Question

说我想在画布上画一个矩形。我希望能够从用户的鼠标中获得坐标。理想情况是用户点击一点并拖动到另一端，就像我们使用油漆绘制的那些矩形一样。如何通过拖动鼠标绘制像我们在绘画中的矩形？（当他点击鼠标并离开时，如何获得鼠标的坐标？）

Answer 1

以下是如何在画布上拖动绘制矩形的大纲：

在mousedown：

保存起始鼠标位置
设置一个标志，指示拖动已经开始

在mousemove中：

清除前一个矩形的画布
根据起始与当前鼠标位置计算矩形宽度/高度
从起始XY到当前鼠标位置绘制一个矩形

在mouseup中：

清除拖动标记，因为拖动已结束

以下是示例代码和演示：http://jsfiddle.net/m1erickson/6E2yd/

<!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(){

    // get references to the canvas and context
    var canvas=document.getElementById("canvas");
    var ctx=canvas.getContext("2d");

    // style the context
    ctx.strokeStyle = "blue";
    ctx.lineWidth=3;

    // calculate where the canvas is on the window
    // (used to help calculate mouseX/mouseY)
    var $canvas=$("#canvas");
    var canvasOffset=$canvas.offset();
    var offsetX=canvasOffset.left;
    var offsetY=canvasOffset.top;
    var scrollX=$canvas.scrollLeft();
    var scrollY=$canvas.scrollTop();

    // this flage is true when the user is dragging the mouse
    var isDown=false;

    // these vars will hold the starting mouse position
    var startX;
    var startY;


    function handleMouseDown(e){
      e.preventDefault();
      e.stopPropagation();

      // save the starting x/y of the rectangle
      startX=parseInt(e.clientX-offsetX);
      startY=parseInt(e.clientY-offsetY);

      // set a flag indicating the drag has begun
      isDown=true;
    }

    function handleMouseUp(e){
      e.preventDefault();
      e.stopPropagation();

      // the drag is over, clear the dragging flag
      isDown=false;
    }

    function handleMouseOut(e){
      e.preventDefault();
      e.stopPropagation();

      // the drag is over, clear the dragging flag
      isDown=false;
    }

    function handleMouseMove(e){
      e.preventDefault();
      e.stopPropagation();

      // if we're not dragging, just return
      if(!isDown){return;}

      // get the current mouse position
      mouseX=parseInt(e.clientX-offsetX);
      mouseY=parseInt(e.clientY-offsetY);

      // Put your mousemove stuff here

      // clear the canvas
      ctx.clearRect(0,0,canvas.width,canvas.height);

      // calculate the rectangle width/height based
      // on starting vs current mouse position
      var width=mouseX-startX;
      var height=mouseY-startY;

      // draw a new rect from the start position 
      // to the current mouse position
      ctx.strokeRect(startX,startY,width,height);

    }

    // 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){handleMouseOut(e);});

}); // end $(function(){});
</script>
</head>
<body>
    <h4>Drag the mouse to create a rectangle</h4>
    <canvas id="canvas" width=300 height=300></canvas>
</body>
</html>

清除

Answer 2

看到这一点，将鼠标移到方块上，并见证Process.js的精彩

http://processingjs.org/learning/topic/pattern/

Answer 3

使用此功能可以获得鼠标坐标

  function getMousePos(canvas, evt) {
    var rect = canvas.getBoundingClientRect();
    return {
      x: evt.clientX - rect.left,
      y: evt.clientY - rect.top
    };
  }

此函数接受canvas对象和事件。现在你只需要在mousedown和mouseup上添加一个eventHandler，你就可以获得这两个位置。

var canvas = document.getElementById('canvasId');
var ctx = canvas.getContext('2d');

var locA, locB;

document.addEventListener('mousedown', function(e) {
     e.preventDefault();
     locA = getMousePos(canvas, e);
});

document.addEventListener('mouseup', function(e) {
     e.preventDefault();
     locB = getMousePos(canvas, e);

     ctx.fillStyle = '#000000';
     ctx.fillRect(locA.x, locA.y, (locB.x - locA.x), (locB.y - locA.y));
});

功能来源：http://www.html5canvastutorials.com/advanced/html5-canvas-mouse-coordinates/

围绕画布坐标与文档坐标仍然存在一些问题，但我相信你能够解决这个问题。

如何在画布上绘制一个矩形，就像我们在油漆上画一样？

3 个答案: