🐛 bug(validation): fix unregsiter error [ci skip]

Closes #336
Closes #338
Closes #347

Author: kazupon

src/components/validity-result-group.js

@@ -87,6 +87,9 @@ export default function (Vue: GlobalAPI): Object {
           this.resetResults(name)
         })
       },
+      validityCount (): Number {
+        return this._validityKeys.length
+      },
       isRegistered (name: string): boolean {
         return name in this._validities
       },

src/components/validity/lifecycles.js

@@ -74,7 +74,7 @@ export default function (Vue: GlobalAPI): Object {
     if (validation) {
       const { instance, name } = validation
       const group = this.group
-      instance.unregister(this.field, this, { named: name, group })
+      instance.unregister(this.field, { named: name, group })
     }
 
     if (this._unwatchResultProp) {

src/validation.js

@@ -83,20 +83,29 @@ export default function (Vue: any): any {
       const namedValidity: ?ValidityGroupComponent = named ? this._getValidityGroup('named', named) : null
       if (named && group && namedValidity && groupValidity) {
         groupValidity.unregister(field)
-        namedValidity.isRegistered(group) && namedValidity.unregister(group)
-        this._validityManager.isRegistered(named) && this._validityManager.unregister(named)
+        if (groupValidity.validityCount() === 0) {
+          namedValidity.isRegistered(group) && namedValidity.unregister(group)
+          this._unregisterValidityGroup('group', group)
+        }
+        if (namedValidity.validityCount() === 0) {
+          this._validityManager.isRegistered(named) && this._validityManager.unregister(named)
+          this._unregisterValidityGroup('named', named)
+        }
       } else if (namedValidity) {
         namedValidity.unregister(field)
-        this._validityManager.isRegistered(named) && this._validityManager.unregister(named)
+        if (namedValidity.validityCount() === 0) {
+          this._validityManager.isRegistered(named) && this._validityManager.unregister(named)
+          this._unregisterValidityGroup('named', named)
+        }
       } else if (groupValidity) {
         groupValidity.unregister(field)
-        this._validityManager.isRegistered(group) && this._validityManager.unregister(group)
+        if (groupValidity.validityCount() === 0) {
+          this._validityManager.isRegistered(group) && this._validityManager.unregister(group)
+          this._unregisterValidityGroup('group', group)
+        }
       } else {
         this._validityManager.unregister(field)
       }
-
-      group && this._unregisterValidityGroup('group', group)
-      named && this._unregisterValidityGroup('named', named)
     }
 
     destroy (): void {