paint method

@override void paint(Canvas canvas, Size size)

Called whenever the object needs to paint. The given Canvas has its coordinate space configured such that the origin is at the top left of the box. The area of the box is the size of the size argument.

Paint operations should remain inside the given area. Graphical operations outside the bounds may be silently ignored, clipped, or not clipped. It may sometimes be difficult to guarantee that a certain operation is inside the bounds (e.g., drawing a rectangle whose size is determined by user inputs). In that case, consider calling Canvas.clipRect at the beginning of paint so everything that follows will be guaranteed to only draw within the clipped area.

Implementations should be wary of correctly pairing any calls to Canvas.save/Canvas.saveLayer and Canvas.restore, otherwise all subsequent painting on this canvas may be affected, with potentially hilarious but confusing results.

To paint text on a Canvas, use a TextPainter.

To paint an image on a Canvas:

1. Obtain an ImageStream, for example by calling ImageProvider.resolve on an AssetImage or NetworkImage object.

2. Whenever the ImageStream's underlying ImageInfo object changes (see ImageStream.addListener), create a new instance of your custom paint delegate, giving it the new ImageInfo object.

3. In your delegate's paint method, call the Canvas.drawImage, Canvas.drawImageRect, or Canvas.drawImageNine methods to paint the ImageInfo.image object, applying the ImageInfo.scale value to obtain the correct rendering size.

Implementation

@override
void paint(Canvas canvas, Size size) {
  // Canvas canvas = Canvas(pictureRecorder); //canvas接受一个图片记录仪

  // 设置背景颜色/图片
  canvas.drawColor(Colors.cyan, BlendMode.color); // 超出 size 的区域不会截断
  // 画出背景边框，便于理解size的大小
  canvas.drawRect(
    Rect.fromLTWH(0, 0, size.width, size.height),
    Paint()..color = Colors.pink,
  );

  Paint paint = Paint()
    ..filterQuality = FilterQuality.high // 创建一个画笔并配置其属性
    ..strokeWidth = 1 // 画笔的宽度
    ..isAntiAlias = true // 是否抗锯齿
    ..color = Colors.amber; // 画笔颜色

  // 设置背景图片
  if (bgImage != null) {
    // canvas.drawImageRect(
    //   bgImage!,
    //   Rect.fromLTWH(
    //       0, 0, bgImage!.width.toDouble(), bgImage!.height.toDouble()),
    //   Rect.fromLTWH(0, 0, size.width, size.height),
    //   paint,
    // );
  }

  // double rectSize = 300;
  // canvas.drawRect(
  //   Rect.fromLTWH((size.width - rectSize) / 2.0, (size.height - rectSize) / 2,
  //       rectSize, rectSize),
  //   paint,
  // );

  // 头像居中显示
  if (avatarImage != null) {
    double srcWidth = avatarImage!.width.toDouble();
    double srcHeight = avatarImage!.height.toDouble();
    double dstSize = 100;

    // ②第二个参数为你需要截取的矩形Rect，举个栗子：截取一张图片，原图的宽高分别为w和h，
    // 将第二个参数设置为：Rect.fromLTRB(0, 0, w/2, h/2)。意思就是从原图片中截取一个矩形，矩形的坐标从(0.0)到（w/2, h/2)。
    // ③第三个为目标矩形Rect，即你想要在canvas上绘制的区域，
    // 举个栗子：接着上面所讲的，将第三个参数设置为Rect.fromLTWH(0, 0, 100, 100), paint);意思就是将上面截取的矩形，在canvas的（0，0）坐标开始绘制，绘制的宽和高为100，
    canvas.drawImageRect(
      avatarImage!,
      Rect.fromLTWH(0, 0, srcWidth, srcHeight),
      Rect.fromLTWH(
        (size.width - dstSize) / 2.0,
        100,
        dstSize,
        dstSize,
      ),
      paint,
    );
  }

  /// Canvas 绘制文字：方法一、使用 Paragraph
  /// 1.生成 ParagraphStyle，可设置文本的基本信息
  final paragraphStyle = ui.ParagraphStyle(
    textAlign: TextAlign.center,
    fontSize: 24.0,
    textDirection: TextDirection.ltr,
    maxLines: 1,
  );

  /// 2.根据 ParagraphStyle 生成 ParagraphBuilder，并添加样式和文字
  ui.ParagraphBuilder shopNameBuilder = ui.ParagraphBuilder(paragraphStyle)
    ..pushStyle(
      ui.TextStyle(
        fontWeight: FontWeight.bold,
        color: Colors.black,
        textBaseline: ui.TextBaseline.alphabetic,
      ),
    )
    ..addText("苹果手机数据线")
    ..pushStyle(ui.TextStyle(color: Colors.green, fontSize: 14))
    ..addText('¥19.90');

  /// 3.通过 build 取到 Paragraph，并根据宽高进行布局layout
  ui.Paragraph shopNameParagraph = shopNameBuilder.build()
    ..layout(ui.ParagraphConstraints(width: size.width));

  /// 4.绘制
  canvas.drawParagraph(shopNameParagraph, const Offset(0, 200.0));

  /// Canvas 绘制文字：方法二、使用 TextPainter 绘制
  final textPainter = TextPainter()
    ..text = const TextSpan(
        text: '可多种不同效果的字体来支持富文本',
        style: TextStyle(color: Colors.white, fontSize: 20))
    ..textDirection = TextDirection.ltr

    /// 可以传入minWidth、maxWidth来限制宽度，若不传文字会绘制在一行
    ..layout(maxWidth: 120);

  /// 绘制矩形框，在文字绘制前可通过textPainter.width和textPainter.height来获取文字绘制的尺寸
  double textOriginX = (size.width - textPainter.width) / 2.0;
  double textOriginY = 250;
  canvas.drawRect(
    Rect.fromLTWH(
      textOriginX,
      textOriginY,
      textPainter.width,
      textPainter.height,
    ),
    Paint()..color = Colors.blue,
  );

  /// 绘制文字
  textPainter.paint(canvas, Offset(textOriginX, textOriginY));

  // 二维码图片
  if (qrCodeImage != null) {
    double srcWidth = qrCodeImage!.width.toDouble();
    double srcHeight = qrCodeImage!.height.toDouble();
    double dstSize = 100;

    canvas.drawImageRect(
      qrCodeImage!,
      Rect.fromLTWH(0, 0, srcWidth, srcHeight),
      Rect.fromLTWH(
        size.width - dstSize - 20,
        size.height - dstSize - 20,
        dstSize,
        dstSize,
      ),
      paint,
    );

    // canvas.drawImage(qrCodeImage!, const ui.Offset(20.0, 30.0), paint);
  }
}