Canvas implements a method example of holding down the mouse and moving to draw a trajectory-CSS Tutorial-php.cn

Requirements

On a canvas canvas, there is nothing on the canvas in the initial state. Now, I want to add some mouse events to the canvas and use the mouse to write on the canvas. The specific effect is to move the mouse to any point on the canvas, then hold down the mouse, move the mouse position, and you can start writing!

This article is the first gain from learning canvas. The first demo that many people do when they first learn canvas is to implement a "clock". Of course, I also implemented one, but I won't talk about this. Let’s talk about a more interesting and simpler thing.

Hold down the mouse to draw the trajectory

Canvas implements a method example of holding down the mouse and moving to draw a trajectory

Principle

Let’s briefly analyze the idea. First, we You need a canvas, then calculate the position of the mouse on the canvas, bind the onmousedown event and onmousemove event to the mouse, draw a path during the movement, and end the drawing when you release the mouse.

Although this idea is very simple, there are some parts that require some tricks to implement.

1. An html file is required, containing canvas elements.

This is a canvas with a width of 800 and a height of 400. Why didn't you write px? Oh, I don’t understand it yet, it’s recommended by the canvas document.

<!doctype html>
<html class="no-js" lang="zh">
    <head>
        <meta charset="utf-8">
        <meta http-equiv="x-ua-compatible" content="ie=edge">
        <title>canvas学习</title>
        <meta name="description" content="">
        <meta name="viewport" content="width=device-width, initial-scale=1">

        <link rel="manifest" href="site.webmanifest">
        <link rel="apple-touch-icon" href="icon.png">
        <link rel="stylesheet" href="css/main.css">
    </head>
    <body>
        <canvas id="theCanvas" width="800" height="400"></canvas>
        <script src="js/main.js"></script>
    </body>
</html>

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2. Determine whether the current environment supports canvas.

In main.js, we write a self-executing function. The following is the code snippet for compatibility judgment. The "code body" will be the core of the implementation requirements.

(function() {
    let theCanvas = document.querySelector('#theCanvas')
    if (!theCanvas || !theCanvas.getContext) {
        //不兼容canvas
        return false
    } else {
        //代码主体
    }
})()

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3. Get the 2d object.

   let context = theCanvas.getContext('2d')

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4. Get the coordinates of the current mouse relative to the canvas.

Why do we need to obtain this coordinate? Because the mouse defaults to obtaining the relative coordinates of the current window, and the canvas can be located anywhere on the page, calculations are required to obtain the real mouse coordinates.

Encapsulates the acquisition of the real coordinates of the mouse relative to the canvas into a function. If you feel abstract, you can draw a picture on a scratch paper to understand why this operation is required.

Normally, it can be x - rect.left and y - rect.top. But why is it actually x - rect.left * (canvas.width/rect.width)?

canvas.width/rect.width means to determine the scaling behavior in canvas and find the scaling multiple.

const windowToCanvas = (canvas, x, y) => {
    //获取canvas元素距离窗口的一些属性，MDN上有解释
    let rect = canvas.getBoundingClientRect()
    //x和y参数分别传入的是鼠标距离窗口的坐标，然后减去canvas距离窗口左边和顶部的距离。
    return {
        x: x - rect.left * (canvas.width/rect.width),
        y: y - rect.top * (canvas.height/rect.height)
    }
}

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5. With the tool function in step 4, we can add mouse events to the canvas!

First bind the onmousedown event to the mouse and use moveTo to draw the starting point of the coordinates.

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)
}

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6. When moving the mouse, there is no mouse long press event, how to monitor it?

The little trick used here is to execute an onmousemove (mouse movement) event inside onmousedown, so that the mouse can be monitored and moved.

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()
    }
}

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7. When the mouse is released, the path will no longer be drawn.

Is there any way to prevent the two events monitored above in the onmouseup event? There are many methods. Setting onmousedown and onmousemove to null is one. I used "switch" here. isAllowDrawLine is set to a bool value to control whether the function is executed. For the specific code, please see the complete source code below.

Source code

is divided into 3 files, index.html, main.js, utils.js. The syntax of es6 is used here. I used parcle to configure the development environment, so no There will be an error. If you copy the code directly and an error occurs when running, if the browser cannot be upgraded, you can change the es6 syntax to es5.

1, index.html
It has been shown above and will not be repeated again.

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
        }
    }
})()

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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
}

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