flutter 用 CustomPaint 繪製自定義圖案

tabbin發表於2019-07-17

什麼是CustomPaint？

在Android中，有時候我們需要使用 Canvas 和 Paint 來進行繪製一些複雜的圖形，在 Flutter 中，當然也有可以讓你自由繪製的方案，他就是 CustomPaint。

CustomPaint 也是一個 Widget，你可以把它嵌到檢視樹的任意一個節點位置。

先看看它的常用屬性：

屬性	型別	說明
painter	CustomPainter	背景畫筆，繪製內容會顯示在child子節點後面
foregroundPainter	CustomPainter	前景畫筆，繪製內容會顯示在child子節點前面
size	Size	設定繪製區域的大小。如果有child，則忽略該引數，且繪製區域為child的尺寸
isComplex	bool	是否複雜的繪製，如果是，Flutter會應用一些快取策略來減少重複渲染的開銷。預設false
willChange	bool	和isComplex配合使用，當啟用快取時，該屬性代表在下一幀中繪製是否會改變。預設false
child	Widget	沒錯，CustomPaint是可以包含一個子節點的

CustomPaint是一個繼承自SingleChildRenderObjectWidget的控制元件，不能用setState的方式來重新整理它。

painter就是我們的主繪製工具，它是一個CustomPainter；foregroundPainter是用來繪製前景的工具；

size為畫布大小，這個size會傳遞給Painter；

isComplex和willChange 是告訴Flutter你的CustomPaint是否複雜到需要使用cache相關的功能；

child屬性我們一般不填，即使你是想要在你的CustomPaint上新增一些其他的佈局，也不建議放在child屬中性，因為你會發現你並不會得到你想要的結果。

CustomPainter 是一個抽象類，你需要繼承它實現自己的邏輯。

class MyPainter extends CustomPainter {

  @override
  paint(Canvas canvas, Size size)  {
       // do
  }

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

繼承 CustomPainter 最重要的是實現 paint(Canvas canvas, Size size) 和 shouldRepaint(CustomPainter oldDelegate) 這兩個函式。

paint() 中是繪製邏輯，可以在這獲得畫布 Canvas 和畫布的大小 Size。
shouldRepaint() 返回 true 才會進行重繪，否則就只會繪製一次。你可以通過一些條件判斷來決定是否每次繪製，這樣能夠節約系統資源。

例如：

@override
  void paint(Canvas canvas, Size size) {
    final outerThickess = max(trackWidth, max(progressWidth, thumbSize));
    Size constrainedSize = new Size(
      size.width - outerThickess,
      size.height - outerThickess
    );
    // paint track.
    final center = new Offset(size.width / 2, size.height /2);
    final radius = min(constrainedSize.width, constrainedSize.height) / 2;
    canvas.drawCircle(center, radius, trackPaint);

    // paint progress.
    final progressAngle = 2 * pi * progressPercent;
    canvas.drawArc(new Rect.fromCircle(
      center: center,
      radius: radius
    ), -pi / 2, progressAngle, false, progressPaint);

    // paint thumb.
    final thumbAngle = 2 * pi * thumbPosition - (pi / 2);
    final thumbX = cos(thumbAngle) * radius;
    final thumbY = sin(thumbAngle) * radius;
    final thumbCenter = new Offset(thumbX, thumbY) + center;
    final thumbRadius = thumbSize / 2;
    canvas.drawCircle(thumbCenter, thumbRadius, thumbPaint);
  }

  @override
  bool shouldRepaint(CustomPainter oldDelegate) {
    // TODO: implement shouldRepaint
    return true;
  }

這裡簡單的列一下，一些常用的canvas繪製API：

// 繪製弧線
drawArc(Rect rect, double startAngle, double sweepAngle, bool useCenter, Paint paint)
// 繪製圖片
drawImage(Image image, Offset p, Paint paint) 
// 繪製圓
drawCircle(Offset c, double radius, Paint paint) 
// 繪製線條
drawLine(Offset p1, Offset p2, Paint paint) 
// 繪製橢圓
drawOval(Rect rect, Paint paint)
// 繪製文字
drawParagraph(Paragraph paragraph, Offset offset)
// 繪製路徑
drawPath(Path path, Paint paint) 
// 繪製點
drawPoints(PointMode pointMode, List<Offset> points, Paint paint)
// 繪製Rect
drawRect(Rect rect, Paint paint) 
// 繪製陰影
drawShadow(Path path, Color color, double elevation, bool transparentOccluder)

1、繪製直線

void paint(Canvas canvas, Size size) {
    Paint paint = Paint()
      ..isAntiAlias = true
      ..color = Colors.pink
      ..strokeWidth = 10
      ..style = PaintingStyle.fill;
    Paint paint1 = Paint()
      ..isAntiAlias = true
      ..color = Colors.blue
      ..strokeWidth = 10
      ..style = PaintingStyle.fill;
    Paint paint2 = Paint()
      ..isAntiAlias = true
      ..color = Colors.green
      ..strokeWidth = 10
      ..style = PaintingStyle.fill;
    /// Offset(),橫縱座標偏移
    canvas.drawLine(Offset(85, 100), Offset(285, 100), paint);
    canvas.drawLine(Offset(85, 100), Offset(200, 300), paint1);
    canvas.drawLine(Offset(285, 100), Offset(200, 300), paint2);
  }

2、繪製弧線

void paint(Canvas canvas, Size size) {
    Paint paint = Paint()
      ..isAntiAlias = true
      ..color = Colors.green
      ..strokeCap = StrokeCap.round
      ..strokeWidth = 10
      ..style = PaintingStyle.stroke;
    Paint paint1 = Paint()
      ..isAntiAlias = true
      ..color = Colors.blue
      ..strokeCap = StrokeCap.round
      ..strokeWidth = 10
      ..style = PaintingStyle.stroke;
    /// Offset(),橫縱座標偏移
    /// void drawArc(Rect rect, double startAngle, double sweepAngle, bool useCenter, Paint paint)
    /// Rect來確認圓弧的位置, 開始的弧度、結束的弧度、是否使用中心點繪製(圓弧是否向中心閉合)、以及paint.
    final center = new Offset(170, 200);
    canvas.drawArc(new Rect.fromCircle(
        center: center,
        radius: size.width / 2
    ), -pi / 2, 2 * pi * 0.5, false, paint);
    canvas.drawArc(new Rect.fromCircle(
        center: Offset(170, 300),
        radius: size.width / 2
    ), pi / 2, 2 * pi * 0.5, false, paint1);
  }

flutter 用CustomPaint畫一個自定義的CircleProgressBar

3、繪製圓

 Paint paint = Paint()
      ..isAntiAlias = true
      ..color = Colors.green
      ..strokeCap = StrokeCap.round
      ..strokeWidth = 10
      ..style = PaintingStyle.fill;
    Paint paint1 = Paint()
      ..isAntiAlias = true
      ..color = Colors.blue
      ..strokeCap = StrokeCap.round
      ..strokeWidth = 10
      ..style = PaintingStyle.stroke;
      canvas.drawCircle(Offset(150.0, 200.0), 50.0, paint1);
      canvas.drawCircle(Offset(150.0, 350.0), 50.0, paint);