prompt/Sources/Prompt/ComposerView.swift

import AppKit
import SwiftUI

/// A chat-style composer backed by AppKit: wraps long prompts, grows to fit up
/// to seven lines, then scrolls. Return submits; Shift+Return inserts a
/// newline. A plain SwiftUI `TextField(axis: .vertical)` cannot keep Return as
/// "send", which is why this is not one.
///
/// Height is driven by the caller: the representable measures the text and
/// writes the clamped height into `height`, which the caller applies as a
/// `.frame(height:)`. Driving the frame is what actually resizes a
/// representable — `intrinsicContentSize` is ignored once SwiftUI offers a
/// bigger size.
struct ComposerView: NSViewRepresentable {
    @Binding var text: String
    @Binding var isFocused: Bool
    @Binding var height: CGFloat
    var placeholder: String
    var onSubmit: () -> Void
    var onEscape: (() -> Void)? = nil

    private let maxLines: CGFloat = 7

    /// The one-line height, so the composer starts at exactly one line instead
    /// of snapping to its real height on the first keystroke.
    static var oneLineHeight: CGFloat {
        let font = NSFont.systemFont(ofSize: NSFont.systemFontSize)
        return font.ascender - font.descender + font.leading + 10 // 2 x 5 inset
    }

    func makeCoordinator() -> Coordinator { Coordinator(self) }

    func makeNSView(context: Context) -> NSScrollView {
        let scrollView = NSScrollView()
        scrollView.drawsBackground = false
        scrollView.borderType = .noBorder
        scrollView.hasVerticalScroller = true
        scrollView.autohidesScrollers = true
        scrollView.horizontalScrollElasticity = .none

        let textView = ComposerTextView()
        textView.delegate = context.coordinator
        textView.placeholderText = placeholder
        textView.isRichText = false
        textView.isVerticallyResizable = true
        textView.isHorizontallyResizable = false
        textView.autoresizingMask = [.width]
        textView.textContainer?.widthTracksTextView = true
        textView.textContainer?.lineFragmentPadding = 0
        textView.textContainerInset = NSSize(width: 0, height: 5)
        textView.font = .systemFont(ofSize: NSFont.systemFontSize)
        textView.textColor = .labelColor
        textView.drawsBackground = false
        // No containerSize: with widthTracksTextView the container follows the
        // view's width, which is what makes long lines wrap.

        scrollView.documentView = textView
        textView.frame = NSRect(origin: .zero, size: scrollView.contentSize)
        context.coordinator.watchWindow(of: textView)
        return scrollView
    }

    func updateNSView(_ scrollView: NSScrollView, context: Context) {
        guard let textView = scrollView.documentView as? ComposerTextView else { return }
        context.coordinator.parent = self
        if textView.string != text {
            textView.string = text
            updateHeight(textView)
        }
        textView.placeholderText = placeholder
        if isFocused, scrollView.window?.firstResponder != textView {
            scrollView.window?.makeFirstResponder(textView)
        }
    }

    fileprivate func updateHeight(_ textView: ComposerTextView) {
        guard let layoutManager = textView.layoutManager,
              let textContainer = textView.textContainer else { return }
        layoutManager.ensureLayout(for: textContainer)
        let content = layoutManager.usedRect(for: textContainer).height + 2 * textView.textContainerInset.height
        let line = Self.oneLineHeight
        let clamped = max(line, min(content, line * maxLines + 1))
        if abs(height - clamped) > 0.5 {
            // Defer so the binding is not written from inside a layout pass.
            DispatchQueue.main.async { self.height = clamped }
        }
    }

    final class Coordinator: NSObject, NSTextViewDelegate {
        var parent: ComposerView
        private var windowObserver: Any?

        init(_ parent: ComposerView) { self.parent = parent }

        /// Re-assert focus every time the panel becomes key again. The focus
        /// binding stays `true` while hidden, so without this a re-shown panel
        /// would not refocus the composer.
        fileprivate func watchWindow(of textView: ComposerTextView) {
            windowObserver = NotificationCenter.default.addObserver(
                forName: NSWindow.didBecomeKeyNotification, object: nil, queue: .main
            ) { [weak self, weak textView] note in
                guard let self, let textView,
                      (note.object as? NSWindow) === textView.window,
                      self.parent.isFocused else { return }
                textView.window?.makeFirstResponder(textView)
            }
        }

        deinit {
            if let windowObserver {
                NotificationCenter.default.removeObserver(windowObserver)
            }
        }

        func textDidChange(_ notification: Notification) {
            guard let textView = notification.object as? ComposerTextView else { return }
            if parent.text != textView.string {
                parent.text = textView.string
            }
            parent.updateHeight(textView)
        }

        // Return sends, Shift+Return inserts a newline. Esc is forwarded so the
        // panel can clear its selection or hide, which a TextField used to do
        // via onExitCommand.
        func textView(_ textView: NSTextView, doCommandBy commandSelector: Selector) -> Bool {
            if commandSelector == #selector(NSResponder.cancelOperation(_:)) {
                parent.onEscape?()
                return true
            }
            guard commandSelector == #selector(NSResponder.insertNewline(_:)) else { return false }
            if NSApp.currentEvent?.modifierFlags.contains(.shift) == true {
                return false
            }
            parent.onSubmit()
            return true
        }
    }
}

private final class ComposerTextView: NSTextView {
    var placeholderText: String = "" {
        didSet { needsDisplay = true }
    }

    override func draw(_ dirtyRect: NSRect) {
        super.draw(dirtyRect)
        guard string.isEmpty, !placeholderText.isEmpty else { return }
        let attributes: [NSAttributedString.Key: Any] = [
            .font: font as Any,
            .foregroundColor: NSColor.placeholderTextColor,
        ]
        let inset = textContainerInset
        let origin = NSPoint(x: inset.width + (textContainer?.lineFragmentPadding ?? 0), y: inset.height)
        placeholderText.draw(at: origin, withAttributes: attributes)
    }
}