singleSourceEffect.js

    /**
     * Creates a single source effect that listens to changes in a computed signal and invokes a callback
     * with the old and new values whenever the computed signal changes.
     *
     * @param {Computed} aComputed - The computed signal to observe.
     * @param {Function} fn - A callback function with the signature (oldValue, newValue).
     * @return {Effect} The created effect.
     */
    fluid.singleSourceEffect = function (aComputed, fn) {
        let oldValue = aComputed.value; // Initialize with the current value of the computed signal.

        return effect(() => {
            const newValue = aComputed.value; // Get the updated value of the computed signal.
            if (oldValue !== newValue) {
                fn(oldValue, newValue); // Notify the callback with old and new values.
                oldValue = newValue; // Update the old value for the next change.
            }
        });
    };