canvas进阶教程（原生canvas制作画图小工具的踩坑和爬坑）

鼠标按下，开始绘制，记录初始鼠标的位置
startPaint = (e)=>{
    this.startPaintFlag = true;
    this.paintstartX = e.clientX;
    this.paintstartY = e.clientY;
}

鼠标抬起，停止绘制
stopPaint = (e)=>{
    this.startPaintFlag = false;    
}

painting = (e)=>{
    const {activeColor, activeShape} = this.state;
    
    绘制过程中
    if(this.startPaintFlag){
      const ctx = this.canvas.getContext("2d");
      ctx.strokeStyle = activeColor;
      ctx.lineWidth = 2;
      ctx.beginPath();
    
      if(activeShape === 'pen'){
        ctx.moveTo(this.paintstartX, this.paintstartY);
        ctx.lineTo(e.clientX, e.clientY);
      }
      if(activeShape === 'circle'){
        const r = Math.sqrt(Math.pow(e.clientX - this.paintstartX, 2) + Math.pow(e.clientY - this.paintstartY, 2));
        ctx.arc(this.paintstartX, this.paintstartY, r, 0, 2*Math.PI)
      }
      if(activeShape === 'rect'){
        ctx.rect(this.paintstartX, this.paintstartY, e.clientX - this.paintstartX, e.clientY - this.paintstartY);
      }
      ctx.stroke();
      
      记录此次移动的最后位置，供下一次绘制使用
      this.paintstartX = e.clientX;
      this.paintstartY = e.clientY;
    }
}

startPaint = (e) => {
  this.startPaintFlag = true;
  this.paintstartX = e.clientX;
  this.paintstartY = e.clientY;

  画矩形和圆形时临时生成一个canvas
  if (this.state.activeShape !== "pen") {
    this.tempCanvas = document.createElement("canvas");
    this.tempCanvas.width = this.canvas.width;
    this.tempCanvas.height = this.canvas.height;
    
    设置一些定位样式
    this.tempCanvas.style.cssText = "position: absolute; top: 0; left: 0; z-index: 0;";
    
    append到需要的容器元素里
    document.querySelector(".contain").appendChild(this.tempCanvas);
  }
};

stopPaint = (e) => {
  this.startPaintFlag = false;
  const { activeShape, activeColor } = this.state;
  if (activeShape !== "pen") {
    
    从容器元素里删除临时画布
    document.querySelector(".contain").removeChild(this.tempCanvas);
    this.tempCanvas = null;
    
    将lastDrawData记录的绘制数据，绘制到展示用的画布上
    const ctx = this.canvas.getContext("2d");
    ctx.strokeStyle = activeColor;
    ctx.lineWidth = 2;
    ctx.beginPath();
    if (activeShape === "circle") {
      const { x, y, r } = this.lastDrawData;
      ctx.arc(x, y, r, 0, 2 * Math.PI);
    } else {
      const { x, y, width, height } = this.lastDrawData;
      ctx.rect(x, y, width, height);
    }
    ctx.stroke();
    this.lastDrawData = null;
  }
};

painting = (e) => {
  const { activeColor, activeShape } = this.state;

  if (this.startPaintFlag) {
    const ctx = this.canvas.getContext("2d");
    ctx.strokeStyle = activeColor;
    ctx.lineWidth = 2;

    画线的逻辑不动
    if (activeShape === "pen") {
      ctx.beginPath();
      ctx.moveTo(this.paintstartX, this.paintstartY);
      ctx.lineTo(e.clientX, e.clientY);
      ctx.stroke();
      this.paintstartX = e.clientX;
      this.paintstartY = e.clientY;
    } else {

      有预览的图形绘制在临时画布上
      const tempCtx = this.tempCanvas.getContext("2d");
      tempCtx.strokeStyle = activeColor;
      tempCtx.lineWidth = 2;

      每次绘制前清除画布
      tempCtx.clearRect(0, 0, this.canvas.width, this.canvas.height);
      tempCtx.beginPath();
      if (activeShape === "circle") {
        const r = Math.sqrt(
          Math.pow(e.clientX - this.paintstartX, 2) +
            Math.pow(e.clientY - this.paintstartY, 2)
        );
        tempCtx.arc(this.paintstartX, this.paintstartY, r, 0, 2 * Math.PI);

        绘制的数据记录在一个变量里
        this.lastDrawData = {
          x: this.paintstartX,
          y: this.paintstartY,
          r,
        };
      } else {
        tempCtx.rect(
          this.paintstartX,
          this.paintstartY,
          e.clientX - this.paintstartX,
          e.clientY - this.paintstartY
        );
        this.lastDrawData = {
          x: this.paintstartX,
          y: this.paintstartY,
          width: e.clientX - this.paintstartX,
          height: e.clientY - this.paintstartY,
        };
      }
      tempCtx.stroke();
    }
  }
};

第一种，用getImageData保存图像，改变大小后，用putImageData绘制，putImageData只能裁剪图像，不能拉伸
const imgData = ctx.getImageData(0, 0, canvas.width, canvas.height);
canvas.width = newWidth;
canvas.height = newHeight;
ctx.putImageData(imgData, 0, 0);

第二种，还是创建一个临时画布，将图形绘制下来，改变大小后，用drawImage绘制，drawImage是可以拉伸图像的
const newCanvas = document.createElement("canvas");
newCanvas.width = canvas.width;
newCanvas.height = canvas.height;
newCanvas.getContext("2d").drawImage(canvas, 0, 0);
canvas.width = newWidth;
canvas.height = newHeight;
ctx.drawImage(newCanvas, 0, 0);