How to implement Vue binding class and binding inline style

.box {
    width: 100px;
    height: 100px;
    background-color: gray;
}
.circle {
    border-radius: 50%;
}

<div id="app">
    <div class="box" @click="isCircle = !isCircle" :class="{circle:isCircle}"></div>
    <!--When isCircle is true, the class name of the div will be added with a circle-->
    <p>{{isCircle}}</p>
</div>
<script src="https://cdn.jsdelivr.net/npm/vue/dist/vue.js"></script>
<script>
    new Vue({
        el:"#app",
        data:{
            isCircle: false
        }
    })
</script>

<div class="box" @click="isCircle = !isCircle" :class="divChange"></div>

computed:{
        divChange:function(){
            return {circle:this.isCircle}
        }
    }

.blue {
    background-color: blue;
}

<div id="app">
    <div class="box" v-on:click="clickFun()" :class="[color,divChange()]"></div>
    <p>{{isCircle}}</p>
</div>
<script src="https://cdn.jsdelivr.net/npm/vue/dist/vue.js"></script>
<script>
    new Vue({
        el:"#app",
        data:{
            isCircle: false,
            color:""
        },
        methods:{
            divChange: function(){
                return {circle:this.isCircle}
            },
            clickFun: function(){
                this.isCircle = !this.isCircle;
                if (this.isCircle){
                    this.color = "blue"
                }else{
                    this.color = ""
                }
            }
        }
    })
</script>

<div id="app">
 <!--Bind inline styles-->
    <div class="box" v-on:click="clickFun()" :style="divStyle"></div>
    <p>{{isCircle}}</p>
</div>

divStyle: function(){
    return {
        backgroundColor:this.color
    }
}

.box {
    width: 100px;
    height: 100px;
    background-color: red;
}

.blue {
    background-color: blue;
}

<div id="app">
    <div class="box" :style="borderRadius" @click="changeStyle" :class="{blue: isBlue}"></div>
    <button @click="startClick">Start</button>
</div>

new Vue({
    el: "#app",
    data: {
        isCircle: false,
        borderRadius: { borderRadius: "50%" },
        isStart: false,
        isBlue: false,
        timer: null
    },

    methods: {
        changeStyle: function () {
            if (this.borderRadius.borderRadius == "0%") {
                this.borderRadius.borderRadius = "50%"
            } else {
                this.borderRadius.borderRadius = "0%"
            }
        },
        startClick: function () {
            this.isStart = !this.isStart;
        }
    },

    watch:
        isStart: function (val) {
            var vm = this;
            if (val) {
                console.log("Enable color switching")
                this.timer = setInterval(function () {
                    vm.isBlue = !vm.isBlue
                }, 1000)
            }else{
                console.log("Cancel timer, stop color switching")
                clearInterval(this.timer);
                this.timer = null;
            }

        }
    }
})