Form Validation

1. Basic Validation

struct SignUpForm: View {
    @State private var email = ""
    @State private var password = ""

    var isEmailValid: Bool {
        email.contains("@") && email.contains(".")
    }

    var isPasswordValid: Bool {
        password.count >= 8
    }

    var isFormValid: Bool {
        isEmailValid && isPasswordValid
    }

    var body: some View {
        Form {
            TextField("Email", text: $email)

            if !email.isEmpty && !isEmailValid {
                Text("Invalid email format")
                    .foregroundColor(.red)
                    .font(.caption)
            }

            SecureField("Password", text: $password)

            if !password.isEmpty && !isPasswordValid {
                Text("Password must be at least 8 characters")
                    .foregroundColor(.red)
                    .font(.caption)
            }

            Button("Sign Up") { }
                .disabled(!isFormValid)
        }
    }
}

2. Real-time Validation

struct ValidatedTextField: View {
    let label: String
    @Binding var text: String
    let validation: (String) -> Bool
    let errorMessage: String

    var body: some View {
        VStack(alignment: .leading, spacing: 4) {
            TextField(label, text: $text)
                .textFieldStyle(.roundedBorder)
                .overlay(
                    RoundedRectangle(cornerRadius: 6)
                        .stroke(
                            !text.isEmpty && !validation(text) ? .red : .clear,
                            lineWidth: 1
                        )
                )

            if !text.isEmpty && !validation(text) {
                Text(errorMessage)
                    .foregroundColor(.red)
                    .font(.caption)
            }
        }
    }
}

// Usage
ValidatedTextField(
    label: "Email",
    text: $email,
    validation: { $0.contains("@") },
    errorMessage: "Please enter a valid email"
)

3. Form với ViewModel

class FormViewModel: ObservableObject {
    @Published var email = ""
    @Published var password = ""
    @Published var confirmPassword = ""

    var emailError: String? {
        guard !email.isEmpty else { return nil }
        return email.contains("@") ? nil : "Invalid email"
    }

    var passwordError: String? {
        guard !password.isEmpty else { return nil }
        if password.count < 8 { return "Too short" }
        return nil
    }

    var confirmError: String? {
        guard !confirmPassword.isEmpty else { return nil }
        return password == confirmPassword ? nil : "Passwords don't match"
    }

    var isValid: Bool {
        emailError == nil && passwordError == nil && confirmError == nil
            && !email.isEmpty && !password.isEmpty && !confirmPassword.isEmpty
    }
}

4. Submit Button State

Button {
    // Submit
} label: {
    if isLoading {
        ProgressView()
    } else {
        Text("Submit")
    }
}
.disabled(!isFormValid || isLoading)
.opacity(isFormValid ? 1 : 0.5)

---

📝 Best Practices

• Validate on change, not just on submit
• Show errors near the field
• Disable submit until valid
• Use ViewModel for complex validation