上传图片文件并立即显示到页面使用 javascript实现鼠标拖动画矩形框以及实现固定区域内随意拖动

首先，你要设计好鼠标事件处理方法，主要是鼠标左键点击，左键释放，还有鼠标移动方法
其次，要了解容什么方式，画一个矩形，设计一个方法：DrawRectgle(左上角，右下角），并且要确定当调用这个方法时，要把原来已经画好的矩形清除掉（或者是根据左上，右下坐标，调整矩形的大小，这样的话，就不用清除原有的矩形）

在鼠标左键按下事件中，获取鼠标位置，保存为左上角坐标，设置开始画矩形的一个标志
在鼠标左键弹起事件中，恢复划矩形的标志为false
在鼠标移动事件中，需要判断是否同时还按下了鼠标左键，如果按下了，并且画矩形标志为true,就获取鼠标位置，保存为右下角坐标（针对鼠标向左上移动的场合，要把左上，右下位置交换一下），然后画一个矩形

// css部分
#canvas {
        background-color: #AAAAAA;
        position: relative;
        background-size:100% 100%;
        border: 2px solid blue;
    }

#canvas>div {
        border: 2px solid green;
        position: absolute;
        background-color: #eaeaea;
    }
#canvas>div>span {
        position: absolute;
        top: 50%;
        left: 50%;
        transform: translate(-50%, -50%);
        font-family: simsun;
        font-size: 9pt;
    }

// html部分
<div id="canvas" style="width:850px;height:477px"></div>

// js部分
var canvasWidth= 850;
var canvasHeight = 477;
var allDivNum = 1;
CanvasExt = {
        drawRect: function(canvas) {//参数canvas--所画素材框的区域
            var that = this;

            // canvas 的矩形框
            var canvasRect = canvas.getBoundingClientRect();
            // 矩形框的左上角坐标
            var canvasLeft = canvasRect.left;
            var canvasTop = canvasRect.top;
            var x = 0;
            var y = 0;

            // 鼠标点击按下事件,画图准备
            $(document).on('mousedown', '#canvas', function(e) {
                // 解决修改时已存在素材框等，删除后再新增存在id重复问题
                var date = new Date().getTime();
                var id = "newDiv" + date + parseInt(Math.random()*10);

                var divEle = "";
                // x y为鼠标的落点
                x = e.clientX - canvasLeft;
                y = e.clientY - canvasTop;

                //创建div
                divEle = document.createElement('div');
                divEle.setAttribute("id", id);

                canvas.append(divEle)
                divEle.style.top = y + "px";
                divEle.style.left = x + "px";

                // 绑定删除操作
                var menu = new BootstrapMenu('#' + id, {
                    actions: [{
                        name: '删除展示框',
                        onClick: function() {
                            del(id);
                        }
                    }]
                });
                var tx = 0;
                var ty = 0;
                var twidth = 0;
                var theight = 0;

                // 添加拖拽操作
                divEle.onmousedown = function(e) {
                    e.stopPropagation();// 阻止时间冒泡
                    var divEleRect = this.getBoundingClientRect();
                    var divEleLeft = e.clientX - divEleRect.left;
                    var divEleTop = e.clientY - divEleRect.top;

                    this.onmousemove = function(e) {
                        e.stopPropagation();
                        tx = e.clientX - canvasLeft - divEleLeft;
                        ty = e.clientY - canvasTop - divEleTop;
                        // 重新获取当前对象的宽和高
                        twidth = document.getElementById(id).style.width;
                        twidth = parseInt(twidth);
                        theight = document.getElementById(id).style.height;
                        theight = parseInt(theight);
                        // 边界检测
                        if(tx <= 0) {
                            tx = 0;
                            this.style.left = 0 + "px";
                        } else if(tx + twidth > canvasWidth) {
                            tx = canvasWidth - twidth;
                            this.style.left = (canvasWidth - twidth) + "px";
                        } else {
                            this.style.left = tx + "px";
                        }
                        if(ty <= 0) {
                            ty = 0;
                            this.style.top = 0 + "px";
                        } else if((ty + theight) > canvasHeight) {
                            y = canvasHeight - theight;
                            this.style.top = (canvasHeight - theight) + "px";
                        } else {
                            this.style.top = ty + "px";
                        }
                    }
                    this.onmouseup = function(e) {
                        var id = $(this).attr("id");
                        e.stopPropagation();
                        this.onmousemove = null;
                    }
                };

                //鼠标移动事件，画图
                var width = 0;
                var height = 0;
                canvas.onmousemove = function(e) {
                    e.stopPropagation();
                    // width height是鼠标移动末尾距鼠标起始位置的差值
                    width = e.clientX - canvasLeft - x;
                    height = e.clientY - canvasTop - y;
                    divEle.style.width = width + "px";
                    divEle.style.height = height + "px";

                    var tw = e.clientX - canvasLeft;
                    var th = e.clientY - canvasTop;
                    if(tw >= (parseInt(canvasWidth)-7) || th >=
                             (parseInt(canvasHeight)-7)){ // 比canvas少3px,为了易于判断

                            allDivNum++;
                            divEle.innerHTML = "<span>素材框" + allDivNum + "</span>";
                        canvas.onmousemove = null;
                        canvas.onmouseup = null;
                        return;
                    }
                }
                canvas.onmouseup = function(e) {
                    if(width < 10 || height < 10) {
                        canvas.removeChild(divEle);
                    } else {
                        allDivNum++;
                        divEle.innerHTML = "<span>素材框" + allDivNum + "</span>";
                    }
                    e.stopPropagation();
                    canvas.onmousemove = null;
                }
            });

        }
    };
window.onload = function() {
        var canvas = document.getElementById("canvas");
        CanvasExt.drawRect(canvas);
        // 取消默认右击事件
        document.oncontextmenu = function(e) {
            e.preventDefault();
        }
    };

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版权声明：本文为CSDN博主「tangdou369098655」的原创文章，遵循 CC 4.0 BY-SA 版权协议，转载请附上原文出处链接及本声明。
原文链接：https://blog.csdn.net/tangdou369098655/article/details/100677663