1.画板的功能
- 修改画笔颜色；
- 修改画笔粗细；
- 橡皮擦；
- 重置画板；
- 撤销上一步；
- 保存成图片；
2.所需知识Element.getBoundingClientRect()方法返回元素的大小及其相对于视口的位置。ctx.moveTo(x, y)将一个新的子路径的起始点移动到(x，y)坐标ctx.lineTo(x, y)使用直线连接子路径的终点到x，y坐标
3.一步步实现第一步，实现基本功能，可以画出来鼠标路径；
class Board {
constructor(id) {
this.canvas = document.getElementById(id);
this.context = this.canvas.getContext('2d');
this.isDrawing = false;
this.posX = 0;
this.posY = 0;
this.init();
}
init() {
const bindDown = this.handleMouseDown.bind(this);
const bindMove = this.handleMouseMove.bind(this);
this.canvas.addEventListener('mousedown', bindDown);
this.canvas.addEventListener('mousemove', bindMove);
window.addEventListener('mouseup', () => {
this.isDrawing = false;
});
}
handleMouseDown(e) {
const rect = this.canvas.getBoundingClientRect();
this.posX = e.clientX - rect.left;
this.posY = e.clientY - rect.top;
this.isDrawing = true;
}
handleMouseMove(e) {
if (this.isDrawing === true) {
const rect = this.canvas.getBoundingClientRect();
this.drawLine(this.context, this.posX, this.posY,
e.clientX - rect.left, e.clientY - rect.top);
this.posX = e.clientX - rect.left;
this.posY = e.clientY - rect.top;
}
}
drawLine(context, x1, y1, x2, y2) {
context.beginPath();
context.strokeStyle = 'black';
context.lineWidth = 1;
context.moveTo(x1, y1);
context.lineTo(x2, y2);
context.stroke();
context.closePath();
}
}
new Board('myCanvas');
第二步，可以修改画笔颜色；
document.getElementById('colorPicker').addEventListener('change', e => {
b.changeColor(e.target.value);
})class Board {
constructor(id, color = '#000') {
this.penColor = color;
}
drawLine(context, x1, y1, x2, y2) {
context.strokeStyle = this.penColor;
}
changeColor(color) {
this.penColor = color;
}
}第三步，修改画笔粗细；
ctx.lineWidth = number;
第四步，橡皮擦；
context.globalCompositeOperation = 'destination-out';
参照刮刮乐功能。
第五步，重置画板；
context.clearRect(0, 0, width, height);
第六步，撤销上一步；
this.canvas.toDataURL()将当前canvas保存为base64的图片，存放在数组中。再设置一个索引，撤销/恢复修改索引的值，从数组中取出对应的图片。
第七步，保存为图片； 【用canvas实现一个简单的画板】创建一个a标签，href为toDataURL()生成的图片，模拟点击事件，点击a链接。
4.完整代码
class Board {
constructor(id, color = '#000', fontsize = 1) {
this.canvas = document.getElementById(id);
this.context = this.canvas.getContext('2d');
this.isDrawing = false;
this.posX = 0;
this.posY = 0;
this.penColor = color;
this.fontsize = fontsize;
this.isErasing = false;
this.step = 0;
this.histroyList = [];
this.init();
}
init() {
const bindDown = this.handleMouseDown.bind(this);
const bindMove = this.handleMouseMove.bind(this);
this.canvas.addEventListener('mousedown', bindDown);
this.canvas.addEventListener('mousemove', bindMove);
window.addEventListener('mouseup', () => {
this.isDrawing = false;
});
this.canvas.addEventListener('mouseup', () => {
this.step++;
if (this.step < this.histroyList.length) {
this.histroyList.length = this.step;
}
this.histroyList.push(this.canvas.toDataURL());
});
this.histroyList.push(this.canvas.toDataURL());
}
handleMouseDown(e) {
const rect = this.canvas.getBoundingClientRect();
this.posX = e.clientX - rect.left;
this.posY = e.clientY - rect.top;
this.isDrawing = true;
}
handleMouseMove(e) {
const rect = this.canvas.getBoundingClientRect();
if (this.isErasing) {
this.context.globalCompositeOperation = 'destination-out';
this.context.beginPath();
this.context.arc(e.clientX - rect.left, e.clientY - rect.top,
10, 0, Math.PI * 2);
this.context.fill();
} else if (this.isDrawing === true) {
this.drawLine(this.context, this.posX, this.posY,
e.clientX - rect.left, e.clientY - rect.top);
this.posX = e.clientX - rect.left;
this.posY = e.clientY - rect.top;
}
}
drawLine(context, x1, y1, x2, y2) {
context.beginPath();
context.strokeStyle = this.penColor;
context.lineWidth = this.fontsize;
context.moveTo(x1, y1);
context.lineTo(x2, y2);
context.stroke();
context.closePath();
}
changeColor(color) {
this.penColor = color;
}
changeFontSize(size) {
this.fontsize = size;
}
switchEraseStatus() {
this.isErasing = !this.isErasing;
}
clearBoard() {
this.context.clearRect(0, 0, window.myCanvas.width,
window.myCanvas.height);
this.step = 0;
this.histroyList = [];
}
revoke() {
if (this.step > 0) {
this.step--;
this.context.clearRect(0, 0, window.myCanvas.width,
window.myCanvas.height);
let pic = new Image();
pic.src = https://www.it610.com/article/this.histroyList[this.step];
pic.addEventListener('load', () => {
this.context.drawImage(pic, 0, 0);
})
} else {
console.log('不能继续撤销了')
}
}
recover() {
if (this.step < this.histroyList.length - 1) {
this.step++;
this.context.clearRect(0, 0, window.myCanvas.width,
window.myCanvas.height);
let pic = new Image();
pic.src = https://www.it610.com/article/this.histroyList[this.step];
pic.addEventListener('load', () => {
this.context.drawImage(pic, 0, 0);
})
} else {
console.log('不能继续恢复了')
}
}
saveAsPic() {
const el = document.createElement('a');
el.href = https://www.it610.com/article/this.canvas.toDataURL();
el.download ='canvas';
const event = new MouseEvent('click');
el.dispatchEvent(event);
}
}
const b = new Board('myCanvas');
window.colorPicker.addEventListener('change', e => {
b.changeColor(e.target.value);
})window.fontsizeSelect.addEventListener('change', e => {
b.changeFontSize(window.fontsizeSelect.value);
})window.eraser.addEventListener('click', () => {
b.switchEraseStatus();
})window.reset.addEventListener('click', () => {
b.clearBoard();
})window.revoke.addEventListener('click', () => {
b.revoke();
})window.recover.addEventListener('click', () => {
b.recover();
})window.saveAsPic.addEventListener('click', () => {
b.saveAsPic();
})
