Add Markdown file previews

ios/Sources/DetailScreens.swift

@@ -1,5 +1,7 @@
 import Highlighter
+import MarkdownUI
 import QuickLook
+import SwiftUI
 import UIKit
 import UniformTypeIdentifiers
 
@@ -336,6 +338,8 @@ final class RepositoryDirectoryViewController: RefreshingTableViewController {
 
 @MainActor
 final class RepositoryFileViewController: UIViewController, QLPreviewControllerDataSource {
+    private enum MarkdownMode: Int { case preview, source }
+
     private let context: AppContext
     private let owner: String
     private let repository: String
@@ -343,6 +347,9 @@ final class RepositoryFileViewController: UIViewController, QLPreviewControllerD
     private let spinner = UIActivityIndicatorView(style: .medium)
     private var loadingTask: Task<Void, Never>?
     private var previewURL: URL?
+    private var markdownSource: String?
+    private var displayedController: UIViewController?
+    private var displayedView: UIView?
     private weak var sourceScrollView: UIScrollView?
     private weak var sourceTextView: UITextView?
 
@@ -370,7 +377,11 @@ final class RepositoryFileViewController: UIViewController, QLPreviewControllerD
                     path: path
                 )
                 if !isMediaFile(path), let source = String(data: data, encoding: .utf8) {
-                    showSource(source)
+                    if isMarkdownFile {
+                        showMarkdown(source)
+                    } else {
+                        showSource(source)
+                    }
                 } else {
                     try showQuickLook(data)
                 }
@@ -396,6 +407,10 @@ final class RepositoryFileViewController: UIViewController, QLPreviewControllerD
             || type.conforms(to: .pdf)
     }
 
+    private var isMarkdownFile: Bool {
+        ["md", "markdown", "mdown", "mkd"].contains((path as NSString).pathExtension.lowercased())
+    }
+
     deinit {
         loadingTask?.cancel()
         if let previewURL { try? FileManager.default.removeItem(at: previewURL) }
@@ -409,7 +424,40 @@ final class RepositoryFileViewController: UIViewController, QLPreviewControllerD
         previewURL! as NSURL
     }
 
+    private func showMarkdown(_ source: String) {
+        markdownSource = source
+        navigationItem.prompt = title
+        let modeControl = UISegmentedControl(items: ["Preview", "Source"])
+        modeControl.selectedSegmentIndex = MarkdownMode.preview.rawValue
+        modeControl.addTarget(self, action: #selector(markdownModeChanged(_:)), for: .valueChanged)
+        modeControl.accessibilityLabel = "Markdown view"
+        navigationItem.titleView = modeControl
+        showMarkdownPreview(source)
+    }
+
+    @objc private func markdownModeChanged(_ sender: UISegmentedControl) {
+        guard let mode = MarkdownMode(rawValue: sender.selectedSegmentIndex),
+              let source = markdownSource else { return }
+        switch mode {
+        case .preview: showMarkdownPreview(source)
+        case .source: showSource(source)
+        }
+    }
+
+    private func showMarkdownPreview(_ source: String) {
+        removeDisplayedView()
+        let preview = UIHostingController(rootView: RepositoryMarkdownPreview(source: source))
+        addChild(preview)
+        preview.view.frame = view.bounds
+        preview.view.autoresizingMask = [.flexibleWidth, .flexibleHeight]
+        displayedController = preview
+        displayedView = preview.view
+        view.addSubview(preview.view)
+        preview.didMove(toParent: self)
+    }
+
     private func showSource(_ source: String) {
+        removeDisplayedView()
         let highlighter = Highlighter()
         highlighter?.setTheme(
             traitCollection.userInterfaceStyle == .dark ? "atom-one-dark" : "atom-one-light"
@@ -445,10 +493,21 @@ final class RepositoryFileViewController: UIViewController, QLPreviewControllerD
         sourceScrollView = scrollView
         sourceTextView = textView
         scrollView.addSubview(textView)
+        displayedView = scrollView
         view.addSubview(scrollView)
         updateSourceLayout()
     }
 
+    private func removeDisplayedView() {
+        displayedController?.willMove(toParent: nil)
+        displayedView?.removeFromSuperview()
+        displayedController?.removeFromParent()
+        displayedController = nil
+        displayedView = nil
+        sourceScrollView = nil
+        sourceTextView = nil
+    }
+
     private func updateSourceLayout() {
         guard let scrollView = sourceScrollView, let textView = sourceTextView else { return }
         textView.textContainer.widthTracksTextView = false
@@ -488,6 +547,20 @@ final class RepositoryFileViewController: UIViewController, QLPreviewControllerD
     }
 }
 
+private struct RepositoryMarkdownPreview: View {
+    let source: String
+
+    var body: some View {
+        ScrollView {
+            Markdown(source)
+                .markdownTheme(.gitHub)
+                .frame(maxWidth: .infinity, alignment: .leading)
+                .padding()
+        }
+        .background(Color(uiColor: .systemBackground))
+    }
+}
+
 enum DiffSource {
     case commit(owner: String, repository: String, sha: String, path: String)
     case pull(owner: String, repository: String, number: Int64, path: String)