本文实例为大家分享了JS canvas实现画板/签字板功能的具体代码，供大家参考，具体内容如下
前言
常见的电子教室里的电子黑板。
本文特点：
原生JS
封装好的模块
最简代码样例
<!DOCTYPE html>
< html lang = "en" >
< head >
< meta charset = "UTF-8" >
< meta http-equiv = "X-UA-Compatible" content = "IE=edge" >
< meta name = "viewport" content = "width=device-width, initial-scale=1.0" >
< title >Document</ title >
</ head >
< body >
< canvas id = "canvas" ></ canvas >
< script >
let c = document.getElementById('canvas');
c.width = window.innerWidth;
c.height = window.innerHeight;
let ctx = c.getContext('2d');
// draw one black board
ctx.fillStyle = "black";
ctx.fillRect(0,0,600,300);
// 按下标记
let onoff = false,
oldx = -10,
oldy = -10;
// 设置颜色
let linecolor = "white";
// 设置线宽
let linw = 4;
// 添加鼠标事件
// 按下
c.addEventListener('mousedown', event => {
onoff = true;
// 位置 - 10是为了矫正位置，把绘图放在鼠标指针的顶端
oldx = event.pageX - 10;
oldy = event.pageY - 10;
},false);
// 移动
c.addEventListener('mousemove', event => {
if(onoff == true){
let newx = event.pageX - 10,
newy = event.pageY - 10;
// 绘图
ctx.beginPath();
ctx.moveTo(oldx,oldy);
ctx.lineTo(newx,newy);
ctx.strokeStyle = linecolor;
ctx.lineWidth = linw;
ctx.lineCap = "round";
ctx.stroke();
// 每次移动都要更新坐标位置
oldx = newx,
oldy = newy;
}
}, true);
// 弹起
c.addEventListener('mouseup', ()=> {
onoff = false;
},false);
</ script >
</ body >
</ html >
结果展示
代码讲解
思路
1、鼠标按下，开始描画。鼠标按下事件。
2、鼠标弹起，结束描画。鼠标弹起事件。
3、鼠标按下移动，路径画线。鼠标移动事件。
代码讲解
整体思路：按下鼠标，触发移动的开关，移动后开始记录线条（用移动后的坐标-移动前的坐标，然后绘线），每次移动都会更新旧坐标。松开鼠标后，释放移动开关。
1、只有在鼠标按下，才会触发移动绘图的效果，所以需要增加一个状态判断。
2、因为鼠标指针和实际位置有一个偏移量，所以在坐标定位的时候，需要增加pagex-10从而使坐标位于指针的尖端处。
3、每次移动都要更新坐标位置，用小段的线段来模拟不规则的线。
封装模块
<canvas id= "canvas" ></canvas>
<script>
class Board{
constructor(canvasName = 'canvas' , data = new Map([
[ "onoff" , false ],
[ "oldx" , -10],
[ "oldy" , -10],
[ "fillStyle" , "black" ],
[ "lineColor" , "white" ],
[ "lineWidth" , 4],
[ "lineCap" , "round" ],
[ "canvasWidth" , window.innerWidth],
[ "canvasHeight" , window.innerHeight]
])){
// this.data = data;
this .c = document.getElementById(canvasName);
this .ctx = this .c.getContext( '2d' );
this .onoff = data.get( "onoff" );
this .oldx = data.get( "oldx" );
this .oldy = data.get( "oldy" );
this .lineColor = data.get( "lineColor" );
this .lineWidth = data.get( "lineWidth" );
this .lineCap = data.get( "lineCap" );
this .c.width = data.get( "canvasWidth" );
this .c.height = data.get( "canvasHeight" );
this .ctx.fillStyle = data.get( "fillStyle" );
this .ctx.fillRect(0,0,600,300);
}
eventOperation(){
// 添加鼠标事件
// 按下
this .c.addEventListener( 'mousedown' , event => {
this .onoff = true ;
// 位置 - 10是为了矫正位置，把绘图放在鼠标指针的顶端
this .oldx = event.pageX - 10;
this .oldy = event.pageY - 10;
}, false );
// 移动
this .c.addEventListener( 'mousemove' , event => {
if ( this .onoff == true ){
let newx = event.pageX - 10,
newy = event.pageY - 10;
// 绘图
this .ctx.beginPath();
this .ctx.moveTo( this .oldx, this .oldy);
this .ctx.lineTo(newx,newy);
this .ctx.strokeStyle = this .lineColor;
this .ctx.lineWidth = this .lineWidth;
this .ctx.lineCap = this .lineCap;
this .ctx.stroke();
// 每次移动都要更新坐标位置
this .oldx = newx,
this .oldy = newy;
}
}, true );
// 弹起
this .c.addEventListener( 'mouseup' , ()=> {
this .onoff = false ;
}, false );
}
}
let board = new Board();
board.eventOperation();
</script>
以上就是本文的全部内容，希望对大家的学习有所帮助，也希望大家多多支持服务器之家。
原文链接：https://blog.csdn.net/u013362192/article/details/113934773
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