todos/App/Sources/View Modifiers/OnNewlineFocusModifier.swift

import SwiftUI

/// A view modifier that intercepts newlines in a text field and transfers focus to another field.
///
/// This modifier monitors text changes in a `TextField` component and prevents newlines
/// from being added. Instead, when a newline is detected, it updates the focus state to move to
/// a specified field.
struct OnNewlineFocusModifier<Field: Hashable>: ViewModifier {

    // MARK: Properties

    /// Binding to the text being edited.
    @Binding
    private var text: String

    /// Binding to the focus state.
    private let focusState: FocusState<Field?>.Binding

    /// The field to focus when a newline is detected.
    private let targetField: Field

    // MARK: Initializers

    /// Creates a modifier that transfers focus on newline.
    ///
    /// - Parameters:
    ///   - text: A binding to the text being edited.
    ///   - focusState: A binding to the focus state.
    ///   - targetField: The field to focus when a newline is detected.
    init(
        text: Binding<String>,
        focusState: FocusState<Field?>.Binding,
        targetField: Field
    ) {
        self._text = text

        self.focusState = focusState
        self.targetField = targetField
    }

    // MARK: Body

    /// The content and behavior of the view modifier.
    func body(content: Content) -> some View {
        content
            .onChange(of: text) { oldValue, newValue in
                guard newValue.contains(.newline) else {
                    return
                }

                text = oldValue
                focusState.wrappedValue = targetField
            }
    }

}

// MARK: - View Extension

extension View {

    /// Transfers focus to another field when the user presses return in a text field.
    ///
    /// This modifier prevents newlines from being added and instead moves keyboard focus
    /// to the specified field. Useful for creating a form-like flow between text fields.
    ///
    /// - Parameters:
    ///   - text: A binding to the text being edited.
    ///   - field: The field to focus when return is pressed.
    ///   - focusState: A binding to the focus state.
    /// - Returns: A modified view with focus transfer behavior.
    func onNewlineFocus<Field: Hashable>(
        _ text: Binding<String>,
        to field: Field,
        on focusState: FocusState<Field?>.Binding
    ) -> some View {
        modifier(
            OnNewlineFocusModifier(
                text: text,
                focusState: focusState,
                targetField: field
            )
        )
    }

}