实际上我不知道发生了什么,因为它昨晚起作用了。无论如何,我正在尝试用html5和javascript做一个绘图应用程序。这是我第一次正确地看着JS并用它创建了一些东西,所以我从各种教程和其他朋友的帮助下获得了我的代码,也就是我是一个n00b。我使用了以下代码:
当我在chrome中测试我的工作时,我得到了这个错误
“未捕获宽度:无法读取null的属性‘TypeError’”
它引用了我的脚本文件的第4行,即
var b_width = canvas.width, b_height = canvas.height;
我试图摆弄它来添加一个文本功能,但就是不能让它工作,所以我只是撤销了所有的东西,现在它产生了这个错误。
完整的脚本:
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;
}
当我把它上传到free server时,我得到了不同的错误。哦,我太困惑了,我希望有个人能明白这一切:/
提前感谢!
发布于 2013-05-12 23:39:57
在浏览器有机会解析文档正文之前,您正在导入脚本。因此,在您通过"id“值查找<canvas>
元素时,它并不存在。
因为无论如何都要导入jQuery,所以可以使用一个“就绪”处理程序,但是在附加事件处理程序的方式上会有一些问题。您的基于属性的事件处理程序依赖于这些处理程序的全局函数,如果您将它们放在一个“就绪”处理程序中,它们就不会是全局的。
然而,简单得多的方法是简单地将<script>
标记(或两者)移到<body>
的末尾。这样,DOM就会被解析,您的getElementById()
调用就会正常工作。许多人建议将脚本放在<body>
的末尾无论如何都应该被认为是最佳实践。
如果做不到这一点,我想您可以这样做,更改脚本顶部的声明:
$(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": []}
});
});
最好是完全采用jQuery并使用它来分配事件处理程序,或者干脆不麻烦地导入它。
发布于 2018-08-27 02:01:54
您还可以在脚本元素上添加defer属性。它指示在页面加载之前不执行script标记的内容。
所以你会得到类似这样的东西:
<script src="myscript.js" defer></script>
https://stackoverflow.com/questions/16509020
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