使用HTML5画布的Brick Breaker无法在画布上显示任何内容

Question

我正在尝试使用HTML5 canvas元素制作破砖游戏。我已经完成了对大多数变量和函数的定义，因此我开始调用drawBall（）;。和drawPaddle（）;功能在主游戏循环中起作用，但是画布上没有任何显示。我已经尝试修复此问题已有一段时间了。任何有关如何解决此问题的建议将不胜感激。

这是我的代码：

var canvas = document.getElementById("canvas");
var ctx = canvas.getContext("2d");

var paddle = {
  height: 15,
  width: 100,
  x: canvas.width / 2 - paddle.width / 2,
  y: canvas.height - paddle.height
}; // defines the variables for the paddle.

document.addEventListener("keydown", keyDownHandler, false);
document.addEventListener("keyup", keyUpHandler, false);

function keyDownHandler(e) {
  if(e.key == "Right" || e.key == "ArrowRight") {
    rightPressed = true;
  } else if (e.key == "Left" || e.key == "ArrowLeft") {
    leftPressed = true;
  }
} // when the right/left arrow key is pressed down, right/leftPressed = true. 

function keyUpHandler(e) {
  if(e.key == "Right" || e.key == "ArrowRight") {
    rightPressed = false;
  } else if (e.key == "Left" || e.key == "ArrowLeft") {
    leftPressed = false;
  }
} // when the left/right key is not being pressed anymore, right/leftPressed = false. 

var bricks = {
  brick1: true,
  brick2: true,
  brick3: true,
  brick4: true,
  brick5: true,
  brick6: true,
  brick7: true,
  brick8: true,
  brick9: true,
  brick10: true
}; // to be put in the "active" parameter of the drawBricks function

var brickHeight = 15;
var brickWidth = 50;

var x = 200;
var y = 200;
var dx = 5;
var dy = -5;
var ballRadius = 10;

var rightPressed = false;
var leftPressed = false;

function drawBall() {
  ctx.beginPath();
  ctx.arc(x, y, ballRadius, 0, Math.PI*2);
  ctx.fillStyle = "#00F";
  ctx.fill();
  ctx.closePath();
}

function drawPaddle() {
  ctx.beginPath();
  ctx.rect(paddle.x, paddle.y, paddle.width, paddle.height);
  ctx.fillStyle = "0F0";
  ctx.fill();
  ctx.closePath();
}

function drawBricks(brickX, brickY, active) { // function for drawing, and the hit detection of the bricks.
  if (active === true) {
    ctx.startPath();
    ctx.fillStyle = "#F00";
    ctx.rect(brickX, brickY, brickWidth, brickHeight);
    ctx.fill();
    ctx.closePath();
  } // draws the bricks, if they are active.
  if (
    active === true &&
    x >= brickX &&
    x <= brickX + brickWidth &&
    y <= brickY + brickHeight &&
    y >= brickY
  ) {
    active = false;
  } // de-activates the bricks if the balls x and y are inside the brick.
}


function draw() {
  ctx.clearRect(0, 0, canvas.width, canvas.height);
  drawBall();
  drawPaddle();

  if (x + dx > canvas.width - ballRadius || x + dx < ballRadius) {
    dx = -dx;
  } // if the ball hits either of the side walls, it goes the other way
  if (y + dy > canvas.height - ballRadius || y + dy < ballRadius) {
    dy = -dy;
  } // if teh ball hits either the top wall or the bottom wall it goes the other way.

  if (rightPressed) {
    paddleX += 10;
  } else if (leftPressed) {
    paddleX -= 10;
  } // changes the x position of the paddle based on which key is being pressed.
  ctx.fillRect(200, 200, 200, 200);

  x += dx;
  y += dy;
} // main loop
setInterval(draw, 10);