使用鼠标事件在画布上绘制圆 [英] Drawing a circle on the canvas using mouse events

问题描述

我试图使用鼠标在画布上使用鼠标事件绘制一个圆，但它不绘制任何东西：

I am trying to draw a circle using mouse on the canvas using mouse events, but it does not draw anything:

tools.circle = function () {
var tool = this;
this.started = false;
this.mousedown = function (ev) {
tool.started = true;
tool.x0 = ev._x;
tool.y0 = ev._y;
ctx.moveTo(tool.x0,tool.y0);
};

this.mousemove = function (ev) {
var centerX = Math.max(tool.x0,ev._x) - Math.abs(tool.x0 - ev._x)/2;
var centerY = Math.max(tool.y0,ev._y) - Math.abs(tool.y0 - ev._y)/2;
var distance = Math.sqrt(Math.pow(tool.x0 - ev._x,2) + Math.pow(tool.y0 - ev._y));
context.circle(tool.x0, tool.y0, distance/2,0,Math.PI*2 ,true);
context.stroke();
};
};

我做错了什么？

推荐答案

好吧，这段代码片段没有告诉我们，但是代码中有一些明显的错误。
首先，DOM事件对象没有_ x 和_ y 属性。而是 clientX 和 clientY 或 pageX 和 pageY 。
要从当前事件对象获取相对鼠标坐标，您可以这样做：

Well, this code snippet doesn't tell us much, but there are a couple of obvious errors in your code. First, DOM Event object doesn't have _x and _y properties. but rather clientX and clientY or pageX and pageY. To get relative mouse coordinates from the current event object, you would do something like:

element.onclick = function(e) {
    var x = e.pageX - this.offsetLeft;
    var y = e.pageY - this.offsetTop;
}

接下来，canvas'2d上下文没有一个名为 circle ，但你可以写自己的，也许像：

Next, canvas' 2d context doesn't have a method called circle, but you could write your own, maybe something like:

var ctx = canvas.context;
ctx.fillCircle = function(x, y, radius, fillColor) {
    this.fillStyle = fillColor;
    this.beginPath();
    this.moveTo(x, y);
    this.arc(x, y, radius, 0, Math.PI*2, false);
    this.fill();
}

无论如何，这里有一个测试html页面来测试这个： http://jsfiddle.net/ArtBIT/kneDX/

我希望这有帮助。
Cheers

I hope this helps. Cheers

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