怎么访问到子组件的实例或者子元素？

Answer 1

通过

this.$refs

Answer 2

写了完整的demo，你可看下

<div id="app">
    <base-input ref="usernameInput"></base-input>
</div>

<script>
    Vue.component("base-input", {
        template: "<input type='input' ref='input'>",
        methods: {
            popUp() {
                alert(11)
            },
            focus: function () {
                this.$refs.input.focus()
            }
        }
    });

    new Vue({
        el: "#app",
        data: {},
        mounted: function () {
            this.$refs.usernameInput.popUp();
            this.$refs.usernameInput.focus();
        }
    });
</script>

完整demo