为什么我的画布不会在动画帧之间清晰？

Question

我决定今天再次开始玩dart，由于某种原因，我不能让我的视口在帧之间清除。我试图使用clearRect和fillRect在整个canvas元素上绘制一个白色矩形。这是我的代码。

import 'dart:html';

void main() {

  var canvasFun = new CanvasFun(query("#Game"));

}

class CanvasFun{

  CanvasElement canvas;

  num _width;
  num _height;

  Square square;

  CanvasFun(this.canvas){
    this.square = new Square(10,10, new Point(50,50));
    requestRedraw();
  }

  void draw(num _){
    var ctx = canvas.context2d;

    //clear the viewport

    ctx.clearRect(0, 0, ctx.canvas.width, ctx.canvas.height);
    ctx.fillStyle = "white";
    ctx.fillRect(0, 0, ctx.canvas.width, ctx.canvas.height);


    //draw the square
    this.square.draw(ctx);
    requestRedraw();

  }


  void requestRedraw() {
    window.requestAnimationFrame(draw);
  }

}
class Point {
  num x, y;

  Point(this.x, this.y);
}
class Square{
  num width;
  num height;

  num vectorX;
  num vectorY;

  Point location;

  Square(this.width, this.height, this.location){
    vectorX = 1;
    vectorY = 1;
  }

  void draw(CanvasRenderingContext2D context){
    context.save(); //I thought this might fix the blue viewport problem
    this.update(context);
    context.rect(this.location.x, this.location.y, this.width, this.height);
    context.fillStyle = "blue";
    context.fill();
  }
  void update(CanvasRenderingContext2D context)
  {
    num xBound = context.canvas.width;
    num yBound = context.canvas.height;

    if((this.location.x + this.width) > xBound){
      this.vectorX = -1;
    }
    else if(this.location.x < 0){
      this.vectorX = 1;
    }

    if((this.location.y + this.height) > yBound){
      this.vectorY = -1;
    }
    else if(this.location.y < 0){
      this.vectorY = 1;
    }

    this.location.x += (this.vectorX * 10);
    this.location.y += (this.vectorY * 20);


  }
}

生成的动画在正确的位置绘制一个矩形，但当它围绕画布移动时，仍会绘制矩形的上一个实例。我希望矩形只在画布上出现一次，看起来像是在屏幕上移动。

以下是输出的屏幕截图（请注意，蓝色方块永远不会被清除）：

Answer 1

我简化了您的代码以使其正常工作：

在CanvasFun：

void draw(num _){
    var ctx = canvas.context2d;

    //clear the viewport
    ctx.clearRect(0, 0, ctx.canvas.width, ctx.canvas.height);

    //draw the square
    this.square.draw(ctx);
    requestRedraw();
}

在Square：

void draw(CanvasRenderingContext2D context){
    this.update(context);
    context.fillStyle = "blue";
    context.fillRect(this.location.x, this.location.y, this.width, this.height);
}

我不确定原始版本的确切问题是什么。 ：）