我能够成功地在 HTML 画布上绘制图像。但我需要能够在画布上拖动图像。
我知道这个功能可以通过一些 JavaScript 库（如 KinectJS）轻松实现。但我只想用简单的 JavaScript 来做。
window.onload = function(){
var canvas = document.getElementById("myCanvas");
var context = canvas.getContext("2d");
var destX = 0;
var destY = 0;
var imageObj = new Image();
imageObj.onload = function(){
context.drawImage(imageObj, destX, destY);
};
imageObj.src = "westeros.png";
<canvas id="myCanvas" height=1024 width=360></canvas>
【问题讨论】：
要进行拖动处理 3 个鼠标事件：
mousedown -- 设置一个标志，指示拖动已经开始。
mouseup -- 清除拖动标志，因为拖动结束
mousemove -- 如果设置了拖动标志，则清除画布并在鼠标位置绘制图像
这里有一些代码：
<!doctype html>
<html>
<head>
<link rel="stylesheet" type="text/css" media="all" href="css/reset.css" /> <!-- reset css -->
<script type="text/javascript" src="http://code.jquery.com/jquery.min.js"></script>
<style>
body{ background-color: ivory; }
canvas{border:1px solid red;}
</style>
<script>
$(function(){
var img = new Image();
img.onload = function(){
ctx.drawImage(img, 0,0);
};
img.src = "http://images.christmastimeclipart.com/images/2/1271716593176_1788/img_1271716593176_17881.jpg";
var canvas=document.getElementById("canvas");
var ctx=canvas.getContext("2d");
var canvasOffset=$("#canvas").offset();
var offsetX=canvasOffset.left;
var offsetY=canvasOffset.top;
var canvasWidth=canvas.width;
var canvasHeight=canvas.height;
var isDragging=false;
function handleMouseDown(e){
canMouseX=parseInt(e.clientX-offsetX);
canMouseY=parseInt(e.clientY-offsetY);
// set the drag flag
isDragging=true;
}
function handleMouseUp(e){
canMouseX=parseInt(e.clientX-offsetX);
canMouseY=parseInt(e.clientY-offsetY);
// clear the drag flag
isDragging=false;
}
function handleMouseOut(e){
canMouseX=parseInt(e.clientX-offsetX);
canMouseY=parseInt(e.clientY-offsetY);
// user has left the canvas, so clear the drag flag
//isDragging=false;
}
function handleMouseMove(e){
canMouseX=parseInt(e.clientX-offsetX);
canMouseY=parseInt(e.clientY-offsetY);
// if the drag flag is set, clear the canvas and draw the image
if(isDragging){
ctx.clearRect(0,0,canvasWidth,canvasHeight);
ctx.drawImage(img,canMouseX-128/2,canMouseY-120/2,128,120);
}
}
$("#canvas").mousedown(function(e){handleMouseDown(e);});
$("#canvas").mousemove(function(e){handleMouseMove(e);});
$("#canvas").mouseup(function(e){handleMouseUp(e);});
$("#canvas").mouseout(function(e){handleMouseOut(e);});
}); // end $(function(){});
</script>
</head>
<body>
<canvas id="canvas" width=400 height=300></canvas>
</body>
</html>
【讨论】：
markE's answer 让我走了大部分路，但我在识别我的鼠标所在的位置时遇到了一些问题。我附上了我的代码，尽管因为我使用的是 OO JS，所以它的结构会有所不同：
hitImage: function() {
return (this.startX > this.imageX && this.startX < this.imageX + this.imageWidth && this.startY > this.imageY && this.startY < this.imageY + this.imageHeight);
},
handleMouseDown: function(e){
this.startX = parseInt(e.clientX - this.offsetX);
this.startY = parseInt(e.clientY - this.offsetY);
// set the drag flag
this.isDragging= this.hitImage;
},
handleMouseUp: function(e,img){
this.startX=parseInt(e.clientX-this.offsetX);
this.startY=parseInt(e.clientY-this.offsetY);
// clear the drag flag
this.isDragging=false;
this.drawImage(e,img);
},
handleMouseOut: function(e,img){
this.handleMouseUp(e,img);
},
handleMouseMove: function(e,img){
// only compute new positions if in drag
if(this.isDragging){
this.canMouseX=parseInt(e.clientX - this.offsetX);
this.canMouseY=parseInt(e.clientY - this.offsetY);
// move the image by the amount of the latest drag
var dx = this.canMouseX - this.startX;
var dy = this.canMouseY - this.startY;
this.imageX += dx;
this.imageY += dy;
// Negative image locations break the pattern in this.fillPattern
this.imageY = Math.max(0, this.imageY);
this.imageX = Math.max(0, this.imageX);
// reset the startXY for next time
this.startX = this.canMouseX;
this.startY = this.canMouseY;
this.drawImage(e,img);
}
【讨论】：