class MyPainter extends CustomPainter {
  @override
  void paint(Canvas canvas, Size size) {
    /// 绘制文本
    const textStyle = TextStyle(color: Colors.black, fontSize: 24);
    const textSpan = TextSpan(text: 'Hello, World', style: textStyle);
    final textPainter = TextPainter(text: textSpan, textDirection: TextDirection.ltr);
    textPainter.layout(minWidth: 0, maxWidth: size.width);
    const offset = Offset(50, 100);
    textPainter.paint(canvas, offset);
  }

  @override
  bool shouldRepaint(covariant CustomPainter oldDelegate) {
    return false;
  }
}

class TextPainter {
  TextPainter({
    InlineSpan? text,//文本内容
    TextAlign textAlign = TextAlign.start,//对齐方式
    TextDirection? textDirection,
    double textScaleFactor = 1.0,
    int? maxLines,
    String? ellipsis,
    Locale? locale,
    StrutStyle? strutStyle,
    TextWidthBasis textWidthBasis = TextWidthBasis.parent,
    ui.TextHeightBehavior? textHeightBehavior,
  })


  void layout({ double minWidth = 0.0, double maxWidth = double.infinity }) {
    if (_paragraph != null && minWidth == _lastMinWidth && maxWidth == _lastMaxWidth) {
      return;
    }

    if (_rebuildParagraphForPaint || _paragraph == null) {
      _createParagraph();
    }
    _lastMinWidth = minWidth;
    _lastMaxWidth = maxWidth;
    // A change in layout invalidates the cached caret and line metrics as well.
    _lineMetricsCache = null;
    _previousCaretPosition = null;
    _layoutParagraph(minWidth, maxWidth);
    _inlinePlaceholderBoxes = _paragraph!.getBoxesForPlaceholders();
  }

  void _layoutParagraph(double minWidth, double maxWidth) {
    _paragraph!.layout(ui.ParagraphConstraints(width: maxWidth));//布局
    if (minWidth != maxWidth) {
      double newWidth;
      switch (textWidthBasis) {
        case TextWidthBasis.longestLine:
          newWidth = _applyFloatingPointHack(_paragraph!.longestLine);
        case TextWidthBasis.parent:
          newWidth = maxIntrinsicWidth;
      }
      newWidth = clampDouble(newWidth, minWidth, maxWidth);
      if (newWidth != _applyFloatingPointHack(_paragraph!.width)) {
        _paragraph!.layout(ui.ParagraphConstraints(width: newWidth));
      }
    }
  }

  void paint(Canvas canvas, Offset offset) {
    final double? minWidth = _lastMinWidth;
    final double? maxWidth = _lastMaxWidth;
    if (_paragraph == null || minWidth == null || maxWidth == null) {
      throw StateError(
        'TextPainter.paint called when text geometry was not yet calculated.\n'
        'Please call layout() before paint() to position the text before painting it.',
      );
    }

    if (_rebuildParagraphForPaint) {
      _createParagraph();
      _layoutParagraph(minWidth, maxWidth);
    }
    canvas.drawParagraph(_paragraph!, offset);//绘制
  }

  ui.Paragraph? _paragraph; //封装的Paragraph

  ui.Paragraph _createParagraph() {
    final InlineSpan? text = this.text;
    if (text == null) {
      throw StateError('TextPainter.text must be set to a non-null value before using the TextPainter.');
    }
    final ui.ParagraphBuilder builder = ui.ParagraphBuilder(_createParagraphStyle());
    text.build(builder, textScaleFactor: textScaleFactor, dimensions: _placeholderDimensions);
    _inlinePlaceholderScales = builder.placeholderScales;
    final ui.Paragraph paragraph = _paragraph = builder.build();
    _rebuildParagraphForPaint = false;
    return paragraph;
  }


}

var deviceData = MediaQuery.of(context); // 返回 MediaQueryData
var width = deviceData.size.width; //返回context所在的窗口宽度
var height = deviceData.size.height;//返回context所在的窗口高度