Vue 计算属性 getting setting

<!DOCTYPE html>
<html>
    <head>
        <meta charset="utf-8">
        <script src="./vue.v2.6.10.dev.js"></script>
    </head>
    <body>
        <div id="app">
            {{fullName}}
        </div>
        
        <script>
            var vm = new Vue({
                el: '#app',
                data: {
                    firstName: 'Muzico',
                    lastName: 'Liang',
                },
                computed: {
                    fullName: function() {
                        return this.firstName + " " + this.lastName
                    }
                }
            })
        </script>
    </body>
</html>

<!DOCTYPE html>
<html>
    <head>
        <meta charset="utf-8">
        <script src="./vue.v2.6.10.dev.js"></script>
    </head>
    <body>
        <div id="app">
            {{fullName}}
        </div>
        
        <script>
            var vm = new Vue({
                el: '#app',
                data: {
                    firstName: 'Muzico',
                    lastName: 'Liang',
                },
                computed: {
                    fullName: {
                        get: function() {
                            return this.firstName + " " + this.lastName
                        },
                        set: function(value) {
                            var arr = value.split(' ')
                            this.firstName = arr[0]
                            this.lastName = arr[1]
                        }
                    }
                }
            })
        </script>
    </body>
</html>