[improvement] add portal mixin (#3583)

packages/mixins/portal.js

@@ -0,0 +1,48 @@
+function getElement(selector) {
+  if (typeof selector === 'string') {
+    return document.querySelector(selector);
+  }
+
+  return selector();
+}
+
+export function PortalMixin({ afterPortal }) {
+  return {
+    props: {
+      getContainer: [String, Function]
+    },
+
+    watch: {
+      getContainer() {
+        this.portal();
+      }
+    },
+
+    mounted() {
+      if (this.getContainer) {
+        this.portal();
+      }
+    },
+
+    methods: {
+      portal() {
+        const { getContainer } = this;
+
+        let container;
+        if (getContainer) {
+          container = getElement(getContainer);
+        } else if (this.$parent) {
+          container = this.$parent.$el;
+        }
+
+        if (container && container !== this.$el.parentNode) {
+          container.appendChild(this.$el);
+        }
+
+        if (afterPortal) {
+          afterPortal.call(this);
+        }
+      }
+    }
+  };
+}