1.畫板的功能
- 修改畫筆顏色；
- 修改畫筆粗細；
- 橡皮擦；
- 重置畫板；
- 撤銷上一步；
儲存成圖片；
2.所需知識
Element.getBoundingClientRect()方法返回元素的大小及其相對於視口的位置。ctx.moveTo(x, y)將一個新的子路徑的起始點移動到(x，y)座標ctx.lineTo(x, y)使用直線連線子路徑的終點到x，y座標3.一步步實現
第一步，實現基本功能，可以畫出來滑鼠路徑；
<canvas id="myCanvas" width="400" height="400"></canvas>
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，可以修改畫筆顏色；
<input id="colorPicker" type="color" />
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，修改畫筆粗細；
ctx.lineWidth = number;第四步，橡皮擦；
context.globalCompositeOperation = 'destination-out';
參照刮刮樂功能。
第五步，重置畫板；
context.clearRect(0, 0, width, height);
第六步，撤銷上一步；
this.canvas.toDataURL()
將當前canvas儲存為base64的圖片，存放在陣列中。再設定一個索引，撤銷/恢復修改索引的值，從陣列中取出對應的圖片。
第七步，儲存為圖片；
建立一個a標籤，href為toDataURL()生成的圖片，模擬點選事件，點選a連結。
4.完整程式碼
<div class="opera">
<input id="colorPicker" type="color" />
<select id="fontsizeSelect">
<option value="1">1</option>
<option value="2">2</option>
<option value="3">3</option>
<option value="4">4</option>
<option value="5">5</option>
</select>
<button id="eraser">橡皮擦</button>
<button id="reset">重置</button>
<button id="revoke">撤銷</button>
<button id="recover">恢復</button>
<button id="saveAsPic">儲存為圖片</button>
</div>
<canvas id="myCanvas" width="400" height="400"></canvas> 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 = this.histroyList[this.step];
pic.addEventListener('load', () => {
this.context.drawImage(pic, 0, 0);
})
} else {
console.log('不能繼續撤銷了')
}
}
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 = this.histroyList[this.step];
pic.addEventListener('load', () => {
this.context.drawImage(pic, 0, 0);
})
} else {
console.log('不能繼續恢復了')
}
}
saveAsPic() {
const el = document.createElement('a');
el.href = 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();
})