Vue3 TypeError: template ref.value is null

Question

如何从以下错误中清除我的控制台：

TypeError: ref.value is null

错误仅在调整大小事件中出现。每次我调整 window 的大小时，我都会渲染图表。所以错误信息一次又一次出现。文档显示模板引用也被初始化为空值 (Source)。所以我必须在初始化之后做一些事情，对吗？

这是我的代码：

<template>
  <canvas
    ref="chartRef"
  />
</template>

<script setup>
// ...
// on resize
export const chartRef = ref(null)
export function createChart () {
  const ctx = chartRef.value.getContext('2d')
  if (ctx !== null) { // fix me
    getDimensions()
    drawChart(ctx)
  }
}
// ...
</script>

如何清理我的控制台以便不再显示错误消息？谢谢

Answer 1

选项A

将其包装在 try...catch

中

选项 2

使用 watch

我发现最好的方法是使用 watch

这是一个可以在多个组件之间重复使用的函数示例。我们可以定义一个函数来生成 canvas 引用，然后可以将其传递给组件 - canvasRef .

const withCanvasRef = () => {
  let onMountCallback = null;
  const onMount = callback => {
    onMountCallback = callback;
  };
  const canvasRef = ref(null);

  watch(canvasRef, (element, prevElement) => {
    if (element instanceof HTMLCanvasElement) {
      canvasRef.value = element;
      if (onMountCallback && prevElement === null) onMountCallback(canvasRef);
    } else {
      ctxRef.value = null;
    }
  });
  return {
    canvasRef,
    onMount
  };
};

然后我们可以获取组件中的canvasRef并将其传递给<canvas>元素。我们还可以使用函数 returns 处理初始渲染的 onMounted 挂钩。

app.component("my-line-chart", {
  setup: props => {
    const { canvasRef, onMount } = withCanvasRef();

    const draw = () => {
      // stuff here,
      // use a check for canvasRef.value if you have conditional rendering
    };

    // on resize
    window.addEventListener("resize", () => draw());

    // on canvas mount
    onMount(() => draw());

    return { canvasRef };
  },
  template: `<div><canvas ref="canvasRef"/></div>`
});

请参阅示例，例如显示此操作的示例。希望您能看到使用 Composition API 作为更好的代码重用和组织解决方案的好处。（尽管它的某些方面看起来有点费力，比如必须手动为道具定义手表）

const app = Vue.createApp({
  setup() {
    const someData = Vue.ref(null);
    let t = null;

    const numPts = 20;
    const generateData = () => {
      const d = [];
      for (let i = 0; i < numPts; i++) {
        d.push(Math.random());
      }

      if (someData.value == null) {
        someData.value = [...d];
      } else {
        const ref = [...someData.value];
        let nMax = 80;
        let n = nMax;
        t !== null && clearInterval(t);

        t = setInterval(() => {
          n = n -= 1;
          n <= 0 && clearInterval(t);
          const d2 = [];
          for (let i = 0; i < numPts; i++) {
            //d2.push(lerp(d[i],ref[i], n/nMax))
            d2.push(ease(d[i], ref[i], n / nMax));
          }
          someData.value = [...d2];
        }, 5);
      }
    };
    generateData();
    return { someData, generateData };
  }
});

const withCanvasRef = () => {
  let onMountCallback = null;
  const onMount = callback => {
    onMountCallback = callback;
  };
  const canvasRef = Vue.ref(null);

  Vue.watch(canvasRef, (element, prevElement) => {
    if (element instanceof HTMLCanvasElement) {
      canvasRef.value = element;
      if (onMountCallback && prevElement === null) onMountCallback(canvasRef);
    } else {
      ctxRef.value = null;
    }
  });
  return {
    canvasRef,
    onMount
  };
};

const drawBarGraph = (canvas, data) => {
  const width = canvas.width;
  const height = Math.min(window.innerHeight, 200);
  const ctx = canvas.getContext("2d");

  const col1 = [229, 176, 84];
  const col2 = [202, 78, 106];

  const len = data.length;
  const mx = 10;
  const my = 10;
  const p = 4;
  const bw = (width - mx * 2) / len;

  const x = i => bw * i + p / 2 + mx;
  const w = () => bw - p;
  const h = num => (height - my * 2) * num;
  const y = num => (height - my * 2) * (1 - num) + my;
  const col = i => {
    const r = lerp(col1[0], col2[0], i / len);
    const g = lerp(col1[1], col2[1], i / len);
    const b = lerp(col1[2], col2[2], i / len);
    return `rgb(${[r, g, b]})`;
  };

  data.forEach((num, i) => {
    ctx.fillStyle = col(i);
    ctx.fillRect(x(i), y(num), w(), h(num));
  });
};

const drawLineGraph = (canvas, data) => {
  const width = canvas.width;
  const height = Math.min(window.innerHeight, 200);
  const ctx = canvas.getContext("2d");

  const col1 = [229, 176, 84];
  const col2 = [202, 78, 106];

  const len = data.length;
  const mx = 10;
  const my = 10;
  const p = 4;
  const bw = (width - mx * 2) / len;

  const x = i => bw * i + p / 2 + mx + bw / 2;
  const y = num => (height - my * 2) * (1 - num) + my;
  const r = 2;

  const col = i => {
    const r = lerp(col1[0], col2[0], i / len);
    const g = lerp(col1[1], col2[1], i / len);
    const b = lerp(col1[2], col2[2], i / len);
    return `rgb(${[r, g, b]})`;
  };

  ctx.lineWidth = 0.2;
  ctx.strokeStyle = "black";
  ctx.beginPath();
  data.forEach((num, i) => {
    i == 0 && ctx.moveTo(x(i), y(num));
    i > 0 && ctx.lineTo(x(i), y(num));
  });
  ctx.stroke();
  ctx.closePath();

  data.forEach((num, i) => {
    ctx.beginPath();
    ctx.fillStyle = col(i);
    ctx.arc(x(i), y(num), r, 0, 2 * Math.PI);
    ctx.fill();
  });
};

const drawSomething = canvas => {
  canvas.width = window.innerWidth / 2 - 5;
  canvas.height = Math.min(window.innerHeight, 200);
  const ctx = canvas.getContext("2d");
  ctx.fillStyle = "rgb(255 241 236)";
  ctx.fillRect(0, 0, window.innerWidth, window.innerHeight);
};

app.component("my-bar-chart", {
  props: ["data"],
  setup: props => {
    const { canvasRef, onMount } = withCanvasRef();

    const draw = () => {
      if (canvasRef.value) {
        drawSomething(canvasRef.value);
        drawBarGraph(canvasRef.value, props.data);
      }
    };

    // on resize
    window.addEventListener("resize", () => draw());

    // on data change
    Vue.watch(
      () => props.data,
      () => draw()
    );

    // on canvas mount
    onMount(() => draw());

    return { canvasRef };
  },
  template: `<div><canvas ref="canvasRef"/></div>`
});

app.component("my-line-chart", {
  props: ["data"],
  setup: props => {
    const { canvasRef, onMount } = withCanvasRef();

    const draw = () => {
      if (canvasRef.value) {
        drawSomething(canvasRef.value);
        drawLineGraph(canvasRef.value, props.data);
      }
    };

    // on resize
    window.addEventListener("resize", () => draw());

    // on data change
    Vue.watch(
      () => props.data,
      () => draw()
    );

    // on canvas mount
    onMount(() => draw());

    return { canvasRef };
  },
  template: `<div><canvas ref="canvasRef"/></div>`
});

app.mount("#app");

const lerp = (start, end, amt) => (1 - amt) * start + amt * end;
const ease = (start, end, amt) => {
  return lerp(start, end, Math.sin(amt * Math.PI * 0.5));
};

body {
  margin: 0;
  padding: 0;
  overflow: hidden;
}
.chart {
  display: inline-block;
  margin-right: 4px;
}

<script src="https://unpkg.com/vue@next/dist/vue.global.prod.js"></script>

<div id="app">
  <button @click="generateData">Scramble</button>
  <div>
    <my-bar-chart class="chart" :data="someData"></my-bar-chart>
    <my-line-chart class="chart" :data="someData"></my-line-chart>
  </div>
</div>

Answer 2

我有点晚了，但我在 Vue 3 中遇到了同样的问题。我通过返回引用解决了 juste :

<template>
  <input ref="myinput">
</template>


<script>
import { onMounted, ref } from 'vue'

export default {
  setup() {

    const myinput = ref(null) // Assign dom object reference to "myinput" variable

    onMounted(() => {
      console.log(myinput.value) // Log a DOM object in console
    })

    return { myinput } // WILL NOT WORK WITHOUT THIS
  }
}
</script>

Answer 3

我也有点晚了，但考虑到它是针对 Vue 3 的，而且现在还是 2021 年，这是我为那些不使用 Composition API:

的人提供的解决方案

在其中一个子组件中，我有这样的东西：

<div ref='popupMenu'>
... some content here ...
</div>

正如预期的那样，this.$refs.popupMenu 是在 mounted 和 activated 回调中设置的，但是，无论出于何种原因，当我试图从 window 滚动侦听器接近它时, 它被设置为 null。我安慰了整个组件以查看发生了什么，我可以看到 $refs.popupMenu 是 null.

不确定这是不是我做的，或者我对 mount/activate 在父组件也重新渲染它的子组件时如何工作的误解，但对我有用的是：

模板

<div :ref='storeReference'>
.... popup content here ...
</div>

数据

data() {
    popupMenu: null
}

方法

methods: {
    storeReference(e) {
        if(e) {
            this.popupMenu = e;
        }
    }
    clearReference(e) {
        this.popupMenu = null;
    }
}

这将确保我始终保留参考资料，并且在我的案例中效果很好。

当控件为 unmounted 或 deactivated 时，将有一个方法 clearReference 将设置 this.popupMenu = null，因此它不会将其保留在内存中。

Answer 4

如果您将 yourRef.value 视为 null，请确保您尝试获取 ref 的元素未隐藏在错误的 [=13] 下=].在实际需要该元素之前，Vue 不会实例化 ref。

Vue3 TypeError: template ref.value is null

Vue3 TypeError: template ref.value is null

templates

canvas

vuejs3

vue-composition-api

refs

选项A

选项 2