如何在HTML5的画布上拖动绘图

How to make drawing draggable on canvas in HTML5

本文关键字：拖动 绘图 HTML5      更新时间：2023-09-26

我是HTML5和JavaScript的新手，我知道有很多库可以做到这一点，但我想知道是否有一种方法可以使用纯JavaScript。

我有一个画布，当画布被点击的时候，会有一个小红点出现在用户点击的地方。我想让每个点都是可拖拽的，有什么想法吗?谢谢。

下面是我的代码:

HTML: 

<div id="primal_plane" style="position:absolute; top:100px; left:10px;">
    <canvas id="canvas_prime" onclick="drawDot('canvas_prime')" width="700" height="500"></canvas>
</div>

:

function drawDot(plane) {
    var canvas = document.getElementById(plane);
    var context = canvas.getContext("2d");
    context.beginPath();
    context.arc(mouseX * 50, mouseY * 50, 4, 0, 2 * Math.PI, false);
    context.fillStyle = 'green';
    context.fill();
    context.lineWidth = 1;
    context.strokeStyle = 'yellow';
    context.stroke();
}

概述您的解决方案:

  • 监听鼠标按下事件，然后(1)如果鼠标不在圆圈上，则创建一个新的圆圈;(2)如果鼠标在圆圈上，则在圆圈上开始拖动操作。
  • 监听鼠标移动事件，并按鼠标自上次鼠标移动事件以来移动的距离移动所拖动的圆圈
  • 监听鼠标上升事件并停止拖动操作
下面是带注释的代码和演示:http://jsfiddle.net/m1erickson/ytUhL/ 
<!doctype html>
<html>
<head>
<link rel="stylesheet" type="text/css" media="all" href="css/reset.css" /> 
<!-- reset css -->
<script type="text/javascript" src="http://code.jquery.com/jquery.min.js">  </script>
<style>
    body{ background-color: ivory; }
    #canvas{border:1px solid red;}
</style>
<script>
    $(function() {
        // canvas related variables
        // references to canvas and its context and its position on the page
        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 scrollX = $canvas.scrollLeft();
        var scrollY = $canvas.scrollTop();
        var cw = canvas.width;
        var ch = canvas.height;
        // flag to indicate a drag is in process
        // and the last XY position that has already been processed
        var isDown = false;
        var lastX;
        var lastY;
        // the radian value of a full circle is used often, cache it
        var PI2 = Math.PI * 2;
        // variables relating to existing circles
        var circles = [];
        var stdRadius = 10;
        var draggingCircle = -1;
        // clear the canvas and redraw all existing circles
        function drawAll() {
            ctx.clearRect(0, 0, cw, ch);
            for(var i=0; i<circles.length; i++){
                var circle = circles[i];
                ctx.beginPath();
                ctx.arc(circle.x, circle.y, circle.radius, 0, PI2);
                ctx.closePath();
                ctx.fillStyle = circle.color;
                ctx.fill();
            }
        }
        function handleMouseDown(e) {
            // tell the browser we'll handle this event
            e.preventDefault();
            e.stopPropagation();
            // save the mouse position
            // in case this becomes a drag operation
            lastX = parseInt(e.clientX - offsetX);
            lastY = parseInt(e.clientY - offsetY);
            // hit test all existing circles
            var hit = -1;
            for (var i=0; i < circles.length; i++) {
                var circle = circles[i];
                var dx = lastX - circle.x;
                var dy = lastY - circle.y;
                if (dx*dx + dy*dy < circle.radius * circle.radius) {
                    hit = i;
                }
            }
            // if no hits then add a circle
            // if hit then set the isDown flag to start a drag
            if (hit < 0) {
                circles.push({x:lastX, y:lastY, radius:stdRadius, color:randomColor()});
                drawAll();
            } else {
                draggingCircle = circles[hit];
                isDown = true;
            }
        }
        function handleMouseUp(e) {
            // tell the browser we'll handle this event
            e.preventDefault();
            e.stopPropagation();
            // stop the drag
            isDown = false;
        }
        function handleMouseMove(e) {
            // if we're not dragging, just exit
            if (!isDown) { return; }
            // tell the browser we'll handle this event
            e.preventDefault();
            e.stopPropagation();
            // get the current mouse position
            mouseX = parseInt(e.clientX - offsetX);
            mouseY = parseInt(e.clientY - offsetY);
            // calculate how far the mouse has moved
            // since the last mousemove event was processed
            var dx = mouseX - lastX;
            var dy = mouseY - lastY;
            // reset the lastX/Y to the current mouse position
            lastX = mouseX;
            lastY = mouseY;
            // change the target circles position by the 
            // distance the mouse has moved since the last
            // mousemove event
            draggingCircle.x += dx;
            draggingCircle.y += dy;
            // redraw all the circles
            drawAll();
        }
        // listen for mouse events
        $("#canvas").mousedown(function(e) { handleMouseDown(e); });
        $("#canvas").mousemove(function(e) { handleMouseMove(e); });
        $("#canvas").mouseup(function(e) { handleMouseUp(e); });
        $("#canvas").mouseout(function(e) { handleMouseUp(e); });
        //////////////////////
        // Utility functions
        function randomColor(){ 
            return('#' + Math.floor(Math.random()*16777215).toString(16));
        }

    }); // end $(function(){});
</script>
</head>
<body>
    <canvas id="canvas" width=300 height=300></canvas>
</body>
</html>

您必须存储创建的点的列表，但是您必须检查点是否已经存在。如果是这样，则设置一个布尔标志，用于标识发生mousedown事件时的拖动行为，另一个标志用于标识被拖动的点。在mouseup事件中清除标志。mousemove方法应该更新存储当前鼠标位置的变量，并更新当前被拖动点的位置。使用jQuery .

相关文章：