<template>
    <transition :name="transitionName">
        <div
            v-show="visible"
            @click="backToTop"
            :style="customStyle"
            class="back-to-ceiling">
            <svg
                width="16"
                height="16"
                aria-hidden="true"
                viewBox="0 0 17 17"
                class="Icon Icon--backToTopArrow"
                xmlns="http://www.w3.org/2000/svg"
                style="height: 16px; width: 16px">
                <path d="M12.036 15.59a1 1 0 0 1-.997.995H5.032a.996.996 0 0 1-.997-.996V8.584H1.03c-1.1 0-1.36-.633-.578-1.416L7.33.29a1.003 1.003 0 0 1 1.412 0l6.878 6.88c.782.78.523 1.415-.58 1.415h-3.004v7.004z"/>
            </svg>
        </div>
    </transition>
</template>
<script>
export default {
    name: "BackToTop",
    props: {
        // 顯示時的高度
        visibilityHeight: {
            type: Number,
            default: 400,
        },
        // 滾動置頂位置
        backPosition: {
            type: Number,
            default: 0,
        },
        // 自定義的樣式
        customStyle: {
            type: Object,
            default: function () {
                return {
                    right: "50px",
                    bottom: "50px",
                    width: "40px",
                    height: "40px",
                    "border-radius": "4px",
                    "line-height": "45px",
                    background: "#e7eaf1",
                };
            },
        },
        // 定義滾動動畫
        transitionName: {
            type: String,
            default: "fade",
        },
    },
    data() {
        return {
            // 是否可見
            visible: false,
            // 滾動定時器
            interval: null,
            // 正在滾動中
            isMoving: false,
        };
    },
    mounted() {
        window.addEventListener("scroll", this.handleScroll);
    },
    beforeDestroy() {
        window.removeEventListener("scroll", this.handleScroll);
        if (this.interval) {
            clearInterval(this.interval);
        }
    },
    methods: {
        // 頁面滾動事件
        handleScroll() {
            this.visible = window.pageYOffset > this.visibilityHeight;
        },
        // 點擊頁面置頂
        backToTop() {
            if (this.isMoving) return;
            const start = window.pageYOffset;
            let i = 0;
            this.isMoving = true;
            this.interval = setInterval(() => {
                const next = Math.floor(
                    this.easeInOutQuad(10 * i, start, -start, 500)
                );
                if (next <= this.backPosition) {
                    window.scrollTo(0, this.backPosition);
                    clearInterval(this.interval);
                    this.isMoving = false;
                } else {
                    window.scrollTo(0, next);
                }
                i++;
            }, 16.7);
        },
        // 速度遞減運算
        easeInOutQuad(t, b, c, d) {
            if ((t /= d / 2) < 1){
                return (c / 2) * t * t + b
            };
            return (-c / 2) * (--t * (t - 2) - 1) + b;
        },
    },
};
</script>
<style scoped>
.back-to-ceiling {
    position: fixed;
    display: inline-block;
    text-align: center;
    cursor: pointer;
}
.back-to-ceiling:hover {
    background: #d5dbe7;
}
.fade-enter-active,
.fade-leave-active {
    transition: opacity 0.5s;
}
.fade-enter,
.fade-leave-to {
    opacity: 0;
}
.back-to-ceiling .Icon {
    fill: #9aaabf;
    background: none;
}
</style>