Flutter Content Rendering (HTML, links, PDFs, local files)

Package-level quirks for rendering dynamic content in Flutter apps (task descriptions, attachment viewers, rich text). Verified against the Vikunja Flutter app on Flutter 3.44.8 / Dart 3.12 / webview_flutter 4.13.

Clickable links inside rendered HTML

flutter_widget_from_html's HtmlWidget:

• Does NOT auto-linkify bare https:// / www. URLs — they render as inert text.
• <a href> tags are only tappable when onTapUrl is supplied.
• onTapUrl signature: FutureOr<bool> Function(String url)? — return true = handled.

Recipe (works for plain text or HTML strings):

1. Linkify first: wrap bare URLs in <a href> (regex below), skipping URLs already inside an href attribute.
2. HtmlWidget(linkifyHtml(desc), onTapUrl: (url) => launchUrl(Uri.parse(url), mode: LaunchMode.externalApplication)) — url_launcher, already available in most apps.
3. AndroidManifest: add a <queries> block (VIEW intent, http + https schemes) for Android 11+ package visibility, or browser launch can fail silently.

Copy-paste linkify (mind the raw-string trap — see below):

final RegExp _bareUrlPattern = RegExp(
  r'(?<![="])((?:https?://|www\.)[^\s<>]+)',
  caseSensitive: false,
);
final RegExp _trailingJunk = RegExp(r'[.,;:!?…)"' r"']+$");

String linkifyHtml(String input) => input.replaceAllMapped(_bareUrlPattern, (m) {
  var url = m.group(1)!.replaceAll(_trailingJunk, '');
  if (url.isEmpty) return m.group(0)!;
  final href = url.startsWith('http') ? url : 'https://$url';
  return '<a href="$href">$url</a>';
});

WebView (webview_flutter 4.x)

• loadFile(): pass 'file://$path'. The 4.x Android impl hands the string straight to loadUrl (no prefix added); the legacy impl prefixes file:// itself. The explicit prefix works for both, and the plugin sets setAllowFileAccess(true) for you.
• setDomStorageEnabled was REMOVED from the API — DOM storage is always on (AndroidWebViewController ctor enables it).
• Pinch zoom is ON by default (ctor: setBuiltInZoomControls(true), setDisplayZoomControls(false)) — no code needed.
• CDN-script pages (Tailwind etc.): setJavaScriptMode(JavaScriptMode.unrestricted).
• Loading UX: NavigationDelegate(onPageStarted/onPageFinished) + spinner; WebView stays blank until the first frame.
• HTML in a WebView renders CSS/JS/RTL exactly like a browser; a plain HtmlWidget view strips styles — choose per use case.

PDFs (flutter_pdfview)

• Android native PdfRenderer → handles Hebrew/RTL PDFs and generated docs well.
• Needs a real local file path (app-private dirs like applicationSupport work).
• Default view: PDFView(filePath: path, autoSpacing: true, pageFling: false); errors surface via onError / onPageError.

Local file download + cache (background_downloader)

• Cache pattern: build the DownloadTask (same URL/headers as the download), call task.filePath() — returns Future<String> (non-nullable) — then File(path).existsSync() to skip re-downloads.
• Open a cached local file in an external app: FileDownloader().openFile(filePath: path, mimeType: mime) (exposes via content URI; plain url_launcher Uri.file does NOT work on Android 7+ for app-private files).

Dart raw strings — no escapes

• In a raw string (r'...'), backslash escaping does NOT exist: r'...\'...' terminates at the quote → parse error (this is NOT a regex bug).
• Include both quote types in one regex by concatenating adjacent raw literals with alternating quotes: r'[.,;:!?…)"' r"']+$".
• The analyzer lint prefer_adjacent_string_concatenation demands adjacent literals (no + operator).

Verification

• flutter analyze must reach zero errors in touched files.
• After any multi-line edit: dart format <file> — it FAILS loudly on unbalanced braces, making it the fastest corruption detector.