Make Binder.validate() work in buffered mode with bean-level validations.

[mvysny]

Describe your motivation

Vaadin 14.9.4.

I am using binder in a buffered mode:

• binder.readBean(bean)
• On save -> if (binder.validate().isOk() && binder.writeBeanIfValid(bean)) { dialog.close() } else {Notification.show("Errors in the form"); }

I have to call binder.validate() otherwise the HasValue.errorMessage wouldn't get populated.

The problem is that binder.validate() fails in buffered mode with

java.lang.IllegalStateException: Cannot validate binder: bean level validators have been configured but no bean is currently set
    at com.vaadin.flow.data.binder.Binder.validate(Binder.java:2479) ~[flow-data-9.0.8.jar:9.0.8]
    at com.vaadin.flow.data.binder.Binder.validate(Binder.java:2463) ~[flow-data-9.0.8.jar:9.0.8]

Describe the solution you'd like

• Instead of failing, validate() could skip bean-level validations. This is preferred: even though it changes the way validate() works now, the change is nowhere being big.
• Alternatively, validate(bean) would run bean-level validations and roll-back changes afterwards. Also works for me.
• Alternatively, add a setBeanLevelValidationsEnabled(false) function which temporarily disables bean-level validations, so that validate() can be called.

Essentially a reopen of https://github.com/vaadin/flow/issues/13393

I don't think that Binder.setValidationStatusHandler(...) would be helpful in this case:

• I have to call validate() at some point, in order to populate HasValue.errorMessage
• If I try implement the bean validations myself (as simple checks after binder.writeBeanIfValid(bean)), I'm hitting a problem soon: there's no way to roll back the changes done to the bean.

[mvysny]

Workaround: clear Binder.validators via reflection, then call validate(), then populate Binder.validators back. I'll prototype this solution then I'll post a demo code.

[mvysny]

Example code of a buffered binder which:

1. calls all validators,
2. writes stuff to the bean, but only if all validators pass
3. If any validator fails, appropriate fields are marked red
4. If any validator fails, a notification error message is shown

public class BinderUtils {
    /**
     * Writes values to given bean, but only if all validations pass. If any of the validation fails, a nice validation error message
     * is shown to the user.
     * @param binder the binder, not null.
     * @param bean the bean to write the values to, not null.
     * @param <T> the type of the bean
     * @return true if everything went okay and the new values are written to the bean, false if not.
     */
    public static <T> boolean writeBeanIfValid(@NotNull Binder<T> binder, @NotNull T bean) {
        // temporarily remove bean-level validators. Workaround for https://github.com/vaadin/flow/issues/15701
        final List<Validator<? super T>> beanValidators = getBeanValidators(binder);
        setBeanValidators(binder, new ArrayList<>());
        final boolean isFieldLevelValidationsOk;
        try {
            isFieldLevelValidationsOk = binder.validate().isOk();
        } finally {
            setBeanValidators(binder, beanValidators);
        }

        String errorMessage = null;
        try {
            if (isFieldLevelValidationsOk) {
                binder.writeBean(bean);
                // all OK, return true.
                return true;
            }
        } catch (ValidationException ex) {
            if (!ex.getBeanValidationErrors().isEmpty()) {
                final ValidationResult err = ex.getBeanValidationErrors().get(0);
                errorMessage = err.getErrorMessage();
            }
        }
        if (errorMessage == null || errorMessage.isBlank()) {
            errorMessage = tr(I18NCommon.ERRORS_IN_FORM);
        }
        MessageNotification.warningMessage(errorMessage);
        return false;
    }

    @SuppressWarnings("unchecked")
    @NotNull
    private static <T> List<Validator<? super T>> getBeanValidators(@NotNull Binder<T> binder) {
        try {
            final Field f = Binder.class.getDeclaredField("validators");
            f.setAccessible(true);
            return new ArrayList<>((List<Validator<? super T>>) f.get(binder));
        } catch (Exception ex) {
            throw new RuntimeException(ex);
        }
    }

    private static <T> void setBeanValidators(@NotNull Binder<T> binder, @NotNull List<Validator<? super T>> validators) {
        try {
            final Field f = Binder.class.getDeclaredField("validators");
            f.setAccessible(true);
            f.set(binder, new ArrayList<>(validators));
        } catch (Exception ex) {
            throw new RuntimeException(ex);
        }
    }
}