正确使用onmousemove [英] proper use of onmousemove
问题描述
我正在尝试编写一些Javascript，通过拖动鼠标来画一条线，然后在放开鼠标左键时将其删除.
I am trying to write some javascript that draws a line by dragging the mouse, and then removes it when you let go of left click.
<!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
This example does not work and throws no errors. If
document.getElementById('e').onmousemove = "mov()";
已被注释掉，并且onmousemove设置为
is commented out and onmousemove is set to
onmousemove="mov()"
然后它可以工作，但是很明显，一条线只能绘制一次.同样，两个示例都无法在FireFox中使用.在Chrome中进行了测试.
then it works, but obviously a line can only be drawn once. Also neither of the examples worked in FireFox. Tested in Chrome.
推荐答案
更改此内容:
document.getElementById('e').onmousemove = "mov()";
对此:
document.getElementById('e').onmousemove = mov;
您想将 .onmousemove 分配给函数引用，而不是字符串.请注意，没有括号:如果您分配 ... onmousemove = mov()，它将运行 mov()函数并分配 onmousemove 返回函数的返回值(此特定函数未定义).没有括号，它将其分配给函数本身.
You want to assign .onmousemove to a function reference, not to a string. Note that there are no parentheses: if you assign ...onmousemove = mov() it will run the mov() function and assign onmousemove to the return from the function (undefined, with this particular function). Without the parentheses it assigns it to the function itself.
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