在React中用canvas对图片标注的实现
作者：大洋洋2020
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：
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