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javascript - 如何在 HTML5 中使绘图在 Canvas 上可拖动

转载 作者：行者123 更新时间：2023-11-29 18:15:55 24 4
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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只使用普通的 javascript 来做到这一点。

我有一个 Canvas ，当在某个地方点击 Canvas 时，用户点击的地方会出现一个小红点。我想让每个小点都可以拖动，有什么想法吗？谢谢。

这是我的代码:

在 HTML 中:

<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>

在 JS 文件中:

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();
}

最佳答案

概述您的解决方案:

  • 监听 mousedown 事件，并且 (1) 如果鼠标不在圆上则创建一个新圆，或者 (2) 如果鼠标在圆上则在圆上开始拖动操作。
  • 监听 mousemove 事件并将拖动的圆圈移动自上次 mousemove 事件以来鼠标移动的距离
  • 监听mouseup事件并停止拖动操作

这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>

关于javascript - 如何在 HTML5 中使绘图在 Canvas 上可拖动，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/23451267/

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