelessWidget {
  const MyApp({super.key});

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(title: const Text('Draw Line')),
        body: Center(
          child: CustomPaint(
            painter: LinePainter(),
            child: Container(),
          ),
        ),
      ),
    );
  }
}

class LinePainter extends CustomPainter {
  @override
  void paint(Canvas canvas, Size size) {
    final paint = Paint()
      ..color = Colors.blue
      ..strokeWidth = 4.0
      ..style = PaintingStyle.stroke;

    final p1 = Offset(0, size.height / 2);
    final p2 = Offset(size.width, size.height / 2);

    canvas.drawLine(p1, p2, paint);
  }

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

Drawing Basic Shapes: Circle

Drawing a circle is just as easy with canvas.drawCircle(). You need a center point, a radius, and a Paint object.

Notice how we use size.width / 2 and size.height / 2 to center our drawings.

import 'package:flutter/material.dart';

void main() {
  runApp(const MyApp());
}

class MyApp extends StatelessWidget {
  const MyApp({super.key});

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(title: const Text('Draw Circle')),
        body: Center(
          child: CustomPaint(
            painter: CirclePainter(),
            child: Container(),
          ),
        ),
      ),
    );
  }
}

class CirclePainter extends CustomPainter {
  @override
  void paint(Canvas canvas, Size size) {
    final paint = Paint()
      ..color = Colors.red
      ..strokeWidth = 5.0
      ..style = PaintingStyle.fill;

    final center = Offset(size.width / 2, size.height / 2);
    final radius = size.width / 4;

    canvas.drawCircle(center, radius, paint);
  }

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

Drawing Basic Shapes: Rectangle

To draw a rectangle, we use canvas.drawRect(). This method takes a Rect object, which defines the rectangle's position and size.

A Rect.fromLTWH constructor is useful for defining a rectangle from its left, top, width, and height.

import 'package:flutter/material.dart';

void main() {
  runApp(const MyApp());
}

class MyApp extends StatelessWidget {
  const MyApp({super.key});

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(title: const Text('Draw Rectangle')),
        body: Center(
          child: CustomPaint(
            painter: RectanglePainter(),
            child: Container(),
          ),
        ),
      ),
    );
  }
}

class RectanglePainter extends CustomPainter {
  @override
  void paint(Canvas canvas, Size size) {
    final paint = Paint()
      ..color = Colors.green
      ..strokeWidth = 3.0
      ..style = PaintingStyle.stroke;

    final rect = Rect.fromLTWH(size.width * 0.2, size.height * 0.2, size.width * 0.6, size.height * 0.6);

    canvas.drawRect(rect, paint);
  }

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

The shouldRepaint Method

The shouldRepaint method in CustomPainter is crucial for performance. It tells Flutter whether your custom painting needs to be redrawn.

Return true if the new delegate (the updated painter) has different data that would change the drawing.
Return false if the drawing would be identical. This prevents unnecessary repaints and saves CPU cycles.

For static drawings, false is often sufficient.

Drawing Complex Shapes with Path

For more complex or irregular shapes, you'll use a Path object. A Path allows you to define a series of connected lines and curves.

You can move to a point (moveTo), draw lines to points (lineTo), and finally close the path (close) before drawing it with canvas.drawPath().

import 'package:flutter/material.dart';

void main() {
  runApp(const MyApp());
}

class MyApp extends StatelessWidget {
  const MyApp({super.key});

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(title: const Text('Draw Triangle')),
        body: Center(
          child: CustomPaint(
            painter: TrianglePainter(),
            child: Container(),
          ),
        ),
      ),
    );
  }
}

class TrianglePainter extends CustomPainter {
  @override
  void paint(Canvas canvas, Size size) {
    final paint = Paint()
      ..color = Colors.purple
      ..style = PaintingStyle.fill;

    final path = Path();
    path.moveTo(size.width / 2, size.height * 0.1);
    path.lineTo(size.width * 0.9, size.height * 0.9);
    path.lineTo(size.width * 0.1, size.height * 0.9);
    path.close();

    canvas.drawPath(path, paint);
  }

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

Test Your Knowledge!

Which of the following statements about Flutter's custom drawing is TRUE?

Recap: Custom Drawing Basics

You've taken your first steps into custom drawing in Flutter! You learned:

CustomPainter is your entry point for custom visuals.
The Canvas object is your drawing surface with a top-left origin.
The Paint object defines the style (color, stroke, fill).
How to draw basic shapes like lines, circles, and rectangles.
shouldRepaint helps optimize performance.
Path allows for drawing complex, custom shapes.

Keep experimenting to bring your unique designs to life!

よくある質問

「CustomPainterとCanvas」レッスンは無料ですか？

はい。「CustomPainterとCanvas」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。

「CustomPainterとCanvas」で何を学びますか？

`CustomPainter`と`Canvas` APIを使ったカスタム描画を学び、独自の図形、グラフ、視覚効果を作成します。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。

「CustomPainterとCanvas」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

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