Watchers and Listeners

This example shows the difference between watch and addStateListener() when a component needs both internal reactions and external integration hooks.

Live Demo

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Component

import { defineComponent } from 'ornata';

export const Counter = defineComponent({
    name: 'Counter',
    state: {
        count: { default: 0, type: Number },
    },
    methods: {
        increment() {
            this.state.count += 1;
        },
    },
    watch: {
        count({ isInitial, newValue, oldValue }) {
            if (isInitial) return;

            console.log(`Internal watcher: ${oldValue} -> ${newValue}`);
        },
    },
});

External subscription

const instance = Counter.mount('[data-counter]');

const cleanup = instance.addStateListener('count', ({ newValue, oldValue }) => {
    console.log(`External listener: ${oldValue} -> ${newValue}`);
});

How to think about it

• watch lives inside the component definition
• addStateListener() is attached to a mounted instance from the outside
• watch is for component-owned side effects
• addStateListener() is for integrations and orchestration

Rule of thumb

• use render for DOM output
• use computed for derived values
• use watch for internal side effects
• use addStateListener() for external subscriptions