var img1 = new Image();
var img2 = new Image();
var img3 = new Image();
var img4 = new Image();
var imageCount = 0, NUM_IMAGES=4;
var ctx;
var WIDTH;
var HEIGHT;
//drag
var draggable = false;
var startX;
var startY;
//array of images
var images=[];
images.push({image:img1, x:0, y:20, width:550, height:466});
images.push({image:img2, x:476, y:170, width:230, height:240});
images.push({image:img3, x:500, y:20, width:635, height:313});
images.push({image:img4, x:910, y:328, width:219, height:134});
function init(){
ctx = $('#myCanvas')[0].getContext("2d");
WIDTH = $("#myCanvas").width();
HEIGHT = $("#myCanvas").height();
ctx.fillStyle = "#A0DCE5";
ctx.fillRect(0,0,WIDTH,HEIGHT);
loadResources();
$('#myCanvas').mousedown(onMouseDown);
$('#myCanvas').mouseup(onMouseUp);
$('#myCanvas').mousemove(onMouseMove);
}
function startInteraction() {
imageCount++;
if (imageCount ==NUM_IMAGES)
render();
}
function loadResources() {
img1.onload = startInteraction;
img1.src = "america.png";
img2.onload = startInteraction;
img2.src = "africa.png";
img3.onload = startInteraction;
img3.src = "europe and asia.png";
img4.onload = startInteraction;
img4.src = "australia.png";
}
function image(image,x,y,width,height) {
ctx.drawImage(image,x,y,width,height)
}
function render() {
for(var i=0;i<images.length;i++){
var r=images[i];
image(r.image,r.x,r.y,r.width,r.height);
}
}
function onMouseDown(e){
// tell browser we're handling this mouse event
e.preventDefault();
e.stopPropagation();
//get current position
var mx=parseInt(e.clientX-$('#myCanvas').offset().left);
var my=parseInt(e.clientY-$('#myCanvas').offset().top);
//test to see if mouse is in image
d = false;
for(var i=0;i<images.length;i++){
var r=images[i];
if(mx>r.x && mx<r.x+r.width && my>r.y && my<r.y+r.height){
//if true set is dragging=true
draggable=true;
}
}
//save mouse position
startX=mx;
startY=my;
}
function onMouseUp(e){
//tell browser we're handling this mouse event
e.preventDefault();
e.stopPropagation();
draggable = false;
for(var i=0;i<images.length;i++){
images[i].isDragging=false;
}
}
function onMouseMove(e){
//if we can drag an image
if(draggable){
//tell brower we're handling this mouse event
e.preventDefault
e.stopPropagation
//get current mouseposition
var mx = parseInt(e.clientX-$('#myCanvas').offset().left);
var my = parseInt(e.clientY-$('#myCanvas').offset().top);
//calculate how far the mouse has moved;
var dx = mx - startX;
var dy = my - startY;
//move each image by how far the mouse moved
for(var i=0;i<images.length;i++){
var r=images[i];
if(r.isDragging){
r.x+=dx;
r.y+=dy;
}
}
//re render the images
render();
//reset the mouse positions for next mouse move;
startX = mx;
startY = my;
}
}
$(document).ready(init);
我在尝试移动图像时遇到了真正的困难，我看到的每个教程我都能理解它。
我做错了有什么令人目眩的事吗？
答案 0 :(得分：3)
在画布上拖动1个以上的图像时，过程为：
在mousedown：
在mousemove中：
在mouseup和mouseout中：
这里带注释的代码和演示：
// canvas related stuff
var canvas=document.getElementById("myCanvas");
var ctx=canvas.getContext("2d");
var WIDTH = canvas.width;
var HEIGHT = canvas.height;
ctx.fillStyle = "#A0DCE5";
$myCanvas=$('#myCanvas');
//drag
var isDragging = false;
var startX;
var startY;
//array of image objects
var images=[];
var NUM_IMAGES=0;
// queue up 4 test images
addImage(20,20,0.50,'https://dl.dropboxusercontent.com/u/139992952/stackoverflow/house204-1.jpg');
addImage(240,20,0.50,'https://dl.dropboxusercontent.com/u/139992952/stackoverflow/house204-2.jpg');
addImage(20,220,0.50,'https://dl.dropboxusercontent.com/u/139992952/stackoverflow/house204-3.jpg');
addImage(240,220,0.50,'https://dl.dropboxusercontent.com/u/139992952/stackoverflow/house204-4.jpg');
// trigger all images to load
for(var i=0;i<images.length;i++){
images[i].image.src=images[i].url;
}
//////////////////////////////
// functions
//////////////////////////////
// queue up another image
function addImage(x,y,scaleFactor,imgURL){
var img=new Image();
img.crossOrigin='anonymous';
img.onload=startInteraction;
images.push({image:img,x:x,y:y,scale:scaleFactor,isDragging:false,url:imgURL});
NUM_IMAGES++;
}
// called after each image fully loads
function startInteraction() {
// return until all images are loaded
if(--NUM_IMAGES>0){return;}
// set all images width/height
for(var i=0;i<images.length;i++){
var img=images[i];
img.width=img.image.width*img.scale;
img.height=img.image.height*img.scale;
}
// render all images
renderAll();
// listen for mouse events
$myCanvas.mousedown(onMouseDown);
$myCanvas.mouseup(onMouseUp);
$myCanvas.mouseout(onMouseUp);
$myCanvas.mousemove(onMouseMove);
}
// flood fill canvas and
// redraw all images in their assigned positions
function renderAll() {
ctx.fillRect(0,0,WIDTH,HEIGHT);
for(var i=0;i<images.length;i++){
var r=images[i];
ctx.drawImage(r.image,r.x,r.y,r.width,r.height);
}
}
// handle mousedown events
function onMouseDown(e){
// tell browser we're handling this mouse event
e.preventDefault();
e.stopPropagation();
//get current position
var mx=parseInt(e.clientX-$myCanvas.offset().left);
var my=parseInt(e.clientY-$myCanvas.offset().top);
//test to see if mouse is in 1+ images
isDragging = false;
for(var i=0;i<images.length;i++){
var r=images[i];
if(mx>r.x && mx<r.x+r.width && my>r.y && my<r.y+r.height){
//if true set r.isDragging=true
r.isDragging=true;
isDragging=true;
}
}
//save mouse position
startX=mx;
startY=my;
}
// handle mouseup and mouseout events
function onMouseUp(e){
//tell browser we're handling this mouse event
e.preventDefault();
e.stopPropagation();
// clear all the dragging flags
isDragging = false;
for(var i=0;i<images.length;i++){
images[i].isDragging=false;
}
}
// handle mousemove events
function onMouseMove(e){
// do nothing if we're not dragging
if(!isDragging){return;}
//tell browser we're handling this mouse event
e.preventDefault
e.stopPropagation
//get current mouseposition
var mx = parseInt(e.clientX-$myCanvas.offset().left);
var my = parseInt(e.clientY-$myCanvas.offset().top);
//calculate how far the mouse has moved;
var dx = mx - startX;
var dy = my - startY;
//move each image by how far the mouse moved
for(var i=0;i<images.length;i++){
var r=images[i];
if(r.isDragging){
r.x+=dx;
r.y+=dy;
}
}
//reset the mouse positions for next mouse move;
startX = mx;
startY = my;
//re render the images
renderAll();
}body{ background-color: ivory; }
#canvas{border:1px solid red;}<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.9.1/jquery.min.js"></script>
<h4>Drag the images.</h4>
<canvas id="myCanvas" width=400 height=400></canvas>