Observer.js

class Observer {
    constructor(data) {
        this.observe(data);
    }

    observe(data) {
        if (!data || typeof data !== 'object') return;
        Object.keys(data).forEach(key => {
            this.defineReactive(data, key, data[key]);
            this.observe(data[key]);
        });
    }

    defineReactive(obj, key, value) {
        let dep = new Dep();
        Object.defineProperty(obj, key, {
            get() {
                Dep.target && dep.addSub(Dep.target);
                return value;
            },
            set: newVal => {
                if (newVal !== value) {
                    this.observe(newVal);
                    value = newVal;
                    dep.notify();
                }
            }
        });
    }
}