javascript - 如何在 Canvas 后面与 HTML 交互？

Question

场景:
我有一个网站，目前在本地托管，我希望能够在网站顶部的 Canvas 上绘图，如下图所示。但是，我不仅要允许用户在网站上绘图，还要让绘图保留在该特定网页上，同时用户与该页面的其余部分进行交互。例如。填写表单或与下拉列表交互等。
注意: HTML Canvas 覆盖整个页面(不仅仅是视口(viewport))。此外，将有一个按钮来区分绘图和与 HTML 交互



具体问题:
如何在存在 Canvas 的情况下与网页上的普通 HTML 元素进行交互。我知道 Canvas 会逐页重置，这很好 - 我不想解决这个问题。我只是想解决单个页面的问题。

尝试过的方法
我试过使用透明度、zIndex 和其他 css 属性。

替代解决方案
1. 我知道这可以使用 SVG 解决，只需将它们覆盖在整个网页上的透明 DIV 中。但我不想使用这种方法，如果可能的话，我会坚持使用 canvas 寻找解决方案。

2. 目前 HTML Canvas 覆盖了整个页面(不仅仅是视口(viewport))所以我想我可以根据页面上绘制的数量动态调整 Canvas 的大小。但是，那么我还不如只使用 SVG 的。

想法？我对 libs/snippets 和任何东西都持开放态度。

Answer 1

解决这个问题的一种方法是:

• 使用 CSS 将默认点事件设置为无
• 为要用于点击等的元素启用指针事件，包括事件捕捉器(背景、正文或元素)
• 根据您的设计要求使用正文或元素，作为上面未使用的事件的事件捕获器。
• 将 Canvas 置于所有指针事件之上，并将指针事件设置为无

任何未被可点击元素(或您要用于事件的元素)消耗的点击现在都将在后面结束。只需将坐标从此支持元素传输到 Canvas 即可进行绘制。

演示

演示并非完美无缺(即不考虑诸如 native 触摸事件/手势之类的事情，它画的是点而不是线等)但它应该足以展示基本原理并让您继续前进。

我们在这里简单地使用 body 来捕获任何未被消耗的事件。向下滚动以绘制并单击文本中的第二个链接。

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

setup();
window.onresize = setup;
window.onmousedown = function(e) {
  isDown = true;
  draw(e);
};
window.onmousemove = function(e) {if (isDown) draw(e);};
window.onmouseup = function() {isDown = false};

function setup() {
  canvas.width = window.innerWidth;
  canvas.height = parseInt(getComputedStyle(document.body).getPropertyValue("height"), 10);
  ctx.fillStyle = "rgba(0,160,255,0.4)";
}

function draw(e) {
  var rect = canvas.getBoundingClientRect();
  ctx.beginPath();
  ctx.arc(e.clientX - rect.left, e.clientY - rect.top, 15, 0, 6.28);
  ctx.fill();
}

* { /* optimize by being more specific, here just for demo */
  pointer-events:none;
  }
a {
  color:#00c;
  pointer-events:auto;
  cursor:pointer;
  text-decoration:underline;
  }
html, body {
  height:3500px;
  width:100%;
  }
body {
  pointer-events:auto;
  -webkit-user-select: none;
  -moz-user-select: none;
  -ms-user-select: none;
  user-select: none;
  }
#poster {
  width:80%;
  margin:50px auto 0 auto;
  background:rgba(0,0,0,0.2);
  padding:20px;
  box-sizing:border-box;
  z-index:1;
  }
#canvas {
  position:absolute;
  left:0;
  top:0;
  z-index:1000;
  }

<h1>Demonstration</h1>

<div id="poster">
  <h2>Drawable page</h2>
  <a onclick="alert('Was clicked');return false">Click me</a>
</div>

<h3>The standard Lorem Ipsum passage, used since the 1500s</h3><p>"Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non proident, sunt in culpa qui officia deserunt mollit anim id est laborum."</p><h3>Section 1.10.32 of "de Finibus Bonorum et Malorum", written by Cicero in 45 BC</h3><p>"Sed ut perspiciatis unde omnis iste natus error sit voluptatem accusantium doloremque laudantium, totam rem aperiam, eaque ipsa quae ab illo inventore veritatis et quasi architecto beatae vitae dicta sunt explicabo. Nemo enim ipsam voluptatem quia voluptas sit aspernatur aut odit aut fugit, sed quia consequuntur magni dolores eos qui ratione voluptatem sequi nesciunt. Neque porro quisquam est, qui dolorem ipsum quia dolor sit amet, consectetur, adipisci velit, sed quia non numquam eius modi tempora incidunt ut labore et dolore magnam aliquam quaerat voluptatem. Ut enim ad minima veniam, quis nostrum exercitationem ullam corporis suscipit laboriosam, nisi ut aliquid ex ea commodi consequatur? Quis autem vel eum iure reprehenderit qui in ea voluptate velit esse quam nihil molestiae consequatur, vel illum qui dolorem eum fugiat quo voluptas nulla pariatur?"</p>
<h3>1914 translation by H. Rackham</h3>
<p>"But I must explain to you how all this mistaken idea of denouncing pleasure and praising pain was born and I will give you a complete account of the system, and expound the actual teachings of the great explorer of the truth, the master-builder of human happiness. No one rejects, dislikes, or avoids pleasure itself, because it is pleasure, but because those who do not know how to pursue pleasure rationally encounter consequences that are extremely painful. Nor again is there anyone <b><a onclick="alert('yeehaw');return false">who loves</a></b> or pursues or desires to obtain pain of itself, because it is pain, but because occasionally circumstances occur in which toil and pain can procure him some great pleasure. To take a trivial example, which of us ever undertakes laborious physical exercise, except to obtain some advantage from it? But who has any right to find fault with a man who chooses to enjoy a pleasure that has no annoying consequences, or one who avoids a pain that produces no resultant pleasure?"</p>
<h3>Section 1.10.33 of "de Finibus Bonorum et Malorum", written by Cicero in 45 BC</h3>
<p>"At vero eos et accusamus et iusto odio dignissimos ducimus qui blanditiis praesentium voluptatum deleniti atque corrupti quos dolores et quas molestias excepturi sint occaecati cupiditate non provident, similique sunt in culpa qui officia deserunt mollitia animi, id est laborum et dolorum fuga. Et harum quidem rerum facilis est et expedita distinctio. Nam libero tempore, cum soluta nobis est eligendi optio cumque nihil impedit quo minus id quod maxime placeat facere possimus, omnis voluptas assumenda est, omnis dolor repellendus. Temporibus autem quibusdam et aut officiis debitis aut rerum necessitatibus saepe eveniet ut et voluptates repudiandae sint et molestiae non recusandae. Itaque earum rerum hic tenetur a sapiente delectus, ut aut reiciendis voluptatibus maiores alias consequatur aut perferendis doloribus asperiores repellat."</p>
<h3>1914 translation by H. Rackham</h3>
<p>"On the other hand, we denounce with righteous indignation and dislike men who are so beguiled and demoralized by the charms of pleasure of the moment, so blinded by desire, that they cannot foresee the pain and trouble that are bound to ensue; and equal blame belongs to those who fail in their duty through weakness of will, which is the same as saying through shrinking from toil and pain. These cases are perfectly simple and easy to distinguish. In a free hour, when our power of choice is untrammelled and when nothing prevents our being able to do what we like best, every pleasure is to be welcomed and every pain avoided. But in certain circumstances and owing to the claims of duty or the obligations of business it will frequently occur that pleasures have to be repudiated and annoyances accepted. The wise man therefore always holds in these matters to this principle of selection: he rejects pleasures to secure other greater pleasures, or else he endures pains to avoid worse pains."</p>

<canvas id="canvas"></canvas>