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[英]why i am getting Cannot read property 'addEventListener' of null javascript
[英]Why do I keep getting “Cannot call method 'addEventListener' of null”?
我對webdev非常缺乏經驗,但是在這里我正在制作一個網站。 我使用了Fabric.js和來自stackoverflow的答案中的腳本,以便用戶可以將圖像從一個div拖動到畫布中。 當我對其進行測試時,我總是收到錯誤“無法調用null的方法'addEventListener'”。 我想這意味着無論腳本嘗試加載什么,都沒有加載,因此將其保留為null。 控制台從我的JS文件中的第77行(canvasContainer.addEventListener('dragenter'...,靠近底部)說出它的位置。但還是不行,有什么想法嗎?
PS。 JavaScript來自natchiketa。
HTML:
<!doctype html>
<html lang="en";>
<head>
<meta charset="utf-8" />
<title>CustomCase</title>
<link rel="stylesheet" href="SkapaDesign.css">
<script src="Jquery.js"></script>
<script src="Modernizr.js"></script>
<script src="Fabric.js"></script>
<script src="Canvas.js"></script>
</head>
<body>
<div id="Wrapper">
<section id="Body">
<div id="CanvasDiv">
<canvas id="Canvas" width="270px" height="519px"></canvas>
</div>
<div id="TextureView">
<div id="TextureViewInside">
<ul class="products">
<img draggable="true" src="FärgadePapper.png">
<img draggable="true" src="Hajar.png">
<img draggable="true" src="Labyrint.png">
<img draggable="true" src="Martini.png">
</ul>
</div>
</div>
</section>
</div>
</body>
</html>
CSS:
#Body{
height: 675px;
}
#Canvas, #TextureView{
float: left;
}
#LeftColumn{
float: left;
width: 355px;
margin: 20px 30px 0px 0px;
}
#RightColumn{
float: left;
width: 600px;
margin-top: 20px;
}
#CanvasDiv{
width: 270px;
height: 519px;
margin-left: 15px;
}
#Canvas{
overflow: hidden;
}
#TextureView{
width: 470px;
height: 559px;
overflow: auto;
}
#TextureViewInside{
padding: 10px;
}
ul.products img{
width: 97px;
display: inline-block;
vertical-align: top;
padding: 4px;
}
JavaScript的:
var canvas = new fabric.Canvas('Canvas');
function handleDragStart(e) {
[].forEach.call(images, function (img) {
img.classList.remove('img_dragging');
});
this.classList.add('img_dragging');
}
function handleDragOver(e) {
if (e.preventDefault) {
e.preventDefault();
}
e.dataTransfer.dropEffect = 'copy';
return false;
}
function handleDragEnter(e) {
this.classList.add('over');
}
function handleDragLeave(e) {
this.classList.remove('over');
}
function handleDrop(e) {
if (e.stopPropagation) {
e.stopPropagation();
}
var img = document.querySelector('#images img.img_dragging');
console.log('event: ', e);
var newImage = new fabric.Image(img, {
width: img.width,
height: img.height,
left: e.layerX,
top: e.layerY
});
canvas.add(newImage);
return false;
}
function handleDragEnd(e) {
[].forEach.call(images, function (img) {
img.classList.remove('img_dragging');
});
}
if (Modernizr.draganddrop) {
var images = document.querySelectorAll('#images img');
[].forEach.call(images, function (img) {
img.addEventListener('dragstart', handleDragStart, false);
img.addEventListener('dragend', handleDragEnd, false);
});
var canvasContainer = document.getElementById('CanvasDiv');
canvasContainer.addEventListener('dragenter', handleDragEnter, false);
canvasContainer.addEventListener('dragover', handleDragOver, false);
canvasContainer.addEventListener('dragleave', handleDragLeave, false);
canvasContainer.addEventListener('drop', handleDrop, false);
} else {
alert("This browser doesn't support the HTML5 Drag and Drop API.");
}
看起來您只需要將部分代碼包裝在document.ready處理程序中,就像這樣……
$(function() {
if (Modernizr.draganddrop) {
var images = document.querySelectorAll('#images img');
[].forEach.call(images, function (img) {
img.addEventListener('dragstart', handleDragStart, false);
img.addEventListener('dragend', handleDragEnd, false);
});
var canvasContainer = document.getElementById('CanvasDiv');
canvasContainer.addEventListener('dragenter', handleDragEnter, false);
canvasContainer.addEventListener('dragover', handleDragOver, false);
canvasContainer.addEventListener('dragleave', handleDragLeave, false);
canvasContainer.addEventListener('drop', handleDrop, false);
} else {
alert("This browser doesn't support the HTML5 Drag and Drop API.");
}
});
document.getElementById("CanvasDiv")
不會返回任何元素,因為當時尚未加載文檔,但是接下來的幾行期望那里會有東西。 將其包裝在document.ready事件處理程序中應該可以解決該問題,因為在您嘗試引用它時將加載DOM。
在加載文檔之前,您無法獲取元素...
window.addEventListener('loaded', function(){
if (Modernizr.draganddrop) {
var images = document.querySelectorAll('#images img');
[].forEach.call(images, function (img) {
img.addEventListener('dragstart', handleDragStart, false);
img.addEventListener('dragend', handleDragEnd, false);
});
var canvasContainer = document.getElementById('CanvasDiv');
canvasContainer.addEventListener('dragenter', handleDragEnter, false);
canvasContainer.addEventListener('dragover', handleDragOver, false);
canvasContainer.addEventListener('dragleave', handleDragLeave, false);
canvasContainer.addEventListener('drop', handleDrop, false);
} else {
alert("This browser doesn't support the HTML5 Drag and Drop API.");
}
});
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