如何利用canvas实现按住鼠标移动绘制出轨迹(2)-木庄网络博客

6、移动鼠标的时候，没有鼠标长按事件，又该怎么监听呢？

这里用到的小技巧是在onmousedown内部再执行一个onmousemove（鼠标移动）事件，这样就能监听按住鼠标并且移动了。

theCanvas.onmousedown = function(e) {
    //获得鼠标按下的点相对canvas的坐标。
    let ele = windowToCanvas(theCanvas, e.clientX, e.clientY)
    //es6的解构赋值
    let { x, y } = ele
    //绘制起点。
    context.moveTo(x, y)
    //鼠标移动事件
    theCanvas.onmousemove = (e) => {
        //移动时获取新的坐标位置，用lineTo记录当前的坐标，然后stroke绘制上一个点到当前点的路径
        let ele = windowToCanvas(theCanvas, e.clientX, e.clientY)
        let { x, y } = ele
        context.lineTo(x, y)
        context.stroke()
    }
}

7、鼠标松开的时候，不再绘制路径。

有什么办法可以让onmouseup事件中阻止掉上面监听的2种事件呢？方法挺多的，设置onmousedown和onmousemove为null算是一种，我这里用到了“开关”。isAllowDrawLine设置为bool值，来控制函数是否执行，具体代码可以看下面完整的源码。

源码

分为3个文件，index.html、main.js、utils.js，这里用到了es6的语法，我是使用parcle配置好了开发环境，所以不会有报错，如果你直接复制代码，运行的时候出现错误，在无法升级浏览器的情况下，可以将es6语法改成es5.

1、index.html

上面已经展示了，不再复述。

2、main.js

import { windowToCanvas } from './utils'
(function() {
    let theCanvas = document.querySelector('#theCanvas')
    if (!theCanvas || !theCanvas.getContext) {
        return false
    } else {
        let context = theCanvas.getContext('2d')
        let isAllowDrawLine = false
        theCanvas.onmousedown = function(e) {
            isAllowDrawLine = true
            let ele = windowToCanvas(theCanvas, e.clientX, e.clientY)
            let { x, y } = ele
            context.moveTo(x, y)
            theCanvas.onmousemove = (e) => {
                if (isAllowDrawLine) {
                    let ele = windowToCanvas(theCanvas, e.clientX, e.clientY)
                    let { x, y } = ele
                    context.lineTo(x, y)
                    context.stroke()
                }
            }
        }
        theCanvas.onmouseup = function() {
            isAllowDrawLine = false
        }
    }
})()

3、utils.js

/*
* 获取鼠标在canvas上的坐标
* */
const windowToCanvas = (canvas, x, y) => {
    let rect = canvas.getBoundingClientRect()
    return {
        x: x - rect.left * (canvas.width/rect.width),
        y: y - rect.top * (canvas.height/rect.height)
    }
}
 
export {
    windowToCanvas
}