我目前正在创建一个类似于绘画的软件，但是我仍然坚持要在画布上放一个跟随鼠标的框。该框的目的是让它显示鼠标位置的位置，显示已选择的颜色以及显示画笔的大小。我在网上找到了这段代码：
<html>
<head>
<script>
function m(e){
var bx = document.getElementById("box");
bx.style.left = e.pageX;
bx.style.top = e.pageY;
}
</script>
</head>
<body onmousemove="m(event)">
<div id="box" style="background-color:#990000; width:20px; height:20px; position:absolute;" />
</body>
我试图将它实现到我的程序中，但它似乎没有在画布上显示...我已将函数m（e）更改为fnTrackMouse（事件）
我只能使用javascript和一些html和CSS。请不要Jquery。这是我正在研究的程序的一部分：
<!doctype html>
<html lang="en">
<head>
<style>
...
</style>
<script>
var oCanvas, oCanvasContext; //declare the global variables used to hold and control the canvas element
var sFillColour; //create a global variable used to hold the active/selected colour
var sCanvasColour; //create a global variable used to hold the canvas colour
var iMouseX, iMouseY; //declare the global variables used to hold the mouse's coordinates
var iBrushWidth, iBrushHeight; //declare the global variables used to hold the selected brush sizes
function fnInitialise(iCanvasWidth, iCanvasHeight) {
//this function is called via the HTML body tag and the onload event
fnDebugMessage("Running fnInitialise"); //debug message
oCanvas = document.getElementById("cw1Canvas"); //create a reference to the HTML canvas element
if (oCanvas.getContext) { //test to see if we can read the canvas' context; if true we have found the canvas
oCanvas.width=iCanvasWidth; //set the canvas width using the width argument passed to the fnInitialise function
oCanvas.height=iCanvasHeight; //set the canvas height using the width argument passed to the fnInitialise function
oCanvasContext = oCanvas.getContext("2d"); //set the context to 2D
fnDebugMessage("Canvas size, width: " + iCanvasWidth + ", height: " + iCanvasHeight); //debug message, if this message appears in
//set some default values`
sCanvasColour=getComputedStyle(oCanvas).getPropertyValue("background-color"); //this instruction automatically detects the background colour of the cavas and stores it in the global sCanvasColour variable
fnDebugMessage("Canvas background colour: " + sCanvasColour); //debug message, if this message appears in
//let set a default brush size
iBrushWidth=5;
iBrushHeight=5;
//set the canvas size to 550px x 550px
oCanvas.width=550;
oCanvas.height=550;
} else {
fnDebugMessage("fnInitialise, failed to get the canvas's context"); //debug message, we were unable to get the canvas' context
}
}
function fnTrackMouse(e) {
//this function is called "everytime" the user's mouse is moved when over the associated element (in this case the canvas)
//we have also added the ability for it to accept a parameter (called e, actually we can call it anything but as event is a reserved work "e" makes some sense
var canvasRect = oCanvas.getBoundingClientRect(); //use this function to dynamically get the size of the canvas and its position
iMouseX=(e.clientX - canvasRect.left - 3); //modify the original position of the mouse by accounting for the position on the canvas; on the x
iMouseY=(e.clientY - canvasRect.top - 3); //modify the original position of the mouse by accounting for the position on the canvas; on the y
var bx = document.getElementById("box");
bx.style.left = iMouseX;
bx.style.top = iMouseY;
fnDebugMessage("Working!"+iMouseX)
if (e.buttons==1) { //this checks to see if the user is pressing the left mouse button (1 = the left mouse button)
//the user has pressed the left button - so lets start painting
fnPaint(iMouseX, iMouseY, iBrushWidth, iBrushHeight); //call the fnPaint function and pass it the coordinates and size to paint
}
fnDebugMessage("Tracking mouse: x: " + iMouseX + ", y: "+iMouseY); //update the console to show the mouse position, dont forget, you may need to include an offset to centre the paint effect
}
</script>
</head>
<body onload="fnInitialise(100, 100);">
<!--
this div block (HTML page divider) is used to hold the entire interactive painting HTML elements
the benefit of putting multiple elements in a single container is that if you set the location of the
container each of the elements held by the container will move relative to it; move one, move all
-->
<div id="cw1MainContainer">
<!-- this div block is only used to hold the HTML canvas element -->
<div id="cw1CanvasContainer">
<canvas id="cw1Canvas" onmousemove="fnTrackMouse(event);" onkeypress="fnBrushSize(event);"></canvas>
<div id="box" style="background-color:#990000; width:20px; height:20px; position:absolute;" />
<!--
by specifing the onmouseover event the canvas will call the "fnTrackMouse" function EVERY time the
mouse moves 1 pixel over the canvas.
by passing the JavaScript "event" we are effectively also passing details about the event,
e.g. where the mouse was, what buttons were pressed etc.
-->
</div>
</div>
</body>
</html>
我已经删除了大部分我认为与此问题无关的代码，但如果有必要，请告诉我。任何帮助，将不胜感激。干杯
你的代码有点问题。
在这里，您尝试根据鼠标位置设置bx元素的位置：
bx.style.left = iMouseX;
bx.style.top = iMouseY;
不幸的是，除非你附加字符串px - 像素的简短形式，否则这不起作用。
bx.style.left = iMouseX+"px";
bx.style.top = iMouseY+"px";
您的责任在于您没有为您的职位提供单位。您可以通过将px后缀附加到鼠标位置的坐标来解决此问题：
bx.style.left = iMouseX +'px';
bx.style.top = iMouseY + 'px';
但是，你仍然会遇到问题。这里的问题是你的canvas正在听onmousemove事件。但是，当您将“box”div移动到鼠标位置时，在您将鼠标从盒子上移开并返回到画布上之前，不会触发画布的onmousemove事件，从而导致“滞后”效果。要解决此问题，您可以将样式pointer-events: none添加到您的框div，这将停止框中的所有鼠标事件。
见下面的例子：
canvas {
border: 1px solid black;
}
#box {
pointer-events: none;
}<!doctype html>
<html lang="en">
<head>
<script>
var oCanvas, oCanvasContext;
var sFillColour;
var sCanvasColour;
var iMouseX, iMouseY;
var iBrushWidth, iBrushHeight;
var multiplier = 1
function fnInitialise(iCanvasWidth, iCanvasHeight) {
oCanvas = document.getElementById("cw1Canvas");
if (oCanvas.getContext) {
oCanvas.width = iCanvasWidth;
oCanvas.height = iCanvasHeight;
oCanvasContext = oCanvas.getContext("2d");
sCanvasColour = getComputedStyle(oCanvas).getPropertyValue("background-color");
iBrushWidth = 5;
iBrushHeight = 5;
} else {
}
}
function fnTrackMouse(e) {
var canvasRect = oCanvas.getBoundingClientRect();
iMouseX = (e.clientX - canvasRect.left - 3);
iMouseY = (e.clientY - canvasRect.top - 3);
var bx = document.getElementById("box");
bx.style.left = iMouseX +'px';
bx.style.top = iMouseY + 'px';
if (e.ctrlKey) {
fnSetFillColour(sCanvasColour);
}
if (e.buttons == 1) {
fnPaint(iMouseX, iMouseY, iBrushWidth, iBrushHeight);
}
}
</script>
</head>
<body onload="fnInitialise(100, 100);">
<div id="cw1MainContainer">
<div id="cw1CanvasContainer">
<canvas id="cw1Canvas" onmousemove="fnTrackMouse(event);" onkeypress="fnBrushSize(event);"></canvas>
<div id="box" style="background-color:#990000; width:20px; height:20px; position:absolute;" />
</div>
</div>
</body>
</html>