canvas里图像拖拽操作

首先想到根据在canvas上鼠标移动，然后再重新画图。但无法确定鼠标前后两次移动的距离，所以无法准确确定图像位置。

而后再根据网上的例子，定义一个div，将div覆盖在图像之上，在移动div的同时，将坐标传给canvas，重新绘制图像。

同时需要熟悉javascript各种坐标

canvas和div标签

<canvas id="myCanvas" width="500" height="400" style="border: 1px solid #c3c3c3;"></canvas>
<div id="cover" style="left: 100px; top: 100px">
</div>

css样式

 body {
            margin: 0;
            padding: 0;
        }

        div {
            border: solid 1px red;
            position: absolute;
            
        }

        canvas,div {
            position: absolute;
            left: 50px;
            top: 50px;
        }

同时注意将javascript代码写在html标签之后

 var canvas = document.getElementById('myCanvas');
        var ctx = canvas.getContext("2d");
        var img = new Image();
        img.src = "Image/jugg.jpg";
        ctx.drawImage(img, 50, 50);
        var divObj = document.getElementById('cover');
        divObj.style.width = img.width + 'px';
        divObj.style.height = img.height + 'px';

        var x = 0;
        var y = 0;

        var moveFlag = false;
        var clickFlag = false;

        divObj.onmousedown = function (e) {
            moveFlag = true;
            clickFlag = true;
            var mWidth = e.clientX - this.offsetLeft;
            var mHeight = e.clientY - this.offsetTop;
           
            document.onmousemove = function (e) {
                clickFlag = false;
                if (moveFlag) {
                    divObj.style.left = e.clientX - mWidth + 'px';
                    divObj.style.top = e.clientY - mHeight + 'px';
                    x = e.clientX - mWidth - canvas.offsetLeft;
                    y = e.clientY - mHeight - canvas.offsetTop;
                    if (e.clientX <= mWidth + canvas.offsetLeft) {
                        divObj.style.left = canvas.offsetLeft + 'px';
                        x = 0;
                    }
                    if (parseInt(divObj.style.left) + divObj.offsetWidth >= canvas.width + canvas.offsetLeft) {
                        divObj.style.left = canvas.width - divObj.offsetWidth + canvas.offsetLeft + "px";
                        x = canvas.width - divObj.offsetWidth;
                    }
                    if (e.clientY <= mHeight + canvas.offsetTop) {
                        divObj.style.top = canvas.offsetTop + "px";
                        y = 0;
                    }
                    if (parseInt(divObj.style.top) + divObj.offsetHeight >= canvas.height + canvas.offsetTop) {
                        divObj.style.top = canvas.height - divObj.offsetHeight + canvas.offsetTop + "px";
                        y = canvas.height - divObj.offsetHeight;
                    }
                    drawImg();

                    divObj.onmouseup = function () {
                        moveFlag = false;
                    }

                }
            }
        }

        function drawImg() {
            ctx.clearRect(0, 0, 500, 400);
            ctx.drawImage(img, x, y);
            ctx.stroke();
        }