var vm = new Vue({
    el: '#app',
    data: {
        a: '1',
        b: 2,
        c: 3
    },
    watch: {
        a: function (val, oldVal) {
            console.log('new: %s, old: %s', val, oldVal)
        },
        // 深度 watcher
        c: {
            handler: function (val, oldVal) {
                console.log(this.a)
                console.log('new: %s, old: %s', val, oldVal)
            },
            deep: true
        }
    },
    methods: {
        click(){
            this.a = 'changed'
            this.b = 'changed'
            this.c = 'changed'
        }
    }

})