javascript - HTML5 Canvas(绘图(矩形)不断消失)

Question

最近怎么样？

最近，在本网站的帮助下，我学会了如何通过单击按钮在 HTML5 Canvas 上绘制矩形...这不是问题:) 问题是这个...不幸的是，每次我单击 Canvas 创建一个新矩形时，旧矩形就会消失(或立即被删除)。当我尝试在 iPod 上使用它时，它也根本不起作用...为什么:(?

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这是我的代码:

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JavaScript:

  // "Rectangle" Button
function rect()
{
var canvas = document.getElementById('canvasSignature'),
    ctx = canvas.getContext('2d'),
    rect = {},
    drag = false;


    function init() {
  canvas.addEventListener('mousedown', mouseDown, false);
  canvas.addEventListener('mouseup', mouseUp, false);
  canvas.addEventListener('mousemove', mouseMove, false);
}


function mouseDown(e) {
  rect.startX = e.pageX - this.offsetLeft;
  rect.startY = e.pageY - this.offsetTop;
  drag = true;
}


function mouseUp() {
  drag = false;
}


function mouseMove(e) {
  if (drag) {
    rect.w = (e.pageX - this.offsetLeft) - rect.startX;
    rect.h = (e.pageY - this.offsetTop) - rect.startY ;
    ctx.clearRect(0,0,canvas.width,canvas.height);
    draw();
  }
}


function draw() {
  ctx.fillRect(rect.startX, rect.startY, rect.w, rect.h);
}


init();
}

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HTML5:

<div id="canvasDiv">
    <canvas id="canvasSignature" width="580px" height="788px" style="border:2px solid #000; background: #FFF;"></canvas>
</div>

<div id="rect">  
    <p><button onclick="rect();">Rectangle</button></p>
</div> 

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任何帮助都将不胜感激:)

Answer 1

您有一个清除 Canvas 的声明

ctx.clearRect(0, 0, canvas.width, canvas.height);