[new feature] add sticky component (#3888)

src/sticky/demo/index.vue

@@ -0,0 +1,76 @@
+<template>
+  <demo-section>
+    <demo-block :title="$t('basicUsage')">
+      <van-sticky>
+        <van-button
+          type="primary"
+          style="margin-left: 15px;"
+        >
+          {{ $t('basicUsage') }}
+        </van-button>
+      </van-sticky>
+    </demo-block>
+
+    <demo-block :title="$t('offsetTop')">
+      <van-sticky :offset-top="50">
+        <van-button
+          type="info"
+          style="margin-left: 115px;"
+        >
+          {{ $t('offsetTop') }}
+        </van-button>
+      </van-sticky>
+    </demo-block>
+
+    <demo-block :title="$t('setContainer')">
+      <div
+        ref="container"
+        style="height: 150px; background-color: #fff;"
+      >
+        <van-sticky :container="container">
+          <van-button
+            type="warning"
+            style="margin-left: 215px;"
+          >
+            {{ $t('setContainer') }}
+          </van-button>
+        </van-sticky>
+      </div>
+    </demo-block>
+  </demo-section>
+</template>
+
+<script>
+export default {
+  i18n: {
+    'zh-CN': {
+      offsetTop: '吸顶距离',
+      setContainer: '指定容器'
+    },
+    'en-US': {
+      offsetTop: 'Offset Top',
+      setContainer: 'Set Container'
+    }
+  },
+
+  data() {
+    return {
+      container: null
+    };
+  },
+
+  mounted() {
+    this.container = this.$refs.container;
+  }
+};
+</script>
+
+<style lang="less">
+.demo-sticky {
+  height: 200vh;
+
+  .van-button {
+    margin-left: 15px;
+  }
+}
+</style>