How can I load a canvas inside a div with Vue?

user_13484913 0
Viewed 197

I am really new to Vue and for this project, I was trying to load canvas inside a div. If the canvas is loaded outside of a div it works correctly but I want to display a canvas if loadModal is true only. In this code I have used two canvas one is inside div and other one is outside of div. It loads canvas correctly outside of div only. Is there a way to load canvas inside div as well? Here is my code below on JsFiddle

JsFiddle Code = https://jsfiddle.net/ujjumaki/6vg7r9oy/9/

View

<div id="app">
  <button @click="runModal()">
  Click Me
  </button>
  <br><br>
  <div v-if="loadModal == true">
    <canvas id="myCanvas" width="200" height="100" style="border:3px solid #d3d3d3; color:red;">
    </canvas>
  </div>
  
  <!-- this one loads correctly -->
  <canvas id="myCanvas" width="200" height="100" style="border:1px solid #d3d3d3;">
  </canvas>
</div>

Method

new Vue({
  el: "#app",
  data: {
    loadModal: false,
  },
  methods: {
    runModal(){
        this.loadModal = true;
      var c = document.getElementById("myCanvas");
      var ctx = c.getContext("2d");
      ctx.beginPath();
      ctx.arc(95, 50, 40, 0, 2 * Math.PI);
      ctx.stroke();
    }
  }
})
vuejs2 vue.js canvas javascript
2 Answers

Try with v-show and with class instead id, so you can select all divs:

new Vue({
  el: "#app",
  data: {
    loadModal: false,
  },
  methods: {
    runModal(){
        this.loadModal = true;
      const c = document.querySelectorAll("canvas");
      c.forEach(can => {
        let ctx = can.getContext("2d");
        ctx.beginPath();
        ctx.arc(95, 50, 40, 0, 2 * Math.PI);
        ctx.stroke();
      })
    }
  }
})

Vue.config.productionTip = false
Vue.config.devtools = false
body {
  background: #20262E;
  padding: 20px;
  font-family: Helvetica;
}
#app {
  background: #fff;
  border-radius: 4px;
  padding: 20px;
  transition: all 0.2s;
}
li {
  margin: 8px 0;
}
h2 {
  font-weight: bold;
  margin-bottom: 15px;
}
del {
  color: rgba(0, 0, 0, 0.3);
}
<script src="https://cdnjs.cloudflare.com/ajax/libs/vue/2.5.17/vue.js"></script>
<div id="app">
  <button @click="runModal()">
  Click Me
  </button>
  <br><br>
  <div v-show="loadModal == true">
    <canvas class="canvas" id="myCanvas" width="200" height="100" style="border:3px solid #d3d3d3; color:red;">
    </canvas>
  </div>
  <canvas class="canvas" id="myCanvas" width="200" height="100" style="border:3px solid #d3d3d3; color:red;">
  </canvas>
</div>

user_119891890

The problem is runModal() sets loadModal to true and immediately tries to reference the <canvas> inside the conditionally rendered <div v-if="loadModal">, but the <div> (and its <canvas>) is still not rendered yet. The effect of the loadModal change is not complete until the next render cycle. The <canvas> outside the <div> "loads" correctly because it's not conditionally rendered.

One solution is to await the next render cycle with the $nextTick() callback before trying to access the <canvas>:

new Vue({
  methods: {
    async runModal() {
      this.loadModal = true;

      // await next render cycle
      await this.$nextTick();

      // <canvas id="myCanvas"> is available here
      var c = document.getElementById("myCanvas");
      ⋮
    }
  }
})

updated fiddle

Alternatively, using <div v-show="loadModal"> (as described in another answer) also works because the <canvas> is already rendered in the document and thus available to query. If you rather prefer to lazily render the <canvas>, stick with v-if="loadModal".

user_62771510
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