fix(csv): stream file export and build clipboard CSV off the UI isolate (#277)

lib/core/csv/result_grid_csv.dart

@@ -1,6 +1,9 @@
 /// RFC 4180–style CSV for a result grid (header + rows).
 library;
 
+import 'dart:io';
+import 'dart:isolate';
+
 String escapeCsvField(String s) {
   final needsQuotes = s.contains(',') ||
       s.contains('"') ||
@@ -12,18 +15,55 @@ String escapeCsvField(String s) {
   return s;
 }
 
+/// Formats a single CSV data row, padding short rows to [columnCount].
+String formatCsvDataRow(List<String> row, int columnCount) {
+  final buf = StringBuffer();
+  for (var i = 0; i < columnCount; i++) {
+    if (i > 0) buf.write(',');
+    if (i < row.length) buf.write(escapeCsvField(row[i]));
+  }
+  return buf.toString();
+}
+
 /// One line per row; pads short rows with empty cells to [columns.length].
+///
+/// Prefer [resultGridAsCsvAsync] for large grids on the UI isolate, and
+/// [writeResultGridCsv] when writing to a file.
 String resultGridAsCsv(List<String> columns, List<List<String>> rows) {
-  final lines = <String>[
-    columns.map(escapeCsvField).join(','),
-    ...rows.map((r) {
-      final cells = List<String>.generate(
-        columns.length,
-        (i) => i < r.length ? escapeCsvField(r[i]) : '',
-        growable: false,
-      );
-      return cells.join(',');
-    }),
-  ];
-  return lines.join('\n');
+  final buf = StringBuffer();
+  buf.write(columns.map(escapeCsvField).join(','));
+  for (final row in rows) {
+    buf.write('\n');
+    buf.write(formatCsvDataRow(row, columns.length));
+  }
+  return buf.toString();
+}
+
+/// Builds CSV off the UI isolate so large grids do not freeze the main thread.
+Future<String> resultGridAsCsvAsync(
+  List<String> columns,
+  List<List<String>> rows,
+) {
+  return Isolate.run(() => resultGridAsCsv(columns, rows));
+}
+
+/// Streams CSV to [sink] without assembling the full document in memory.
+Future<void> writeResultGridCsv(
+  IOSink sink, {
+  required List<String> columns,
+  required List<List<String>> rows,
+}) async {
+  sink.write(columns.map(escapeCsvField).join(','));
+  var written = 0;
+  for (final row in rows) {
+    sink.write('\n');
+    sink.write(formatCsvDataRow(row, columns.length));
+    written++;
+    // Yield periodically so a large export does not starve the event loop
+    // when this runs on the UI isolate.
+    if (written % 500 == 0) {
+      await Future<void>.delayed(Duration.zero);
+    }
+  }
+  await sink.flush();
 }

lib/core/csv/save_result_grid_csv.dart

@@ -16,13 +16,12 @@ enum SaveResultGridCsvOutcome {
   error,
 }
 
-/// Opens a platform save dialog and writes [columns]/[rows] as CSV.
+/// Opens a platform save dialog and streams [columns]/[rows] as CSV to disk.
 Future<SaveResultGridCsvOutcome> saveResultGridCsvFile({
   required List<String> columns,
   required List<List<String>> rows,
   String? suggestedName,
 }) async {
-  final csv = resultGridAsCsv(columns, rows);
   final name = suggestedName ??
       'querya_results_${DateTime.now().toIso8601String().replaceAll(':', '-')}.csv';
   final location = await getSaveLocation(
@@ -35,10 +34,21 @@ Future<SaveResultGridCsvOutcome> saveResultGridCsvFile({
   if (path == null || path.isEmpty) {
     return SaveResultGridCsvOutcome.cancelled;
   }
+  IOSink? sink;
   try {
-    await File(path).writeAsString(csv);
+    sink = File(path).openWrite();
+    await writeResultGridCsv(
+      sink,
+      columns: columns,
+      rows: rows,
+    );
+    await sink.close();
+    sink = null;
     return SaveResultGridCsvOutcome.written;
   } on Object {
+    try {
+      await sink?.close();
+    } catch (_) {}
     return SaveResultGridCsvOutcome.error;
   }
 }

lib/features/main_screen/results_tab.dart

@@ -123,8 +123,10 @@ class ResultsTab extends StatelessWidget {
                 OutlineButton(
                   size: ButtonSize.small,
                   onPressed: () {
-                    final csv = resultGridAsCsv(columns, rows);
-                    Clipboard.setData(ClipboardData(text: csv));
+                    unawaited(() async {
+                      final csv = await resultGridAsCsvAsync(columns, rows);
+                      await Clipboard.setData(ClipboardData(text: csv));
+                    }());
                   },
                   leading: const material.Icon(
                     material.Icons.copy_rounded,

test/core/csv/result_grid_csv_test.dart

@@ -1,3 +1,5 @@
+import 'dart:io';
+
 import 'package:flutter_test/flutter_test.dart';
 import 'package:querya_desktop/core/csv/result_grid_csv.dart';
 
@@ -29,4 +31,61 @@ void main() {
       );
     });
   });
+
+  group('resultGridAsCsvAsync', () {
+    test('matches synchronous result off the UI isolate', () async {
+      const columns = ['a', 'b'];
+      const rows = [
+        ['1', 'two,comma'],
+        ['quote', 'say "hi"'],
+      ];
+      final asyncCsv = await resultGridAsCsvAsync(columns, rows);
+      expect(asyncCsv, resultGridAsCsv(columns, rows));
+    });
+  });
+
+  group('writeResultGridCsv', () {
+    test('streams the same content as resultGridAsCsv', () async {
+      const columns = ['a', 'b'];
+      const rows = [
+        ['1', 'two,comma'],
+        ['quote', 'say "hi"'],
+      ];
+      final file = File(
+        '${Directory.systemTemp.path}/querya_csv_stream_${DateTime.now().microsecondsSinceEpoch}.csv',
+      );
+      try {
+        final sink = file.openWrite();
+        await writeResultGridCsv(sink, columns: columns, rows: rows);
+        await sink.close();
+        expect(await file.readAsString(), resultGridAsCsv(columns, rows));
+      } finally {
+        if (await file.exists()) await file.delete();
+      }
+    });
+
+    test('streams large grids without building one giant string first', () async {
+      const columns = ['id', 'value'];
+      final rows = List.generate(
+        2500,
+        (i) => ['$i', 'value_$i'],
+        growable: false,
+      );
+      final file = File(
+        '${Directory.systemTemp.path}/querya_csv_large_${DateTime.now().microsecondsSinceEpoch}.csv',
+      );
+      try {
+        final sink = file.openWrite();
+        await writeResultGridCsv(sink, columns: columns, rows: rows);
+        await sink.close();
+        final lines = await file.readAsLines();
+        expect(lines.length, 2501);
+        expect(lines.first, 'id,value');
+        expect(lines[1], '0,value_0');
+        expect(lines.last, '2499,value_2499');
+      } finally {
+        if (await file.exists()) await file.delete();
+      }
+    });
+  });
 }