vue点击元素变色兄弟元素不变色

<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>点击变色，兄弟元素不变色</title>
</head>
<script src="https://cdn.jsdelivr.net/npm/vue/dist/vue.js"></script>
<style>
    .color{
        color: red;
    }
</style>
<body>
    <div id="app">
        <ul>
            <li :class="{color: list.checked}" @click="click_btn(list, index)" v-for="(list, index) in arr" :key="index">
                -------- {{list.name}} --------
            </li>
        </ul>
    </div>
    <script>
        var app = new Vue({
            el: '#app',
            data: {
                color_red: true,
                arr: [
                    {
                        name: '111111',
                        checked: false
                    },
                    {
                        name: '2222222222',
                        checked: false
                    },
                    {
                        name: '333333333',
                        checked: false
                    }
                ]
            },
            methods: {
                click_btn: function (list, index) {
                    var this_1 = this
                    console.log(list)
                    for(var i in this_1.arr){
                        if( i == index){
                            this_1.arr[i].checked = true
                        }else{
                            this_1.arr[i].checked = false
                        }
                    }
                    console.log(this_1.arr)
                }
            },
            mounted() {

            }
        })
    </script>
</body>

</html>