如何通过请求动画帧从其新实例中移动形状

How to move shapes from its new instance by requestAnimationFrame?

本文关键字：实例 移动 新实例 请求 何通过 动画      更新时间：2023-09-26

这是小提琴。将创建小矩形以模拟按下空格键（键码 32）时的项目符号。我遇到了一些问题：如何将它们移动到顶部（减少 y 坐标）？谁能帮我？感谢！

window.requestAnimationFrame = window.requestAnimationFrame || window.mozRequestAnimationFrame || window.webkitRequestAnimationFrame || window.msRequestAnimationFrame;
var canvas = document.getElementById("myCanvas");
var context = canvas.getContext("2d");
var cw = canvas.width;
var ch = canvas.height;
var ps = false;
init();
function init(){
    context.rect((cw-5)/2, ch-5, 5, 5);
  context.fill();
  update();
}
function update(){
  if(ps){
    playerShoot();
  }
  requestAnimationFrame(update);
}
function playerShoot(){
    var b = new bullet(2);
}
function bullet(speed){
    this.speed = speed;
  speed++;
  context.ellipse((cw-1)/2, ch-10-speed, 1, 3, 0, 0, Math.PI*2);
  context.fill();
}
document.addEventListener("keydown", function(e){
  switch(e.keyCode){
    case 32:
        ps = true;
      break;
  };
});
document.addEventListener("keyup", function(e){
  switch(e.keyCode){
    case 32:
        ps = false;
      break;
  };
});
我已经在

代码本身的注释中解释了很多代码。

其他几点：

  • 某些浏览器（包括我的浏览器，即Firefox v44.0.2）不绘制省略号。所以我把你的项目符号变成了另一个矩形。
  • 我用fillRect而不是rect只是因为我更了解这一点。
  • 我通过在具有不透明背景颜色的旧项目符号上绘制来重新绘制项目符号。但是，如果需要，您也可以清除上一个项目符号周围的矩形。
  • 您在示例中提高了速度。从概念的角度来看，这可能不是你想要的，即使你已经得到了你想要的视觉结果。我怀疑你希望你的子弹以恒定的速度移动。因此，speed变量应该是常数，即不变。相反，您应该使用 speed 常量来定期更改项目符号的位置。我换了bulletY，这是子弹的垂直位置。
  • 为简单起见，我一次只允许屏幕上显示一个项目符号。
  • 我将代码限制为运行 500 个周期。这主要是为了不惹恼尝试代码的堆栈溢出用户......他们不希望无限循环发生。

window.requestAnimationFrame = window.requestAnimationFrame || window.mozRequestAnimationFrame || window.webkitRequestAnimationFrame || window.msRequestAnimationFrame;
var canvas = document.getElementById("myCanvas");
var context = canvas.getContext("2d");
var cw = canvas.width;
var ch = canvas.height;
var ps = false;
// some new variables
var bulletShowing = false; // is a bullet currently showing?
var bulletY; // the vertical position of the bullet
var speed = 8; // the bullet speed
var time = 500; // the time remaining
init();
function init() {
  
  // draw background
  context.fillStyle = "yellow";
  context.fillRect(0, 0, cw, ch);
  
  // draw gun
  context.fillStyle = "black";
  context.fillRect((cw - 5) / 2, ch - 5, 5, 5);
  // update the scene
  update();
}
function update() {
  if (ps) {
    playerShoot();
  }
  
  // if a bullet is supposed to be showing then, well, show it
  if (bulletShowing) {
    
    // redraw the bullet (erase the old, draw the new)
    drawBullet();
    
    // if the bullet has gone off-screen, allow a new shot
    if (bulletY < -5) {
      bulletShowing = false;
    }
  }
  
  // give a visual indicator of time remaining
  document.querySelector("div").innerHTML = "Time: " + time;
  
  // decrement the time
  time -= 1;
  
  // if there is still time remaining, do it all again
  if (time >= 0) {
    requestAnimationFrame(update);
  }
}
function playerShoot() {
  // indicate a bullet will now be showing
  bulletShowing = true;
  // start the bullet out near the gun
  bulletY = ch - 10;
}
function drawBullet() {
  
  // erase the old bullet by drawing over it with the background color
  // this rectangle is slightly larger than the bullet itself
  // to ensure the entire old bullet is drawn over
  context.fillStyle = "yellow";
  context.fillRect((cw - 1) / 2 - 2, bulletY - 1, 5, 7);
  
  // move the bullet position
  bulletY -= speed;
  
  // draw the new bullet
  context.fillStyle = "black";
  context.fillRect((cw - 1) / 2 - 1, bulletY, 3, 5);
}
document.addEventListener("keydown", function(e) {
  switch (e.keyCode) {
    case 32:
      
      // only allow one bullet on the screen at a time
      // (for the sake of coding simplicity)
      if (!bulletShowing) {
        ps = true;
      }
      break;
  };
});
document.addEventListener("keyup", function(e) {
  switch (e.keyCode) {
    case 32:
      ps = false;
      break;
  };
});
#myCanvas {
  position: absolute;
  top: 0;
  left: 50%;
  transform: translate(-50%, 5%);
  background-color: #cccccc;
  z-index: -1;
}
<p>Click on the canvas, then use the space bar to fire bullets one at a time.</p>
<div></div>
<canvas id="myCanvas" width=300 height=150></canvas>

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