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Form Binding with Dynamic Validation

Combine Vaadin Binder with signals for forms where validation rules depend on other field values, enabling reactive cross-field validation with dynamic error messages.

This guide shows how to combine Vaadin Binder with signals for forms that have validation rules depending on other field values. You’ll learn how to use signals for cross-field validation and bind UI elements to validation status, all while leveraging Binder’s built-in validation features.

The Use Case

Forms often need validation rules that depend on other field values. For example, a registration form where age requirements change based on account type: personal accounts need users to be 14 or older, while business accounts require 18 or older. Additionally, you want to show real-time validation status and enable the submit button only when all fields are valid.

Signals simplify this by making validation rules and UI state reactive. When a user changes the account type, validation rules automatically update and the UI reflects the new state immediately—no manual coordination needed.

The Solution

Source code
BinderIntegrationExample.java

How It Works

Signals for Dynamic Values

Create ValueSignal instances for fields that affect validation logic:

Source code
Java
ValueSignal<AccountType> accountTypeSignal = new ValueSignal<>(AccountType.PERSONAL);
ValueSignal<Integer> ageSignal = new ValueSignal<>(0);

These signals track the current values of fields that other validation rules depend on. When these values change, dependent validations update automatically.

Two-Way Binding with Form Fields

Use bindValue() to connect signals to form fields:

Source code
Java
accountTypeSelect.bindValue(accountTypeSignal);
ageField.bindValue(ageSignal);

This creates a two-way connection: when the user changes the field, the signal updates, and when code updates the signal, the field updates. See Two-Way Signal Mapping for more details about bidirectional bindings.

Computed Signals for Dynamic Validation

Create a computed signal that derives the validation result from multiple signals:

Source code
Java
Signal<Boolean> ageValidSignal = Signal.computed(() -> {
    Integer age = ageSignal.value();
    AccountType accountType = accountTypeSignal.value();
    if (age == null) {
        return false;
    }
    return accountType == AccountType.BUSINESS ? age >= 18 : age >= 13;
});

This signal automatically recalculates whenever ageSignal or accountTypeSignal changes. The validation logic is centralized and reactive.

Using Signals in Binder Validators

Reference the computed signal in your Binder validator:

Source code
Java
binder.forField(ageField)
    .withValidator(value -> ageValidSignal.value(), value -> {
        AccountType accountType = accountTypeSignal.value();
        return accountType == AccountType.BUSINESS
            ? "Business accounts require age 18 or older"
            : "Personal accounts require age 13 or older";
    })
    .bind("age");

The validator checks ageValidSignal.value() and provides a dynamic error message based on the current account type. When the user changes the account type, Binder automatically re-runs validation because the signal value changed.

Cross-Field Validation with Binding Values

For simpler cross-field validation that doesn’t need signals, use Binder.Binding.value() - binding validator re-runs always when a binding value changes:

Source code
Java
Binder.Binding<UserRegistration, String> pwBinding = binder
    .forField(passwordField)
    .withValidator(value -> value != null && value.length() >= 8,
        "Password must be at least 8 characters")
    .bind("password");

binder.forField(confirmPasswordField)
    .withValidator(value -> value != null && value.equals(pwBinding.value()),
        "Passwords do not match")
    .bind("confirmPassword");

You can mix binding.value() and signal.value() in the same validator - Binder re-validates whenever any referenced binding or signal value changes. Use signals when you need to share validation state across multiple components or derive complex logic from multiple sources.

Reactive Submit Button

Enable the submit button only when the form is valid using bindEnabled():

Source code
Java
submitButton.bindEnabled(
    binder.getValidationStatus().map(BinderValidationStatus::isOk));

The getValidationStatus() method returns a signal that updates whenever validation state changes. The map() method transforms it to a boolean that controls the button’s enabled state.

Reactive Status Display

Show form status with reactive text and styling:

Source code
Java
Span statusLabel = new Span();
statusLabel.bindText(binder.getValidationStatus()
    .map(status -> status.isOk() ? "Form is valid - Ready to submit"
        : "Please complete all required fields correctly"));
statusLabel.getStyle().bind("color", binder.getValidationStatus()
    .map(status -> status.isOk() ? "green" : "orange"));

Multiple bindings to the same signal are efficient - the signal value is cached and shared across all dependent components.

Key Patterns

Use signals for dynamic validation

When validation rules depend on other field values, use a binding value or a signal that holds a dependency state. Binder automatically re-validates when binding or signal values change.

Two-way binding with bindValue()

Connect form fields to signals so changes flow in both directions automatically.

Computed signals for derived logic

Create computed signals that derive validation state from multiple source signals. This centralizes logic and makes dependencies explicit.

Reactive UI elements

Bind buttons, labels, and styles to Binder’s validation status for real-time feedback.

When to Use This Pattern

This pattern is ideal when:

  • Validation rules depend on other form fields

  • Error messages need to be dynamic based on field values

  • You want real-time validation feedback as users type

  • You need to enable/disable submit buttons based on form validity

For simpler forms with static validation rules, standard Binder validators without signals may be sufficient.