table of Contents:
Article Directory
Preface
An electronic blackboard in a common electronic classroom.
Features of this article:
- Primitive
JS - Packaged module
Simplest code example
<!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>
Result display
Code explanation
Ideas
- Press the mouse to start drawing. Mouse down event.
- The mouse bounces up to end the drawing. The mouse up event.
- The mouse is pressed and moved, and the path is drawn. Mouse movement event.
Code explanation
The overall idea: press the mouse to trigger the switch to move, and start recording the line after moving (using the coordinate after the movement-the coordinate before the movement, and then drawing the line), and the old coordinates will be updated every time you move. After releasing the mouse, release the move switch.
- Only when the mouse is pressed, will the effect of moving the drawing be triggered, so a state judgment needs to be added.
- Because there is an offset between the mouse pointer and the actual position, when positioning the coordinate, it needs to be increased
pagex-10so that the coordinate is at the tip of the pointer. - The coordinate position must be updated every time you move, and a small line segment is used to simulate an irregular line.
Package module
<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>