HTML5 Canvas Listen or Don't Listen to Events with Konva

To listen or don't listen to events with Konva, we can set the listening property of the config object to true or false when a shape is instantiated, or we can set the listening property with the setListening() method. Once we've set the listening property for one or more nodes, we'll also need to redraw the hit graph for each affected layer with the drawHit() method.

Instructions: Mouseover the oval to observe that the event handler is not executed. Click on "Listen" to start listening for events and observe that the event handler is now executed.

Vanilla

import Konva from 'konva';

const stage = new Konva.Stage({
  container: 'container',
  width: window.innerWidth,
  height: window.innerHeight,
});

const layer = new Konva.Layer();
stage.add(layer);

const text = new Konva.Text({
  x: 10,
  y: 10,
  fontFamily: 'Calibri',
  fontSize: 24,
  text: '',
  fill: 'black',
});
layer.add(text);

const oval = new Konva.Ellipse({
  x: stage.width() / 2,
  y: stage.height() / 2,
  radiusX: 100,
  radiusY: 50,
  fill: 'yellow',
  stroke: 'black',
  strokeWidth: 4,
  listening: false,
});

oval.on('mouseover', function () {
  writeMessage('Mouseover oval');
});
oval.on('mouseout', function () {
  writeMessage('');
});

function writeMessage(message) {
  text.text(message);
  layer.draw();
}

layer.add(oval);

// add button to toggle listening
const button = document.createElement('button');
button.innerHTML = 'Listen';
document.body.appendChild(button);
button.addEventListener('click', () => {
  const listening = !oval.listening();
  oval.listening(listening);
  button.innerHTML = listening ? 'Stop listening' : 'Listen';
  layer.drawHit();
});

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React

import { Stage, Layer, Ellipse, Text } from 'react-konva';
import { useState } from 'react';

const App = () => {
  const [message, setMessage] = useState('');
  const [listening, setListening] = useState(false);

  return (
    <>
      <button onClick={() => setListening(!listening)}>
        {listening ? 'Stop listening' : 'Listen'}
      </button>
      <Stage width={window.innerWidth} height={window.innerHeight}>
        <Layer>
          <Text
            x={10}
            y={10}
            fontFamily="Calibri"
            fontSize={24}
            text={message}
            fill="black"
          />
          <Ellipse
            x={window.innerWidth / 2}
            y={window.innerHeight / 2}
            radiusX={100}
            radiusY={50}
            fill="yellow"
            stroke="black"
            strokeWidth={4}
            listening={listening}
            onMouseover={() => setMessage('Mouseover oval')}
            onMouseout={() => setMessage('')}
          />
        </Layer>
      </Stage>
    </>
  );
};

export default App;

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Vue

<template>
  <div>
    <button @click="toggleListening">
      {{ listening ? 'Stop listening' : 'Listen' }}
    </button>
    <v-stage :config="stageSize">
      <v-layer>
        <v-text :config="textConfig" />
        <v-ellipse
          :config="ovalConfig"
          @mouseover="handleMouseover"
          @mouseout="handleMouseout"
        />
      </v-layer>
    </v-stage>
  </div>
</template>

<script setup>
import { ref, computed } from 'vue';

const message = ref('');
const listening = ref(false);

const stageSize = {
  width: window.innerWidth,
  height: window.innerHeight
};

const textConfig = computed(() => ({
  x: 10,
  y: 10,
  fontFamily: 'Calibri',
  fontSize: 24,
  text: message.value,
  fill: 'black'
}));

const ovalConfig = computed(() => ({
  x: window.innerWidth / 2,
  y: window.innerHeight / 2,
  radiusX: 100,
  radiusY: 50,
  fill: 'yellow',
  stroke: 'black',
  strokeWidth: 4,
  listening: listening.value
}));

const handleMouseover = () => {
  message.value = 'Mouseover oval';
};

const handleMouseout = () => {
  message.value = '';
};

const toggleListening = () => {
  listening.value = !listening.value;
};
</script>

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