Vue2 初始化流程 初始化属性，变量方法，监听事件等等=> $mount => mountComponent => new Watcher和_update 数据响应式 observe => new Observer => dep => setter最终触发watcher._update 枚举：方法变异 批量初步更新 watcher.update => queueWatcher => queue.push(watcher) + nextTick （