如何在React中用canvas对图片标注

  在审核业务中难免会有需要对图片进行标注的需求，本次用一个最小demo来演示如何对图片进行矩形标注。

  首先我们要理解canvas是一块画布，而这块画布需要在我们要标注的图片上层，图片和canvas外层的div用相对位置，内层的图片和canvas用绝对位置，即可保证canvas重叠于图片之上。如图：

  我们来看下canvas的初始化，在img、canvas中都有ref属性，不同的是img的ref属性直接就是一个useRef引用，而canvas中的ref是一个回调函数。它在组件被加载或卸载时会立即执行，加载时ref回调接收当前组件实例作为参数，卸载时ref回调接收null作为参数。在initCanvas函数中，用canvas的ref引用承接了canvas节点，并且通过drawImage函数，初始化了一块400*400的画布，第一个参数为需要绘制到的上下文元素：

<img src={lancome} ref={imgInstance} className="App-logo" alt="logo" />
<canvas className="canvas" ref={initCanvas} width="400px" height="400px" />

const canvasRef = useRef(null);
const imgInstance = useRef(null);

const initCanvas =  useCallback((node) => {
      canvasRef.current = node;
      const context = node.getContext('2d');
      context.drawImage(imgInstance.current, 0, 0, 400, 400);
  }, []);

  接下来，我们通过invalidLocations来保存之前的标注位置信息，addInvalidLocation函数是为了添加标注位置信息。最需要注意的是我们在useEffect中所监听的三个函数，startDraw、drawingDeal和drawingEnd。

  鼠标落下时，startDraw为起始点的x，y坐标赋值，并且拖拽状态位isDrawing置为true。鼠标移动时，drawingDeal函数会边通过clearRect函数更新画布，边根据鼠标的最新位置通过highlightInvalid来更新标注，经过确定矩形位置大小，内容填充，描边三个步骤来绘制出矩形。鼠标抬起时，drawingEnd函数会通过addInvalidLocation函数添加标注位置，然后初始化参数。

const [invalidLocations, setInvalidLocations] = useState([]);

const addInvalidLocation = useCallback((newMark) => {
    setInvalidLocations([...invalidLocations, newMark]);
}, [invalidLocations])

const highlightInvalid = (context, x1, y1, x2, y2) => {
    context.beginPath();
    context.rect(x1, y1, x2 - x1, y2 - y1);
    context.fillStyle = 'rgba(255, 0, 0, 0.2)';
    context.fill();
    context.strokeStyle = '#FF0070';
    context.lineWidth = 1;
    context.stroke();
    console.log('drawing', x2, y2);
};

const clearRect = (drawContext) => {
    drawContext.clearRect(0, 0, canvasRef.current.width, canvasRef.current.height);
};

useEffect(() => {
    const canvasElem = canvasRef.current;
    let x = 0; let y = 0;
    let isDrawing = false;
    const drawContext = canvasRef.current.getContext('2d');
    let canvasRect;
    const lastCursorPosition = {
      x: 0,
      y: 0,
    };
    const startDraw = (e) => {
      console.log(e.type, 'start');
      canvasRect = canvasRef.current.getBoundingClientRect();
      x = e.clientX - canvasRect.left;
      y = e.clientY - canvasRect.top;
      if (x < 0) x = 0;
      if (y < 0) y = 0;
      isDrawing = true;
    };
    const drawingDeal = (e) => {
      console.log(e.type, 'move');
      if (isDrawing) {
        const x1 = e.clientX - canvasRect.left;
        const y1 = e.clientY - canvasRect.top;
        clearRect(drawContext);
        highlightInvalid(drawContext, x, y, x1, y1);
        lastCursorPosition.x = x1;
        lastCursorPosition.y = y1;
      }
    };
    const drawingEnd = () => {
      if (isDrawing) {
        if (lastCursorPosition.x && lastCursorPosition.y) {
          const width = lastCursorPosition.x - x + 1;
          const height = lastCursorPosition.y - y + 1;
          addInvalidLocation({ x, y, width, height });
          lastCursorPosition.x = 0;
          lastCursorPosition.y = 0;
        }
        clearRect(drawContext);
        isDrawing = false;
        x = 0;
        y = 0;
      }
    };
    canvasElem.addEventListener('mousedown', startDraw);
    canvasElem.addEventListener('mousemove', drawingDeal);
    canvasElem.addEventListener('mouseup', drawingEnd);
    return () => {
      canvasElem.removeEventListener('mousedown', startDraw);
      canvasElem.removeEventListener('mousemove', drawingDeal);
      canvasElem.removeEventListener('mouseup', drawingEnd);
    };
}, [invalidLocations, addInvalidLocation]);
  

  在添加完标注位置之后，模板中我们通过迭代返回绝对定位的div来实现已经标注过的矩形。

<div className="img-wrap">
    <img src={lancome} ref={imgInstance} className="App-logo" alt="logo" />
    <canvas className="canvas" ref={initCanvas} width="400px" height="400px" />
    {invalidLocations && invalidLocations.map((location, index) => { 
        const { width, height, x, y } = location;
            return <div
                     key={`${width}_${height}_${x}_${y}`}
                     tabIndex={-1}
                     className={'remark'}
                     style={{ width: `${width}px`, height: `${height}px`, left: `${x}px`, top: `${y}px` }}
            ></div>
    })}
</div>

  最后效果：