如何使放置的图像在 canvas 内可拖动？

Question

**我正在使用这段代码，我怎样才能使拖放的图像成为可拖动的。我试过 jquery 但我需要在 canvas 中拖动相同的内容。开导我。 **

var canvas = document.getElementById("canvas");
var ctx = canvas.getContext("2d");
var $canvas = $("#canvas");
var canvasOffset = $canvas.offset();
var offsetX = canvasOffset.left;
var offsetY = canvasOffset.top;

var image1 = new Image();
image1.src = "https://dl.dropboxusercontent.com/u/139992952/Whosebug/house32x32transparent.png";

var $house = $("#house");
var $canvas = $("#canvas");

$house.draggable({
    helper: 'clone',
});
// set the data payload
$house.data("image", image1); // key-value pair

$canvas.droppable({
    drop: dragDrop,
});


function dragDrop(e, ui) {
    var element = ui.draggable;
    var data = element.data("url");
    var x = parseInt(ui.offset.left - offsetX);
    var y = parseInt(ui.offset.top - offsetY);
    ctx.drawImage(element.data("image"), x - 1, y);
}

Answer 1

一旦您将图像绘制到 canvas 上，房子就会在 canvas 上变成不可记忆的、不可移动的像素。不能再用jQuery拖房子了

Canvas "moves" 通过擦除 canvas 并在改变的位置重新绘制内容。

Canvas "drags" 通过监听鼠标事件：

In mousedown，检查鼠标是否在 canvas 上绘制的任何房屋上。为此，您要记住房屋的绘制位置，然后使用鼠标位置使用房屋的边界框进行命中测试。设置一个 isDragging 标志以指示房子被鼠标击中并应被拖动。
In mousemove，重新绘制自上次鼠标移动后鼠标移动距离的房子。
In mouseup & mouseout，拖动操作结束，清除isDragging标志。

要创建一个边界框来进行命中测试，您至少需要跟踪房屋图像的宽度和高度以及它在 canvas 上放置位置的 x、y 坐标。您通常将此信息保存在 javascript 对象中。

这是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/Whosebug/house204-1.jpg');
addImage(240,20,0.50,'https://dl.dropboxusercontent.com/u/139992952/Whosebug/house204-2.jpg');
addImage(20,220,0.50,'https://dl.dropboxusercontent.com/u/139992952/Whosebug/house204-3.jpg');
addImage(240,220,0.50,'https://dl.dropboxusercontent.com/u/139992952/Whosebug/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 a house with the mouse.</h4>
<canvas id="myCanvas" width=400 height=400></canvas>

如何使放置的图像在 canvas 内可拖动？

How can make a dropped image draggable within the canvas ?

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jquery

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