vant-weapp/dist/mixins/observer/props.js

export function observeProps(props) {
    if (!props) {
        return;
    }
    Object.keys(props).forEach(key => {
        let prop = props[key];
        if (prop === null || !prop.type) {
            prop = { type: prop };
        }
        let { observer } = prop;
        prop.observer = function () {
            if (observer) {
                if (typeof observer === 'string') {
                    observer = this[observer];
                }
                observer.apply(this, arguments);
            }
            this.setData();
        };
        props[key] = prop;
    });
}