验证规则与错误消息
在 rules 属性中应用 required、minLength 和 pattern 等内置验证规则，并在每个字段下方渲染错误对象。
验证规则与错误消息 是 CoddyKit 上的免费 React Native Academy 课时。 这是第 2 节课，共 4 节。 你可以在下方免费阅读本课时的完整内容 — 然后在浏览器中使用内置代码编辑器和全天候 AI 导师进行实践。 这是 React Native Academy 学习路径的一部分，你的进度在网页和 CoddyKit 应用中同步。 React Native Academy 课程共包含 4 节课。
本课时的部分内容尚未翻译，以英文显示。
Built-In Validation Rules Overview
react-hook-form provides several built-in validation rules that cover the most common requirements without writing custom code. They are: required, minLength, maxLength, min (for numeric minimum), max (for numeric maximum), and pattern (regex match). Each rule can take a simple value or an object with value and message for a custom error message.
required Rule
The required rule ensures a field is not left empty. You can pass a boolean true (which uses a generic error message) or a string (which becomes the error message). For required fields in mobile forms, always pass a descriptive message so users immediately know what is missing without guessing.
// Simple boolean — uses generic message
rules={{ required: true }}
// String shorthand — message is the string
rules={{ required: 'Please enter your email' }}
// Full object form
rules={{ required: { value: true, message: 'Email is required' } }}minLength and maxLength Rules
Use minLength to enforce a minimum number of characters and maxLength to cap input length. These are essential for username fields (minimum 3 characters), passwords (minimum 8 characters), and bio fields (maximum 200 characters). Both rules compare the string's length property against the value you specify.
rules={{
required: 'Username is required',
minLength: {
value: 3,
message: 'Username must be at least 3 characters',
},
maxLength: {
value: 20,
message: 'Username cannot exceed 20 characters',
},
}}pattern Rule for Regex Validation
The pattern rule tests the field value against a regular expression. Use it for email format, phone numbers, postal codes, or any structured text format. Pass the regex as the value property. React-hook-form tests the entire string against the pattern, so you typically do not need anchors.
// Email validation
rules={{
pattern: {
value: /^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$/,
message: 'Please enter a valid email address',
},
}}
// Phone number (digits only, 10-15 chars)
rules={{
pattern: {
value: /^[0-9]{10,15}$/,
message: 'Enter a valid phone number (digits only)',
},
}}min and max Rules for Numbers
For numeric inputs — quantities, ages, prices — use min and max rules. They compare the numeric value of the input string, not its length. Set keyboardType='numeric' on the TextInput to help users enter numbers. Combine with pattern to ensure only digits are entered.
rules={{
required: 'Age is required',
min: { value: 18, message: 'You must be at least 18 years old' },
max: { value: 120, message: 'Please enter a valid age' },
pattern: {
value: /^[0-9]+$/,
message: 'Age must be a whole number',
},
}}Custom Validation with validate
The validate rule accepts a function (or an object of named functions) that receives the current field value and returns true if valid or an error message string if invalid. Use it for complex requirements that built-in rules cannot express, such as confirming passwords match or checking whether a username is already taken via an async API call.
// Single custom validator
rules={{
validate: (value) =>
value.includes('@') || 'Email must contain an @ symbol',
}}
// Multiple named validators
rules={{
validate: {
notEmpty: (v) => v.trim().length > 0 || 'Cannot be blank',
noSpaces: (v) => !v.includes(' ') || 'No spaces allowed',
},
}}Async validate for Server-Side Checks
The validate function can be async. This is useful for checking whether a username or email is already taken. The function receives the input value, makes an API call, and returns an error string if the value is already in use or true (or undefined) if it is available. The input is marked invalid until the async check resolves.
rules={{
validate: async (email) => {
try {
const { available } = await api.checkEmailAvailability(email);
return available || 'This email is already registered';
} catch {
return true; // Allow if check fails — server will catch it later
}
},
}}Accessing Errors from formState
formState.errors is a nested object where each key is a field name and the value is an error object with a type (which rule failed) and a message (the error string). Access a field's error with errors.fieldName?.message — the optional chaining prevents runtime errors when the field has no error.
const { formState: { errors } } = useForm();
// Accessing errors:
console.log(errors.email?.type); // 'required' | 'pattern' | 'validate'
console.log(errors.email?.message); // 'Email is required'
// Checking if any error exists:
const hasError = !!errors.email;
// Getting error from a named validate:
console.log(errors.username?.type); // 'noSpaces'Styling Inputs Based on Error State
Provide immediate visual feedback by changing the input's border color when it has an error. Pass the error state as a conditional style using an array of styles. A red border on the invalid field plus a red error message text below creates a clear, standard pattern that users recognize from web forms.
const styles = StyleSheet.create({
input: {
borderWidth: 1,
borderColor: '#ccc',
borderRadius: 8,
padding: 12,
},
inputError: {
borderColor: '#d32f2f',
},
errorText: {
color: '#d32f2f',
fontSize: 12,
marginTop: 4,
},
});
<TextInput style={[styles.input, errors.email && styles.inputError]} />
{errors.email && <Text style={styles.errorText}>{errors.email.message}</Text>}Controlling When Errors Appear
By default, errors only show after the first submit attempt. After that, react-hook-form re-validates in real time as the user fixes each field. You can change this with the mode option passed to useForm. The 'onBlur' mode shows errors as soon as a field loses focus, which gives earlier feedback without distracting red messages while the user is still typing.
const form = useForm<SignUpForm>({
mode: 'onBlur', // validate on field blur
reValidateMode: 'onChange', // re-validate on every change after first error
defaultValues: { name: '', email: '', password: '' },
});Server-Side Errors with setError
After the form passes client-side validation and the API call fails with a server error — like 'Email already exists' — use setError to inject an error into a specific field. This re-triggers the error display for that field, giving the user precise feedback without re-running all validation rules from scratch.
const { setError, handleSubmit } = useForm<RegisterForm>();
async function onSubmit(data: RegisterForm) {
try {
await api.register(data);
navigation.replace('Home');
} catch (err: any) {
if (err.code === 'EMAIL_EXISTS') {
setError('email', {
type: 'server',
message: 'This email is already registered. Please log in.',
});
}
}
}Quick Check
Test your understanding of React Native Mobile Development concepts from this lesson.
Lesson Recap
In this lesson you learned: built-in rules (required, minLength, maxLength, pattern, min, max) cover most validation needs, validate handles complex and async custom checks, and setError injects server-side errors into specific fields after a failed API call. Next up we explore building multi-step forms with form state.
常见问题解答
「验证规则与错误消息」课时是免费的吗？
是的 — 「验证规则与错误消息」的完整文本可在网页上免费阅读。要进行交互式练习（内置代码编辑器和全天候 AI 导师）并解锁 React Native Academy 课程的其余内容，请升级到 CoddyKit PRO。 React Native Academy 课程共包含 4 节课。
「验证规则与错误消息」这节课中我会学到什么？
在 rules 属性中应用 required、minLength 和 pattern 等内置验证规则，并在每个字段下方渲染错误对象。 你通过在浏览器中直接运行的动手代码来练习 React Native Academy，全天候 AI 导师会在你学习这节课的过程中回答你的问题。
学习 React Native Academy 需要有经验吗？
无需任何先前经验。CoddyKit 上的 React Native Academy 课程适合初学者到高级学习者，你可以从这里开始或从头开始，按照自己的节奏学习。 这是第 2 节课，共 4 节。
「验证规则与错误消息」课时需要多长时间？
大多数 CoddyKit 课程大约需要 5–10 分钟。每节课都很精短且互动，所以你能稳步进步，并在网页和应用中从离开的地方继续。
我能在这节 React Native Academy 课中编写并运行代码吗？
能。每节 React Native Academy 课都包含内置代码编辑器，你可以在浏览器中直接编写并运行真实代码，并获得即时 AI 反馈 — 无需本地设置。