Question

我正在尝试编写一些通过拖动鼠标绘制线条的javascript，然后在松开左键时将其删除。

<!DOCTYPE html>
<html>
<head>
<script type="text/javascript">

window.onload = function() {
  window.stop = false
  window.canvas = document.getElementById("e");
  window.context = canvas.getContext("2d");
  canvas.width = document.documentElement.clientWidth;
  canvas.height = document.documentElement.clientHeight;
  window.pos = Shift();
}

function Shift() {
  e = window.event
  var posx = 0;
  var posy = 0;
  if (!e) var e = window.event;
  if (e.pageX || e.pageY)   {
    posx = e.pageX;
    posy = e.pageY;
  }
    else if (e.clientX || e.clientY)    {
    posx = e.clientX + document.body.scrollLeft
                 + document.documentElement.scrollLeft;
    posy = e.clientY + document.body.scrollTop;
                     + document.documentElement.scrollTop;
  }
  posx -= document.getElementById('e').offsetLeft;
  posy -= document.getElementById('e').offsetTop;
  return[posx,posy];
}

function up(){
  document.getElementById('e').onmousemove = null;
  canvas.width = canvas.width;
}

function mov(){
  canvas.width = canvas.width;
  window.pos = Shift();
  context.moveTo(window.start[0],window.start[1]);
  context.lineTo(pos[0],pos[1]);
  context.stroke();
}

function down(){
  window.start = Shift();
  document.getElementById('e').onmousemove = "mov()";
}

</script>
</head>
<body>
  <canvas id="e" onMouseDown="down()" onmousemove="" onMouseup="up()"></canvas>
</body>
</html>

此示例不起作用并且不会引发任何错误。如果

   document.getElementById('e').onmousemove = "mov()";

被注释掉，onmousemove被设置为

onmousemove="mov()"

然后它可以工作，但显然一条线只能画一次。 FireFox中也没有这两个例子。在Chrome中测试过。

Answer 1

改变这个：

document.getElementById('e').onmousemove = "mov()";

对此：

document.getElementById('e').onmousemove = mov;

您希望将.onmousemove分配给函数引用，而不是字符串。请注意，没有括号：如果指定...onmousemove = mov()，它将运行mov()函数并将onmousemove分配给函数的返回值（未定义，具有此特定函数）。如果没有括号，它会将其分配给函数本身。

Answer 2

使用

document.getElementById('e').addEventListener('mousemove', mov, false);

document.getElementById('e').removeEventListener('mousemove', mov);

做了一些修复，但这是疯狂的代码：）

<!DOCTYPE html>
<html>
<head>
<script type="text/javascript">
window.onload = function() {
window.stop = false
  window.canvas = document.getElementById("e");
  window.context = canvas.getContext("2d");
  canvas.width = document.documentElement.clientWidth;
  canvas.height = document.documentElement.clientHeight;
}

function Shift(e) {

var posx = 0;
var posy = 0;
if (!e) var e = window.event;
if (e.pageX || e.pageY)     {
    posx = e.pageX;
    posy = e.pageY;
}
else if (e.clientX || e.clientY)    {
    posx = e.clientX + document.body.scrollLeft
        + document.documentElement.scrollLeft;
    posy = e.clientY + document.body.scrollTop;
        + document.documentElement.scrollTop;
}
posx -= document.getElementById('e').offsetLeft;
posy -= document.getElementById('e').offsetTop;
return[posx,posy];
}
function up(){
 document.getElementById('e').removeEventListener('mousemove', mov);
canvas.width = canvas.width;
}
function mov(e){
canvas.width = canvas.width;
window.pos = Shift(e);
    context.moveTo(window.start[0],window.start[1]);
context.lineTo(pos[0],pos[1]);
    context.stroke();
}
function down(e){
window.start = Shift(e);
document.getElementById('e').addEventListener('mousemove', mov, false);

}
</script>
</head>
<body>
<canvas id="e" onMouseDown="down()" onmousemove="" onMouseup="up()"></canvas>
</body>
</html>

Answer 3

您正在正确分配功能

http://jsfiddle.net/wmTYr/

<div id="meh">hi</div>
<script>
function go() {
    alert();
}
document.getElementById("meh").onmouseover = go
</script>

正确使用onmousemove

3 个答案: