React 不会在 useEffect 的 canvas 中绘制任何东西

Question

我用 matter.js 构建了一个 React 网站。我正在使用 useEffect 挂钩将内容渲染到 canvas 和 matter.js（我发现大部分代码是 here）。但是，当我尝试向 canvas 绘制任何其他内容时，什么也没有出现。所有 matter.js 相关的作品。

const scene = useRef()
const isDragging = useRef(false)
const engine = useRef(Engine.create())

useEffect(() => {
  const cw = 1100;
  const ch = 700;
  const render = Render.create({
    canvas: scene.current,
    engine: engine.current,
    options: {
      width: cw,
      height: ch,
      wireframes: false,
      background: 'transparent'
    }
  })

  console.log("gravity " + engine.current.gravity.y + "x : " + engine.current.gravity.x)

  let mouse = Mouse.create(render.canvas);
  let mouseConstraint = MouseConstraint.create(engine.current, {
    mouse: mouse,
    constraint: {
      render: {
        visible: false
      }
    }
  })
  render.mouse = mouse;

  World.add(engine.current.world, [
    Bodies.rectangle(cw / 2, 0, cw, 20, {
      isStatic: true,
      density: 1
    }),
    Bodies.rectangle(cw / 2, ch, cw, 20, {
      isStatic: true,
      density: 1
    }),
    Bodies.rectangle(0, ch / 2, 20, ch, {
      isStatic: true,
      density: 1
    }),
    Bodies.rectangle(cw, ch / 2, 20, ch, {
      isStatic: true,
      density: 1
    }),

    mouseConstraint,
  ])

  Runner.run(engine.current)
  Render.run(render)

  Events.on(mouseConstraint, "mousedown", function(event) {
    handleSelections(mouseConstraint.body)
  })
  Events.on(mouseConstraint, "startdrag", function(event) {
    isDragging.current = true
  })
  Events.on(mouseConstraint, "enddrag", function() {
    isDragging.current = false
  })
  Events.on(engine.current, 'afterUpdate', function() {
    countTen.current = countTen.current + 1;

    if (countTen.current == 10) {
      countTen.current = 0;

      if (selectedObjectRef.current != null) {

        setGraphingPos(selectedObjectRef.current.velocity.y * -1);

        setTicks(ticks + 1)
      }
    }
  })

  // ******************* This part doesn't work **************************
  scene.current.getContext('2d').beginPath();
  scene.current.getContext('2d').arc(100, 100, 20, 0, 2 * Math.PI, false);
  scene.current.getContext('2d').fillStyle = 'red';
  scene.current.getContext('2d').fill();
  // **********************************************************************
  return () => {
    Render.stop(render)
    World.clear(engine.current.world)
    Engine.clear(engine.current)
    render.canvas.remove()
    render.canvas = null
    render.context = null
    render.textures = {}
  }
}, [])

<canvas ref={scene} onClick={ handleMouseDown} className='main-canvas'></canvas>

非常感谢任何形式的帮助！

Answer 1

目前，您在 MJS wipes per frame 的同一个 canvas 上预先绘制了一次。因此，您绘制圆圈，然后 MJS 在呈现第一帧时立即擦除 canvas。随着引擎和渲染器运行向前，您永远不会尝试再次绘制。

我看到了一些解决方案（至少！）。哪个最合适取决于您的具体用例。

在 afterRender callback for the render object 内的每一帧上绘制 canvas。这是您现在所在位置的最简单、最直接的解决方案，我在下面提供了一个示例。
在您提供给 MJS 的那个之上添加一个 second transparent canvas。这是通过绝对定位完成的。现在你可以在这个叠加层中做任何你想做的事情而不受 MJS 的干扰。
运行 MJS headlessly，这给了你最大的控制权，因为 MJS built-in 渲染器是 mostly intended for protoyping (it's a physics engine, not a graphics engine). Once you're headless, you can render whatever you want, whenever you want, however you want. I have many demos of headless rendering in my other answers, both to the plain DOM and with React and p5.js.

顺便说一句，这似乎与 React 或 useEffect 没有任何关系；即使你在 vanilla JS 中注入一个普通的 canvas 也会出现同样的问题。

此外，请记住，无论您绘制什么，除了碰巧在同一个 canvas 上之外，都与 MJS 完全无关。不要指望物理学能对它起作用，除非你使用的是 body MJS 知道的坐标。

虽然您没有提供完整的组件，但这里是上述 afterRender 方法的最小 proof-of-concept：

const {useEffect, useRef} = React;
const {Bodies, Engine, Events, Render, Runner, Composite} = Matter;

const Scene = () => {
  const canvasRef = useRef();

  useEffect(() => {
    const cw = 200;
    const ch = 200;
    const engine = Engine.create();
    const ctx = canvasRef.current.getContext("2d");
    const render = Render.create({
      canvas: canvasRef.current,
      engine,
      options: {
        width: cw,
        height: ch,
        wireframes: false,
        background: "transparent",
      }
    });
    Events.on(render, "afterRender", () => {
      const x = Math.cos(Date.now() / 300) * 80 + 100;
      const y = Math.sin(Date.now() / 299) * 80 + 100;
      ctx.beginPath();
      ctx.arc(x, y, 20, 0, 2 * Math.PI);
      ctx.fillStyle = "red";
      ctx.fill();
    });
    Composite.add(engine.world, [
      Bodies.rectangle(cw / 2, 0, cw, 20, {
        isStatic: true,
        density: 1,
      }),
      Bodies.rectangle(cw / 2, ch, cw, 20, {
        isStatic: true,
        density: 1,
      }),
      Bodies.rectangle(0, ch / 2, 20, ch, {
        isStatic: true,
        density: 1,
      }),
      Bodies.rectangle(cw, ch / 2, 20, ch, {
        isStatic: true,
        density: 1,
      }),
    ]);
    Runner.run(engine);
    Render.run(render);

    return () => {
      Render.stop(render);
      Composite.clear(engine.world);
      Engine.clear(engine);
      render.canvas.remove();
      render.canvas = null;
      render.context = null;
      render.textures = {};
    };
  }, []);
  return <canvas ref={canvasRef}></canvas>;
};

ReactDOM.render(<Scene />, document.querySelector("#app"));

<script src="https://cdnjs.cloudflare.com/ajax/libs/react/17.0.2/umd/react.production.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react-dom/17.0.2/umd/react-dom.production.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/matter-js/0.18.0/matter.min.js"></script>
<div id="app"></div>

React 不会在 useEffect 的 canvas 中绘制任何东西

React won't draw anything in the canvas in useEffect

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javascript

reactjs

matter.js

react-hooks