การตั้งค่า react-hook-form ร่วมกับ Controller

react-hook-form is a performant, flexible library for building forms in React and React Native. Unlike manually wiring every input to useState, react-hook-form uses uncontrolled inputs to minimize re-renders. It integrates validation, error handling, and form submission into a single coherent API, and it works seamlessly with any UI component library through the Controller wrapper.

Install react-hook-form from npm. It has no dependencies and works with any React version 16.8+. No special configuration is needed — import its hooks and components directly into your React Native components after installation.

Call useForm() at the top of your component to create a form instance. It returns several utilities: control (for wiring inputs via Controller), handleSubmit (for form submission), formState.errors (for displaying validation errors), and reset (to clear all fields). Pass default values to useForm to pre-populate the form.

In React Native, inputs are controlled components that need to be wrapped in Controller so react-hook-form can manage their values. Pass the control object, the field name, and a render prop. The render prop receives field which contains value, onChange, and onBlur to wire to the TextInput.

Pass a rules prop to Controller to define validation constraints. The required rule marks a field as mandatory, minLength enforces a minimum character count, maxLength caps input length, and pattern validates against a regex. Each rule takes an object with a value and a message string shown when the rule fails.

After Controller validates a field, any error is available in formState.errors[fieldName]. Access its message property and render it below the input in a red Text component. Errors only appear after the field has been touched (onBlur) or after the first submit attempt, preventing premature error messages.

Wrap your submit handler with handleSubmit. react-hook-form will run all validation rules first; if any field is invalid, it populates formState.errors and does not call your handler. If all fields are valid, your handler receives a typed object containing all field values — ready to send to an API.

For password fields, set secureTextEntry to hide the typed characters and configure an appropriate keyboard type. Wire the field to Controller exactly like the email field. Many apps also add a 'show/hide' icon button that toggles secureTextEntry via local state — this improves accessibility for users with typos.

The watch function returned by useForm lets you subscribe to a field's current value and react to changes in real time. A common use case is a 'confirm password' field that validates its value matches the password field. Pass a field name to watch and use the returned value in the confirm field's validate rule.

Call reset() to clear all field values and errors, returning the form to its default values. You can also pass a new set of values to reset({ email: 'prefilled@example.com' }) to re-initialize the form with different defaults, which is useful for edit-profile forms that pre-populate with existing user data.

react-hook-form offers several mode settings that control when validation runs: 'onSubmit' (default, validates only on submit), 'onBlur' (validates when a field loses focus), 'onChange' (validates on every keystroke), and 'all' (combines onBlur and onChange). For mobile forms, 'onBlur' offers the best balance between immediate feedback and avoiding distracting errors while typing.

Test your understanding of React Native Mobile Development concepts from this lesson.

In this lesson you learned: useForm creates the form instance with control, handleSubmit, and formState.errors, Controller wires native TextInput components to the form with onChange, onBlur, and value props, and rules define built-in validation constraints that populate errors automatically. Next up we explore validation rules and error message handling in depth.