improved performance and battery usage on older devices

lib/widget/file_viewer.dart

@@ -4,6 +4,7 @@ import 'dart:io';
 import 'dart:math';
 
 import 'package:file_picker/file_picker.dart';
+import 'package:flutter/foundation.dart';
 import 'package:flutter/material.dart';
 import 'package:flutter_bloc/flutter_bloc.dart';
 import 'package:flutter_svg/flutter_svg.dart';
@@ -15,6 +16,7 @@ import '../routing/app_routes.dart';
 import '../share_intent/remote_file_ref.dart';
 import '../state/app/modules/settings/bloc/settings_cubit.dart';
 import '../utils/downloads/download_manager.dart';
+import '../utils/screen_bound_image.dart';
 import 'app_progress_indicator.dart';
 import 'async_action_button.dart';
 import 'centered_leading.dart';
@@ -49,6 +51,7 @@ class FileViewer extends StatefulWidget {
 enum FileViewingActions { openExternal, share, save, sendToChat, saveToCloud }
 
 class _FileViewerState extends State<FileViewer> {
+  Future<_TextPayload>? _textPayload;
   final PhotoViewController photoViewController = PhotoViewController();
 
   late SettingsCubit settings = context.read<SettingsCubit>();
@@ -302,7 +305,13 @@ class _FileViewerState extends State<FileViewer> {
       controller: photoViewController,
       maxScale: 3.0,
       minScale: 0.1,
-      imageProvider: Image.file(File(widget.path)).image,
+      // 2× the screen stays sharp while zooming in; the 4096 px cap only
+      // bites on camera-sized photos.
+      imageProvider: screenBoundImage(
+        context,
+        FileImage(File(widget.path)),
+        scale: 2,
+      ),
       backgroundDecoration: BoxDecoration(
         color: Theme.of(context).colorScheme.surface,
       ),
@@ -348,13 +357,17 @@ class _FileViewerState extends State<FileViewer> {
   Widget _buildTextView() => Scaffold(
     appBar: _appbar(),
     body: FutureBuilder<_TextPayload>(
-      future: _readTextPayload(),
+      // Cached: a future created in build re-read the file on every rebuild.
+      future: _textPayload ??= compute(
+        _loadTextPayload,
+        (widget.path, _textViewMaxBytes),
+      ),
       builder: (context, snapshot) {
         if (!snapshot.hasData) {
           return const Center(child: AppProgressIndicator.large());
         }
         final payload = snapshot.data!;
-        final lines = const LineSplitter().convert(payload.content);
+        final lines = payload.lines;
         // Stable gutter width — sized by the highest line number's digit count.
         final gutterWidth = (lines.length.toString().length * 9.0) + 16;
         return SelectionArea(
@@ -443,38 +456,6 @@ class _FileViewerState extends State<FileViewer> {
   }
 
   static const int _textViewMaxBytes = 5 * 1024 * 1024;
-
-  Future<_TextPayload> _readTextPayload() async {
-    final file = File(widget.path);
-    final size = await file.length();
-    final ext = widget.path.split('.').last.toLowerCase();
-    if (size <= _textViewMaxBytes) {
-      final raw = await file.readAsString();
-      return _TextPayload(content: _maybePrettify(raw, ext), truncated: false);
-    }
-    final raf = await file.open();
-    try {
-      final bytes = await raf.read(_textViewMaxBytes);
-      // Truncated payloads stay raw — a parser would choke on the dangling tail.
-      return _TextPayload(
-        content: utf8.decode(bytes, allowMalformed: true),
-        truncated: true,
-      );
-    } finally {
-      await raf.close();
-    }
-  }
-
-  /// Falls through to the original text on parse errors.
-  String _maybePrettify(String content, String ext) {
-    if (ext != 'json') return content;
-    try {
-      final parsed = jsonDecode(content);
-      return const JsonEncoder.withIndent('  ').convert(parsed);
-    } on Object {
-      return content;
-    }
-  }
 }
 
 class _ActionDescriptor {
@@ -489,7 +470,47 @@ class _ActionDescriptor {
 }
 
 class _TextPayload {
-  final String content;
+  final List<String> lines;
   final bool truncated;
-  const _TextPayload({required this.content, required this.truncated});
+  const _TextPayload({required this.lines, required this.truncated});
+}
+
+/// Reads, prettifies (JSON) and splits a text file on a background isolate:
+/// up to 5 MB of decoding and line splitting would otherwise freeze the UI.
+Future<_TextPayload> _loadTextPayload((String, int) args) async {
+  final (path, maxBytes) = args;
+  final file = File(path);
+  final size = await file.length();
+  final ext = path.split('.').last.toLowerCase();
+  if (size <= maxBytes) {
+    final raw = await file.readAsString();
+    return _TextPayload(
+      lines: const LineSplitter().convert(_maybePrettify(raw, ext)),
+      truncated: false,
+    );
+  }
+  final raf = await file.open();
+  try {
+    final bytes = await raf.read(maxBytes);
+    // Truncated payloads stay raw — a parser would choke on the dangling tail.
+    return _TextPayload(
+      lines: const LineSplitter().convert(
+        utf8.decode(bytes, allowMalformed: true),
+      ),
+      truncated: true,
+    );
+  } finally {
+    await raf.close();
+  }
+}
+
+/// Falls through to the original text on parse errors.
+String _maybePrettify(String content, String ext) {
+  if (ext != 'json') return content;
+  try {
+    final parsed = jsonDecode(content);
+    return const JsonEncoder.withIndent('  ').convert(parsed);
+  } on Object {
+    return content;
+  }
 }