HTML Canvas 触摸屏帮助

Question

您好，我正在开发一个特定于移动设备的网站，该网站涉及用户在其移动设备上绘制形状和图案。我在为移动设备实现此功能时遇到了问题 canvas 绘图在桌面上完美运行，但在触摸屏设备上却完全无法运行。我试过使用触摸事件，但它们似乎根本不起作用。下面是我的代码。任何帮助将不胜感激

<!DOCTYPE html>
<html>
<head>
<title>Page Title</title>


<style>
canvas { border: 5px solid blue }

</style>

</head>
<body>
<canvas id="c" width="500" height="300"></canvas>
<h1>This is a Heading</h1>
<p>This is a paragraph.</p>


<script>
function midPointBtw(p1, p2) {
  return {
    x: p1.x + (p2.x - p1.x) / 2,
    y: p1.y + (p2.y - p1.y) / 2
  };
 }

function getPattern() {
  return ctx.createPattern(img, 'repeat');
 }


var el = document.getElementById('c');
var ctx = el.getContext('2d');

ctx.lineWidth = 50;
ctx.lineJoin = ctx.lineCap = 'round';

var img = new Image;
img.onload = function() {
  ctx.strokeStyle = getPattern();
};
img.src = "dick2.png";

var isDrawing, points = [ ];

el.ontouchstart = function(e) {
  isDrawing = true;
  points.push({ x: e.clientX, y: e.clientY });

};

el.ontouchmove = function(e) {
  if (!isDrawing) return;

  points.push({ x: e.clientX, y: e.clientY });
  ctx.clearRect(0, 0, ctx.canvas.width, ctx.canvas.height);

  var p1 = points[0];
  var p2 = points[1];

  ctx.beginPath();
  ctx.moveTo(p1.x, p1.y);

  for (var i = 1, len = points.length; i < len; i++) {
    var midPoint = midPointBtw(p1, p2);
    ctx.quadraticCurveTo(p1.x, p1.y, midPoint.x, midPoint.y);
    p1 = points[i];
    p2 = points[i+1];
  }
  ctx.lineTo(p1.x, p1.y);
  ctx.stroke();
};

el.onmouseup = function() {
  isDrawing = false;
  points.length = 0;
};




function showInput() {
document.getElementById('display').innerHTML = 
document.getElementById("user_input").value;
    }







</script>


<form id ="search">
<input type = "text" name="message" id="user_input">
<input type = "button" value= "Submit" onclick="showInput();">
</form>



<p><span id='display'></span></p>

</body>
</html>

Answer 1

触摸事件与鼠标事件的工作方式不同。由于触摸屏能够在不同的灵敏度下支持多个触摸点，而不仅仅是 clientX 和 clientY 出现在事件本身上，因此可以在 touches 下的不同位置访问它们。如果您只关心列表中的第一个触摸，您可以通过最少的更改轻松地使您的代码与桌面和移动设备兼容：

function midPointBtw(p1, p2) {
  return {
    x: p1.x + (p2.x - p1.x) / 2,
    y: p1.y + (p2.y - p1.y) / 2
  };
}

function getPattern() {
  return ctx.createPattern(img, 'repeat');
}


var el = document.getElementById('c');
var ctx = el.getContext('2d');

ctx.lineWidth = 50;
ctx.lineJoin = ctx.lineCap = 'round';

var img = new Image;
img.onload = function() {
  ctx.strokeStyle = getPattern();
};
img.src = "https://placekitten.com/200";

var isDrawing, points = [];

var getXY = function(e) {
  var source = e.touches ? e.touches[0] : e;

  return {
    x: source.clientX,
    y: source.clientY
  };
};

var startDrawing = function(e) {
  isDrawing = true;
  points.push(getXY(e));
};
var keepDrawing = function(e) {
  if (!isDrawing) return;

  points.push(getXY(e));
  ctx.clearRect(0, 0, ctx.canvas.width, ctx.canvas.height);

  var p1 = points[0];
  var p2 = points[1];

  ctx.beginPath();
  ctx.moveTo(p1.x, p1.y);

  for (var i = 1, len = points.length; i < len; i++) {
    var midPoint = midPointBtw(p1, p2);
    ctx.quadraticCurveTo(p1.x, p1.y, midPoint.x, midPoint.y);
    p1 = points[i];
    p2 = points[i + 1];
  }
  ctx.lineTo(p1.x, p1.y);
  ctx.stroke();
};
var stopDrawing = function() {
  isDrawing = false;
  points = [];
};

el.addEventListener('touchstart', startDrawing);
el.addEventListener('mousedown', startDrawing);

el.addEventListener('touchmove', keepDrawing);
el.addEventListener('mousemove', keepDrawing);

el.addEventListener('touchend', stopDrawing);
el.addEventListener('mouseup', stopDrawing);

canvas {
  border: 5px solid blue
}

<canvas id="c" width="500" height="300"></canvas>

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