paint method

  1. @override
void paint (
  1. Canvas canvas,
  2. Size size
)
override

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) {
  canvasSize = size;

  //初始化歌词的Y坐标在正中央
  lyricTextPaints[currentLyricIndex]
    //设置歌词
    ..text = TextSpan(
        text: lyrics[currentLyricIndex].lyric, style: currLyricTextStyle)
    ..layout(maxWidth: lyricMaxWidth);
  var currentLyricY = _offset +
      size.height / 2 -
      lyricTextPaints[currentLyricIndex].height / 2;

  //遍历歌词进行绘制
  for (int lyricIndex = 0; lyricIndex < lyrics.length; lyricIndex++) {
    var currentLyricTextPaint = lyricTextPaints[lyricIndex];

    var currentLyric = lyrics[lyricIndex];
    //仅绘制在屏幕内的歌词
    if (currentLyricY < size.height && currentLyricY > 0) {
      //绘制歌词到画布
      currentLyricTextPaint
        //设置歌词
        ..text = TextSpan(
            text: currentLyric.lyric,
            style: currentLyricIndex == lyricIndex
                ? currLyricTextStyle
                : lyricTextStyle)
        //计算文本宽高
        ..layout(maxWidth: lyricMaxWidth)
        //绘制 offset=横向居中
        ..paint(
            canvas,
            Offset((size.width - currentLyricTextPaint.width) / 2,
                currentLyricY));
    }
    currentLyricTextPaint..layout(maxWidth: lyricMaxWidth);
    var currentLyricHeight = currentLyricTextPaint.height;
    //当前歌词结束后调整下次开始绘制歌词的y坐标
    currentLyricY += currentLyricHeight + lyricGapValue;
    //如果有翻译歌词时,寻找该行歌词以后的翻译歌词
    if (subLyrics != null) {
      List<Lyric> remarkLyrics = subLyrics
          .where((subLyric) =>
              subLyric.startTime >= currentLyric.startTime &&
              subLyric.endTime <= currentLyric.endTime)
          .toList();
      remarkLyrics.forEach((remarkLyric) {
        //获取位置
        var subIndex = subLyrics.indexOf(remarkLyric);

        //仅绘制在屏幕内的歌词
        if (currentLyricY < size.height && currentLyricY > 0) {
          subLyricTextPaints[subIndex] //设置歌词
            ..text =
                TextSpan(text: remarkLyric.lyric, style: subLyricTextStyle)
            //计算文本宽高
            ..layout(maxWidth: lyricMaxWidth)
            //绘制 offset=横向居中
            ..paint(
                canvas,
                Offset((size.width - subLyricTextPaints[subIndex].width) / 2,
                    currentLyricY));
        }
        //当前歌词结束后调整下次开始绘制歌词的y坐标
        currentLyricY += _subLyricHeight + subLyricGapValue;
      });
    }
  }
}