watcher implements in lit-element

attributeChangedCallback.js

	attributeChangedCallback(attr, oldValue, newValue) {
		super.attributeChangedCallback(attr, oldValue, newValue)
		if (!this.watch || !this.watch.apply) {
			return null
		}
		if (!this._watch) {
			this._watch = this.watch()
		}
		if (this._watch.hasOwnProperty(attr)) {
			const watcher = this._watch[attr]
			watcher.bind(this, newValue, oldValue)()
		}
	}

mutationObserver.js

watch(targetNode, prop, cback) {
    const callback = function (mutationsList, observer) {
        const mutation = mutationsList.find(item => item.attributeName === prop)
        if (mutation) {
            cback(mutation)
        }
    }
    // observer.disconnect()
    const observer = new MutationObserver(callback)
    observer.observe(targetNode, { attributes: true, childList: false, subtree: false })
}

updated.js

	updated(changedProperties) {
		if (!this.watch || !this.watch.apply) {
			return null
		}
		if (!this._watch) {
			this._watch = this.watch()
		}
		changedProperties.forEach((oldValue, attr) => {
			if (this._watch.hasOwnProperty(attr)) {
				const watcher = this._watch[attr]
				watcher.bind(this, this[attr], oldValue)()
			}
		})
	}