Change isSubmitting behaviour to mimic v1 (#1987)

In version 2 the behaviour of isSubmitting was changed from having the
user handle its state after submitting to always resetting it to false
after the onSubmit handler had returned (or it's promise).

This adds back to the version 1 behaviour of not touching the
isSubmitting state after the onSubmit handler has been called if we
don't return a promise in the onSubmit handler.

Solves: #1957

src/Form.tsx

@@ -8,7 +8,7 @@ export type FormikFormProps = Pick<
     'onReset' | 'onSubmit'
   >
 >;
-    
+
 type FormProps = React.ComponentPropsWithoutRef<'form'>;
 
 // @todo tests

src/Formik.tsx

@@ -710,7 +710,12 @@ export function useFormik<Values extends FormikValues = FormikValues>({
       (combinedErrors: FormikErrors<Values>) => {
         const isActuallyValid = Object.keys(combinedErrors).length === 0;
         if (isActuallyValid) {
-          return Promise.resolve(executeSubmit())
+          const promiseOrUndefined = executeSubmit();
+          if (promiseOrUndefined === undefined) {
+            return;
+          }
+
+          return Promise.resolve(promiseOrUndefined)
             .then(() => {
               if (!!isMounted.current) {
                 dispatch({ type: 'SUBMIT_SUCCESS' });

src/types.tsx

@@ -203,7 +203,10 @@ export interface FormikConfig<Values> extends FormikSharedConfig {
   /**
    * Submission handler
    */
-  onSubmit: (values: Values, formikHelpers: FormikHelpers<Values>) => void;
+  onSubmit: (
+    values: Values,
+    formikHelpers: FormikHelpers<Values>
+  ) => void | Promise<void>;
   /**
    * A Yup Schema or a function that returns a Yup schema
    */

test/Formik.test.tsx

@@ -1144,7 +1144,7 @@ describe('<Formik>', () => {
     expect(getProps().submitCount).toEqual(1);
   });
 
-  it('isSubmitting is fired when submit is attempted', async () => {
+  it('isSubmitting is fired when submit is attempted (v1)', async () => {
     const onSubmit = jest.fn();
     const validate = jest.fn(() => Promise.resolve({}));
 
@@ -1164,6 +1164,34 @@ describe('<Formik>', () => {
     // do it again async
     await validatePromise;
     // done validating and submitting
+    expect(getProps().isSubmitting).toBe(true);
+    expect(getProps().isValidating).toBe(false);
+    expect(validate).toHaveBeenCalled();
+    expect(onSubmit).toHaveBeenCalled();
+    expect(getProps().submitCount).toEqual(1);
+  });
+
+  it('isSubmitting is fired when submit is attempted (v2, promise)', async () => {
+    const onSubmit = jest.fn().mockResolvedValue(undefined);
+    const validate = jest.fn(() => Promise.resolve({}));
+
+    const { getProps } = renderFormik({
+      onSubmit,
+      validate,
+    });
+
+    expect(getProps().submitCount).toEqual(0);
+    expect(getProps().isSubmitting).toBe(false);
+    expect(getProps().isValidating).toBe(false);
+    // we call set isValidating synchronously
+    const validatePromise = getProps().submitForm();
+    // so it should change
+    expect(getProps().isSubmitting).toBe(true);
+    expect(getProps().isValidating).toBe(true);
+    // do it again async
+    await validatePromise;
+    // done validating and submitting
+    expect(getProps().isSubmitting).toBe(false);
     expect(getProps().isValidating).toBe(false);
     expect(validate).toHaveBeenCalled();
     expect(onSubmit).toHaveBeenCalled();