我在html画布上绘制了一些圆圈，我的代码看起来像这样：
HTML：
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>WebTunings</title>
<link href="index.css" rel="stylesheet">
<script src="index.js"></script>
</head>
<body>
<canvas id="canvas" width="600" height="600"></canvas>
<div id="controls">
<p><label>Fill: <input id="fillBox" type="checkbox" checked="checked"></label></p>
<div class="lightBorder">
<p><input type="radio" name="shape" value="circle" checked="checked">Circle</p>
</div>
<p><input id="clearCanvas" type="button" value="reset"></p>
</div>
</body>
</html>
使用Javascript：
var canvas,
context,
dragging = false,
dragStartLocation,
snapshot;
function getCanvasCoordinates(event) {
var x = event.clientX - canvas.getBoundingClientRect().left,
y = event.clientY - canvas.getBoundingClientRect().top;
return {x: x, y: y};
}
function takeSnapshot() {
snapshot = context.getImageData(0, 0, canvas.width, canvas.height);
}
function restoreSnapshot() {
context.putImageData(snapshot, 0, 0);
}
function drawCircle(position) {
var radius = Math.sqrt(Math.pow((dragStartLocation.x - position.x), 2) + Math.pow((dragStartLocation.y - position.y), 2));
context.beginPath();
context.arc(dragStartLocation.x, dragStartLocation.y, radius, 0, 2 * Math.PI, false);
context.fillStyle = getRndColor();
}
function draw(position) {
var fillBox = document.getElementById("fillBox"),
shape = document.querySelector('input[type="radio"][name="shape"]:checked').value;
if (shape === "circle") {
drawCircle(position);
}
if (fillBox.checked) {
context.fill();
} else {
context.stroke();
}
}
function dragStart(event) {
dragging = true;
dragStartLocation = getCanvasCoordinates(event);
takeSnapshot();
}
function drag(event) {
var position;
if (dragging === true) {
restoreSnapshot();
position = getCanvasCoordinates(event);
draw(position, "polygon");
}
}
function dragStop(event) {
dragging = false;
restoreSnapshot();
var position = getCanvasCoordinates(event);
draw(position, "polygon");
}
function getRndColor() {
var r = 255*Math.random()|0,
g = 255*Math.random()|0,
b = 255*Math.random()|0;
return 'rgb(' + r + ',' + g + ',' + b + ')';
}
function eraseCanvas(){
context.clearRect(0, 0, canvas.width, canvas.height);
}
function init() {
canvas = document.getElementById("canvas");
context = canvas.getContext('2d');
context.strokeStyle = 'green';
context.lineWidth = 4;
context.lineCap = 'round';
clearCanvas = document.getElementById("clearCanvas");
canvas.addEventListener('mousedown', dragStart, false);
canvas.addEventListener('mousemove', drag, false);
canvas.addEventListener('mouseup', dragStop, false);
clearCanvas.addEventListener("click", eraseCanvas, false);
}
window.addEventListener('load', init, false);
现在我想做的是每当我绘制多个圆圈时，我想选择一个随机圆圈并将其拖动到画布上的另一个地方，适用于每个圆圈。我只想使用html5和javascript其他第三方库。有没有办法做到这一点？任何帮助都会很棒......
答案 0 :(得分：8)
Canvas不记得＆＃34;它画了你的圆圈或矩形，所以你必须记住。这通常通过在javascript对象中定义每个圆或矩形并将所有这些形状保存在shapes []数组中来完成。
// an array of objects that define different shapes
var shapes=[];
// define 2 rectangles
shapes.push({x:10,y:100,width:30,height:30,fill:"#444444",isDragging:false});
shapes.push({x:80,y:100,width:30,height:30,fill:"#ff550d",isDragging:false});
// define 2 circles
shapes.push({x:150,y:100,r:10,fill:"#800080",isDragging:false});
shapes.push({x:200,y:100,r:10,fill:"#0c64e8",isDragging:false});
然后你可以在javascript中监听鼠标事件。当浏览器发出鼠标事件时，您可以调用函数作为响应。这是告诉您想要收听鼠标事件的浏览器：
// listen for mouse events
canvas.onmousedown = myDown;
canvas.onmouseup = myUp;
canvas.onmousemove = myMove;
您可以使用鼠标事件功能（myDown，myUp，myMove）进行拖动。
在mousedown事件（由myDown函数处理）时，您测试每个形状以查看鼠标是否在阵列中的一个形状内。如果鼠标位于1+形状内，请在这些1+形状上设置isDragging标记，并设置dragok标记以指示您需要跟踪鼠标以进行拖动。
在鼠标移动事件（由myMove函数处理）后，您可以移动任何被拖动的形状，即自上一次鼠标移动后鼠标拖动的距离。
在鼠标事件发生时（由myUp函数处理），您可以通过清除dragok标志来停止拖动操作。
以下是带注释的示例代码和演示：
// get canvas related references
var canvas=document.getElementById("canvas");
var ctx=canvas.getContext("2d");
var BB=canvas.getBoundingClientRect();
var offsetX=BB.left;
var offsetY=BB.top;
var WIDTH = canvas.width;
var HEIGHT = canvas.height;
// drag related variables
var dragok = false;
var startX;
var startY;
// an array of objects that define different shapes
var shapes=[];
// define 2 rectangles
shapes.push({x:10,y:100,width:30,height:30,fill:"#444444",isDragging:false});
shapes.push({x:80,y:100,width:30,height:30,fill:"#ff550d",isDragging:false});
// define 2 circles
shapes.push({x:150,y:100,r:10,fill:"#800080",isDragging:false});
shapes.push({x:200,y:100,r:10,fill:"#0c64e8",isDragging:false});
// listen for mouse events
canvas.onmousedown = myDown;
canvas.onmouseup = myUp;
canvas.onmousemove = myMove;
// call to draw the scene
draw();
// draw a single rect
function rect(r) {
ctx.fillStyle=r.fill;
ctx.fillRect(r.x,r.y,r.width,r.height);
}
// draw a single rect
function circle(c) {
ctx.fillStyle=c.fill;
ctx.beginPath();
ctx.arc(c.x,c.y,c.r,0,Math.PI*2);
ctx.closePath();
ctx.fill();
}
// clear the canvas
function clear() {
ctx.clearRect(0, 0, WIDTH, HEIGHT);
}
// redraw the scene
function draw() {
clear();
// redraw each shape in the shapes[] array
for(var i=0;i<shapes.length;i++){
// decide if the shape is a rect or circle
// (it's a rect if it has a width property)
if(shapes[i].width){
rect(shapes[i]);
}else{
circle(shapes[i]);
};
}
}
// handle mousedown events
function myDown(e){
// tell the browser we're handling this mouse event
e.preventDefault();
e.stopPropagation();
// get the current mouse position
var mx=parseInt(e.clientX-offsetX);
var my=parseInt(e.clientY-offsetY);
// test each shape to see if mouse is inside
dragok=false;
for(var i=0;i<shapes.length;i++){
var s=shapes[i];
// decide if the shape is a rect or circle
if(s.width){
// test if the mouse is inside this rect
if(mx>s.x && mx<s.x+s.width && my>s.y && my<s.y+s.height){
// if yes, set that rects isDragging=true
dragok=true;
s.isDragging=true;
}
}else{
var dx=s.x-mx;
var dy=s.y-my;
// test if the mouse is inside this circle
if(dx*dx+dy*dy<s.r*s.r){
dragok=true;
s.isDragging=true;
}
}
}
// save the current mouse position
startX=mx;
startY=my;
}
// handle mouseup events
function myUp(e){
// tell the browser we're handling this mouse event
e.preventDefault();
e.stopPropagation();
// clear all the dragging flags
dragok = false;
for(var i=0;i<shapes.length;i++){
shapes[i].isDragging=false;
}
}
// handle mouse moves
function myMove(e){
// if we're dragging anything...
if (dragok){
// tell the browser we're handling this mouse event
e.preventDefault();
e.stopPropagation();
// get the current mouse position
var mx=parseInt(e.clientX-offsetX);
var my=parseInt(e.clientY-offsetY);
// calculate the distance the mouse has moved
// since the last mousemove
var dx=mx-startX;
var dy=my-startY;
// move each rect that isDragging
// by the distance the mouse has moved
// since the last mousemove
for(var i=0;i<shapes.length;i++){
var s=shapes[i];
if(s.isDragging){
s.x+=dx;
s.y+=dy;
}
}
// redraw the scene with the new rect positions
draw();
// reset the starting mouse position for the next mousemove
startX=mx;
startY=my;
}
}body{ background-color: ivory; }
#canvas{border:1px solid red;}<h4>Drag one or more of the shapes</h4>
<canvas id="canvas" width=300 height=300></canvas>