@exmg/lit-base/src/observer.js

Lit Base Elements for ExMachina

export const observer = (observer) => (proto, propName) => {
    // if we haven't wrapped `updated` in this class, do so
    if (!proto.constructor._observers) {
        proto.constructor._observers = new Map();
        const userUpdated = proto.updated;
        proto.updated = function (changedProperties) {
            userUpdated.call(this, changedProperties);
            changedProperties.forEach((v, k) => {
                const o = this.constructor._observers.get(k);
                if (o !== undefined) {
                    o.call(this, this[k], v);
                }
            });
        };
        // clone any existing observers (superclasses)
        // eslint-disable-next-line no-prototype-builtins
    }
    else if (!proto.constructor.hasOwnProperty('_observers')) {
        const observers = proto.constructor._observers;
        proto.constructor._observers = new Map();
        observers.forEach(
        // eslint-disable-next-line @typescript-eslint/no-explicit-any
        (v, k) => proto.constructor._observers.set(k, v));
    }
    // set this method
    proto.constructor._observers.set(propName, observer);
};
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