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javascript - 未捕获的类型错误 : Cannot read property 'width' of null

转载 作者:太空狗 更新时间:2023-10-29 13:29:47 28 4
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我实际上不知道发生了什么,因为它昨晚还在工作。无论如何,我正在尝试使用 html5 和 javascript 制作绘图应用程序。这是我第一次正确地看待 JS 并用它创建一些东西,所以我从各种教程中获得了我的代码,并从其他 friend 那里得到了帮助 a.k.a 我是一个 n00b。我使用了以下内容:

  1. github.com/jaseemkp/paint-app-with-save-facility
  2. codetheory.in/different-tools-for-our-sketching-application/
  3. HTML5 Canvas Cookbook - 第 6 章:与 Canvas 交互:将事件监听器附加到形状和区域 - 创建绘图应用。 (我从这里获得了保存功能)

当我在 chrome 中测试我的工作时,我得到了错误

"Uncaught TypeError: Cannot read property 'width' of null"

这是指我的脚本文件的第 4 行

var b_width = canvas.width, b_height = canvas.height;

我试图摆弄它来添加一个文本功能,但就是无法让它工作,所以我只是撤消了所有操作,现在它产生了这个错误。

完整脚本:

var canvas = document.getElementById("realCanvas");
var tmp_board = document.getElementById("tempCanvas");
var b_width = canvas.width, b_height = canvas.height;
var ctx = canvas.getContext("2d");
var tmp_ctx = tmp_board.getContext("2d");
var x, y;
var saved = false, hold = false, fill = false, stroke = true, tool = 'rectangle';

var data = {"rectangle": [], "circle": [], "line": []};


function curr_tool(selected){tool = selected;}

function attributes(){
if (document.getElementById("fill").checked)
fill = true;
else
fill = false;
if (document.getElementById("outline").checked)
stroke = true;
else
stroke = false;
}


function clears(){
ctx.clearRect(0, 0, b_width, b_height);
tmp_ctx.clearRect(0, 0, b_width, b_height);
data = {"rectangle": [], "circle": [], "line": []};
}

//colour function
function color(scolor){
tmp_ctx.strokeStyle = scolor;
if (document.getElementById("fill").checked)
tmp_ctx.fillStyle = scolor;
}



//line

tmp_board.onmousedown = function(e) {
attributes();
hold = true;
x = e.pageX - this.offsetLeft;
y = e.pageY -this.offsetTop;
begin_x = x;
begin_y = y;
tmp_ctx.beginPath();
tmp_ctx.moveTo(begin_x, begin_y);
}


tmp_board.onmousemove = function(e) {
if (x == null || y == null) {
return;
}
if(hold){
x = e.pageX - this.offsetLeft;
y = e.pageY - this.offsetTop;
Draw();
}
}

tmp_board.onmouseup = function(e) {
ctx.drawImage(tmp_board,0, 0);
tmp_ctx.clearRect(0, 0, tmp_board.width, tmp_board.height);
end_x = x;
end_y = y;
x = null;
y = null;
Draw();
hold = false;
}



//draw function
function Draw(){

//rectangle
if (tool == 'rectangle'){

if(!x && !y){
data.rectangle.push({"x": begin_x, "y": begin_y, "width": end_x-begin_x, "height": end_y-begin_y, "stroke": stroke, "strk_clr": tmp_ctx.strokeStyle, "fill": fill, "fill_clr": tmp_ctx.fillStyle });
return;
}
tmp_ctx.clearRect(0, 0, b_width, b_height);
tmp_ctx.beginPath();

if(stroke)
tmp_ctx.strokeRect(begin_x, begin_y, x-begin_x, y-begin_y);
tmp_ctx.lineWidth = $('#selWidth').val();
if(fill)
tmp_ctx.fillRect(begin_x, begin_y, x-begin_x, y-begin_y);
tmp_ctx.closePath();

}

//line

if (tool == 'line'){
if(!x && !y){
data.line.push({"x": begin_x, "y": begin_y, "width": end_x-begin_x, "height": end_y-begin_y, "stroke": stroke, "strk_clr": tmp_ctx.strokeStyle,});
return;
}
tmp_ctx.beginPath();
if(stroke)
tmp_ctx.strokeRect(begin_x, begin_y, x-begin_x, y-begin_y);
tmp_ctx.clearRect(0, 0, tmp_board.width, tmp_board.height);
tmp_ctx.beginPath();
tmp_ctx.moveTo(begin_x, begin_y);
tmp_ctx.lineTo(x, y);
tmp_ctx.lineWidth = $('#selWidth').val();
tmp_ctx.stroke();
tmp_ctx.closePath();

}

//circle

else if (tool == 'circle'){
if(!x && !y){
data.circle.push({"x": begin_x, "y": begin_y, "radius": end_x-begin_x, "stroke": stroke, "strk_clr": tmp_ctx.strokeStyle, "fill": fill, "fill_clr": tmp_ctx.fillStyle });
return;
}
tmp_ctx.clearRect(0, 0, b_width, b_height);
tmp_ctx.beginPath();
tmp_ctx.arc(begin_x, begin_y, Math.abs(x-begin_x), 0 , 2 * Math.PI, false);

if(stroke)
tmp_ctx.stroke();
tmp_ctx.lineWidth = $('#selWidth').val();
if(fill)
tmp_ctx.fill();
tmp_ctx.closePath();

}

}

//save function
//set up image of canvas
function getCanvasImg(canvas){
var img = new Image();
img.src = canvas.toDataURL();
return img;
}
//get image of canvas
window.onload = function (){
var events = new Events("tempCanvas");
var canvas = events.getCanvas();
var context = events.getContext();
}
//open image of canvas in new window in click of button
document.getElementById("saveButton").addEventListener("click", function(evt){
//open new window with saved image, right click and save
window.open(canvas.toDataURL());
}, false);

完整的 HTML

<!doctype html>
<html>
<head>
<title>LogoMakr</title>
<link rel="stylesheet" href="style.css" type="text/css">
<script src="jquery.min.js"></script>
<script src="script.js"> </script>
</style>
</head>
<body>
<div class="tools">
<button type="button" id="saveButton" value="Save">Save</button>
<button type="button" onclick="clears()">CLEAR</button>
<button type="button" onclick="curr_tool('rectangle')">Rectangle</button>
<button type="button" onclick="curr_tool('circle')">Circle</button>
<button type="button" onclick="curr_tool('line')">Line</button>

</div>

<table id="table1" >
<tr>
<tr>
<td><button onclick="color('black')" style="background-color: black; height: 20px; width: 20px;"></button></td>
<td><button onclick="color('white')" style="background-color: white; height: 20px; width: 20px;"></button></td>
<td><button onclick="color('green')" style="background-color: green; height: 20px; width: 20px;"></button></td>
<td><button onclick="color('blue')" style="background-color: blue; height: 20px; width: 20px;"></button></td>
<td><button onclick="color('yellow')" style="background-color: yellow; height: 20px; width: 20px;"></button></td>
<td><button onclick="color('red')" style="background-color: red; height: 20px; width: 20px;"></button></td>
<td><button onclick="color('#ff6600')" style="background-color: #ff6600; height: 20px; width: 20px;"></button></td>
<td><button onclick="color('#663300')" style="background-color: #663300; height: 20px; width: 20px;"></button></td>
<td><button onclick="color('grey')" style="background-color: grey; height: 20px; width: 20px;"></button></td>
<td><button onclick="color('#FF6699')" style="background-color: #FF6699; height: 20px; width: 20px;"></button></td>
<td><button onclick="color('#8b00ff')" style="background-color: #8b00ff; height: 20px; width: 20px;"></button></td>
<td><input type="checkbox" id="fill"/>Fill</td>
<td><input type="checkbox" id="outline" checked="checked"/>Outline</td>

<td>Line Width:</td>
<td><select id="selWidth">
<option value="1">1</option>
<option value="3">3</option>
<option value="5" selected="selected">5 </option>
<option value="7">7</option>
<option value="9" >9</option>
<option value="11">11</option>
</select>
</td>
</tr>
</table>


<div>
<canvas id="realCanvas" width="680" height="460" style=" background-color: #ffffff; z-index: 0" ></canvas>
<canvas id="tempCanvas" width="680" height="460" style="z-index: 1"></canvas>
</div>



</body>
</html>

CSS

#realCanvas, #tempCanvas {
position: absolute;
left:280px;
top:50px;
border: 5px solid;
cursor: crosshair;
}



#table1{
position: absolute;
left:400px;
top:5px;
}

当我将其上传到 free server 时,虽然我有不同的错误。哦,我很困惑,我希望有人能理解这一切:/

提前致谢!

最佳答案

您在浏览器有机会解析文档正文之前导入您的脚本。因此,<canvas>在您通过“id”值查找元素时元素不存在。

由于无论如何您都在导入 jQuery,因此您可以使用“现成的”处理程序,但是您在附加事件处理程序的方式上会遇到一些问题。您的基于属性的事件处理程序依赖于那些全局的处理程序函数,如果您将它们放在“就绪”处理程序中,它们就不会是全局的。

然而,更简单的方法是简单地移动您的 <script>标记(或两者)到 <body> 的最后.这样 DOM 将被解析并且你的 getElementById()电话应该工作。很多人建议将脚本放在 <body> 的末尾无论如何都应该被视为最佳实践。

如果做不到这一点,我想你可以做这样的事情,改变脚本顶部的声明:

$(function() {
$.extend(window, {
canvas: document.getElementById("realCanvas"),
tmp_board: document.getElementById("tempCanvas"),
b_width: canvas.width, b_height = canvas.height,
ctx: canvas.getContext("2d"),
tmp_ctx: tmp_board.getContext("2d"),
x: undefined,
y: undefined,
saved: false,
hold: false,
fill: false,
stroke: true,
tool: 'rectangle',
data: {"rectangle": [], "circle": [], "line": []}
});
});

最好要么完全采用 jQuery 并使用它来分配事件处理程序,要么就不用费心导入它了。

关于javascript - 未捕获的类型错误 : Cannot read property 'width' of null,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/16509020/

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