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packages/backtop/index.js

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+import Backtop from './src/main';
+
+/* istanbul ignore next */
+Backtop.install = function(Vue) {
+  Vue.component(Backtop.name, Backtop);
+};
+
+export default Backtop;

packages/backtop/src/main.vue

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+<template>
+  <transition name="el-fade-in">
+    <div
+      v-if="visible"
+      @click.stop="handleClick"
+      :style="{
+        'right': styleRight,
+        'bottom': styleBottom
+      }"
+      class="el-backtop">
+      <slot>
+        <el-icon name="caret-top"></el-icon>
+      </slot>
+    </div>
+  </transition>
+</template>
+
+<script>
+import throttle from 'throttle-debounce/throttle';
+
+const cubic = value => Math.pow(value, 3);
+const easeInOutCubic = value => value < 0.5
+  ? cubic(value * 2) / 2
+  : 1 - cubic((1 - value) * 2) / 2;
+
+export default {
+  name: 'ElBacktop',
+
+  props: {
+    visibilityHeight: {
+      type: Number,
+      default: 200
+    },
+    target: [String],
+    right: {
+      type: Number,
+      default: 40
+    },
+    bottom: {
+      type: Number,
+      default: 40
+    }
+  },
+
+  data() {
+    return {
+      el: null,
+      container: null,
+      visible: false
+    };
+  },
+
+  computed: {
+    styleBottom() {
+      return `${this.bottom}px`;
+    },
+    styleRight() {
+      return `${this.right}px`;
+    }
+  },
+
+  mounted() {
+    this.init();
+    this.throttledScrollHandler = throttle(300, this.onScroll);
+    this.container.addEventListener('scroll', this.throttledScrollHandler);
+  },
+
+  methods: {
+    init() {
+      this.container = document;
+      this.el = document.documentElement;
+      if (this.target) {
+        this.el = document.querySelector(this.target);
+        if (!this.el) {
+          throw new Error(`target is not existed: ${this.target}`);
+        }
+        this.container = this.el;
+      }
+    },
+    onScroll() {
+      const scrollTop = this.el.scrollTop;
+      this.visible = scrollTop >= this.visibilityHeight;
+    },
+    handleClick(e) {
+      this.scrollToTop();
+      this.$emit('click', e);
+    },
+    scrollToTop() {
+      const el = this.el;
+      const beginTime = Date.now();
+      const beginValue = el.scrollTop;
+      const rAF = window.requestAnimationFrame || (func => setTimeout(func, 16));
+      const frameFunc = () => {
+        const progress = (Date.now() - beginTime) / 500;
+        if (progress < 1) {
+          el.scrollTop = beginValue * (1 - easeInOutCubic(progress));
+          rAF(frameFunc);
+        } else {
+          el.scrollTop = 0;
+        }
+      };
+      rAF(frameFunc);
+    }
+  },
+
+  beforeDestroy() {
+    this.container.removeEventListener('scroll', this.throttledScrollHandler);
+  }
+};
+</script>