squash pdf export + storage permissions

lib/export/export_pdf.dart

@@ -0,0 +1,58 @@
+import 'dart:io';
+import 'dart:ui';
+
+import 'package:pdf/pdf.dart';
+import 'package:pdf/widgets.dart' as pw;
+
+import '../canvas/document_types.dart';
+import '../savesystem/path.dart';
+
+const _a4width = 210 * PdfPageFormat.mm;
+const _a4height = 297 * PdfPageFormat.mm;
+
+class _StrokePDFPaint extends pw.Widget {
+  List<Stroke> strokes;
+
+  @override
+  void layout(pw.Context context, pw.BoxConstraints constraints,
+      {bool parentUsesSize = false}) {
+    box =
+        PdfRect.fromPoints(PdfPoint.zero, const PdfPoint(_a4width, _a4height));
+  }
+
+  @override
+  void paint(pw.Context context) {
+    super.paint(context);
+
+    for (final stroke in strokes) {
+      context.canvas.setStrokeColor(PdfColor.fromInt(stroke.color.value));
+      for (int i = 0; i < stroke.points.length - 1; i++) {
+        Offset pt1 = stroke.points[i].point * PdfPageFormat.mm;
+        pt1 = Offset(pt1.dx, _a4width - pt1.dy);
+        Offset pt2 = stroke.points[i + 1].point * PdfPageFormat.mm;
+        pt2 = Offset(pt2.dx, _a4width - pt2.dy);
+
+        context.canvas.setLineWidth(stroke.points[i].thickness);
+        context.canvas.drawLine(pt1.dx, pt1.dy, pt2.dx, pt2.dy);
+        context.canvas.strokePath();
+      }
+    }
+  }
+
+  _StrokePDFPaint(this.strokes);
+}
+
+void exportPDF(List<Stroke> strokes, String name) async {
+  final pdf = pw.Document();
+
+  const PdfPageFormat a4 = PdfPageFormat(_a4width, _a4height);
+
+  pdf.addPage(pw.MultiPage(
+    pageFormat: a4,
+    build: (context) => [_StrokePDFPaint(strokes)],
+  ));
+
+  final path = await getSavePath();
+  final file = File('${path.path}${Platform.pathSeparator}$name');
+  await file.writeAsBytes(await pdf.save(), flush: true);
+}