Use constructor instead of componentWillMount to register Field, Fiel…

…ds, FieldArray. PR comment: #3956 (comment)

src/createField.js

@@ -31,13 +31,6 @@ const createField = (structure: Structure<*, *>) => {
           'Field must be inside a component decorated with reduxForm()'
         )
       }
-    }
-
-    shouldComponentUpdate(nextProps: Props, nextState?: Object) {
-      return shallowCompare(this, nextProps, nextState)
-    }
-
-    componentDidMount() {
       this.context._reduxForm.register(
         this.name,
         'Field',
@@ -46,6 +39,10 @@ const createField = (structure: Structure<*, *>) => {
       )
     }
 
+    shouldComponentUpdate(nextProps: Props, nextState?: Object) {
+      return shallowCompare(this, nextProps, nextState)
+    }
+
     componentWillReceiveProps(nextProps: Props, nextContext: any) {
       const oldName = prefixName(this.context, this.props.name)
       const newName = prefixName(nextContext, nextProps.name)

src/createFieldArray.js

@@ -43,9 +43,6 @@ const createFieldArray = (structure: Structure<*, *>) => {
           'FieldArray must be inside a component decorated with reduxForm()'
         )
       }
-    }
-
-    componentDidMount() {
       this.context._reduxForm.register(
         this.name,
         'FieldArray',

src/createFields.js

@@ -35,18 +35,14 @@ const createFields = (structure: Structure<*, *>) => {
       if (error) {
         throw error
       }
+      const { _reduxForm: { register } } = context
+      this.names.forEach(name => register(name, 'Field'))
     }
 
     shouldComponentUpdate(nextProps: Props) {
       return shallowCompare(this, nextProps)
     }
 
-    componentDidMount() {
-      const { context } = this
-      const { _reduxForm: { register } } = context
-      this.names.forEach(name => register(name, 'Field'))
-    }
-
     componentWillReceiveProps(nextProps: Props) {
       if (!plain.deepEqual(this.props.names, nextProps.names)) {
         const { context } = this