<template>
  <view class="">
    <view :class="[elClass]" :style="{
			height: fixed ? height + 'px' : 'auto',
			backgroundColor: bgColor
		}" class="u-sticky-wrap">
      <view :style="{
				position: fixed ? 'fixed' : 'static',
				top: stickyTop + 'px',
				left: left + 'px',
				width: width == 'auto' ? 'auto' : width + 'px',
				zIndex: uZIndex
			}" class="u-sticky">
        <slot></slot>
      </view>
    </view>
  </view>
</template>

<script>
/**
 * sticky 吸顶
 * @description 该组件与CSS中position: sticky属性实现的效果一致，当组件达到预设的到顶部距离时， 就会固定在指定位置，组件位置大于预设的顶部距离时，会重新按照正常的布局排列。
 * @tutorial https://www.uviewui.com/components/sticky.html
 * @property {String Number} offset-top 吸顶时与顶部的距离，单位rpx（默认0）
 * @property {String Number} index 自定义标识，用于区分是哪一个组件
 * @property {Boolean} enable 是否开启吸顶功能（默认true）
 * @property {String} bg-color 组件背景颜色（默认#ffffff）
 * @property {String Number} z-index 吸顶时的z-index值（默认970）
 * @property {String Number} h5-nav-height 导航栏高度，自定义导航栏时(无导航栏时需设置为0)，需要传入此值，单位px（默认44）
 * @event {Function} fixed 组件吸顶时触发
 * @event {Function} unfixed 组件取消吸顶时触发
 * @example <u-sticky offset-top="200"><view>塞下秋来风景异，衡阳雁去无留意</view></u-sticky>
 */
export default {
  name: "u-sticky",
  props: {
    // 吸顶容器到顶部某个距离的时候，进行吸顶，在H5平台，NavigationBar为44px
    offsetTop: {
      type: [Number, String],
      default: 0
    },
    //列表中的索引值
    index: {
      type: [Number, String],
      default: ''
    },
    // 是否开启吸顶功能
    enable: {
      type: Boolean,
      default: true
    },
    // h5顶部导航栏的高度
    h5NavHeight: {
      type: [Number, String],
      default: 44
    },
    // 吸顶区域的背景颜色
    bgColor: {
      type: String,
      default: '#ffffff'
    },
    // z-index值
    zIndex: {
      type: [Number, String],
      default: ''
    }
  },
  data() {
    return {
      fixed: false,
      height: 'auto',
      stickyTop: 0,
      elClass: this.$u.guid(),
      left: 0,
      width: 'auto',
    };
  },
  watch: {
    offsetTop(val) {
      this.initObserver();
    },
    enable(val) {
      if (val == false) {
        this.fixed = false;
        this.disconnectObserver('contentObserver');
      } else {
        this.initObserver();
      }
    }
  },
  computed: {
    uZIndex() {
      return this.zIndex ? this.zIndex : this.$u.zIndex.sticky;
    }
  },
  mounted() {
    this.initObserver();
  },
  methods: {
    initObserver() {
      if (!this.enable) return;
      // #ifdef H5
      this.stickyTop = this.offsetTop != 0 ? uni.upx2px(this.offsetTop) + this.h5NavHeight : this.h5NavHeight;
      // #endif
      // #ifndef H5
      this.stickyTop = this.offsetTop != 0 ? uni.upx2px(this.offsetTop) : 0;
      // #endif

      this.disconnectObserver('contentObserver');
      this.$uGetRect('.' + this.elClass).then((res) => {
        this.height = res.height;
        this.left = res.left;
        this.width = res.width;
        this.$nextTick(() => {
          this.observeContent();
        });
      });
    },
    observeContent() {
      this.disconnectObserver('contentObserver');
      const contentObserver = this.createIntersectionObserver({
        thresholds: [0.95, 0.98, 1]
      });
      contentObserver.relativeToViewport({
        top: -this.stickyTop
      });
      contentObserver.observe('.' + this.elClass, res => {
        if (!this.enable) return;
        this.setFixed(res.boundingClientRect.top);
      });
      this.contentObserver = contentObserver;
    },
    setFixed(top) {
      const fixed = top < this.stickyTop;
      if (fixed) this.$emit('fixed', this.index);
      else if (this.fixed) this.$emit('unfixed', this.index);
      this.fixed = fixed;
    },
    disconnectObserver(observerName) {
      const observer = this[observerName];
      observer && observer.disconnect();
    },
  },
  beforeDestroy() {
    this.disconnectObserver('contentObserver');
  }
};
</script>

<style lang="scss" scoped>
@import "../../libs/css/style.components.scss";

.u-sticky {
  z-index: 9999999999;
}
</style>
