fix(safeId): trigger id creation/update after mount (fixes #1978) (#2161)

* fix(safeId): trigger id update on mount.

Uses computed prop to generate a method that returns the ID, which has the same call arguments as the original safeID(suffix) method

Author: tmorehouse

src/mixins/id.js

@@ -1,6 +1,7 @@
 /*
  * SSR Safe Client Side ID attribute generation
- *
+ * id's can only be generated client side, after mount.
+ * this._uid is not synched between server and client.
  */
 
 export default {
@@ -10,21 +11,35 @@ export default {
       default: null
     }
   },
-  methods: {
-    safeId (suffix = '') {
-      const id = this.id || this.localId_ || null
-      if (!id) {
-        return null
-      }
-      suffix = String(suffix).replace(/\s+/g, '_')
-      return suffix ? id + '_' + suffix : id
+  data () {
+    return {
+      localId_: null
     }
   },
+  mounted () {
+    // mounted only occurs client side
+    this.$nextTick(() => {
+      // Update dom with auto ID after dom loaded to prevent
+      // SSR hydration errors.
+      this.localId_ = `__BVID__${this._uid}`
+    })
+  },
   computed: {
-    localId_ () {
-      if (!this.$isServer && !this.id && typeof this._uid !== 'undefined') {
-        return '__BVID__' + this._uid
+    safeId () {
+      // Computed property that returns a dynamic function for creating the ID.
+      // Reacts to changes in both .id and .localId_ And regens a new function
+      const id = this.id || this.localId_
+
+      // We return a function that accepts an optional suffix string
+      // So this computed prop looks and works like a method!!!
+      const fn = (suffix) => {
+        if (!id) {
+          return null
+        }
+        suffix = String(suffix || '').replace(/\s+/g, '_')
+        return suffix ? id + '_' + suffix : id
       }
+      return fn
     }
   }
 }