Add Social Preview image

+import 'dart:io';
+
+import 'package:pdf/pdf.dart';
+import 'package:pdf/widgets.dart';
+import 'package:string_scanner/string_scanner.dart';
+
+const dpi = 72.0;
+const px = dpi / PdfPageFormat.inch * PdfPageFormat.point;
+
+void main() {
+  // Open self
+  final source = File('../test/github_social_preview.dart').readAsStringSync();
+  final code = DartSyntaxHighlighter(
+    SyntaxHighlighterStyle.dark(),
+  ).format(
+    source.substring(
+      source.lastIndexOf('// START') + 9,
+      source.lastIndexOf('// END') - 3,
+    ),
+  );
+
+  // START
+  // Enable debug painting
+  Document.debug = true;
+
+  // Create a pdf document
+  final pdf = Document(
+    title: 'Github Social Preview',
+    author: 'David PHAM-VAN',
+  );
+
+  // New page
+  pdf.addPage(
+    Page(
+      pageFormat: PdfPageFormat(
+        1280 * px,
+        640 * px,
+        marginAll: 78 * px,
+      ),
+      build: (context) => Row(
+        mainAxisSize: MainAxisSize.max,
+        children: [
+          // Display the source code on the first half of the page
+          Flexible(
+            fit: FlexFit.tight,
+            child: Container(
+              color: PdfColors.grey800,
+              padding: EdgeInsets.all(10 * px),
+              child: ClipRect(child: RichText(text: code)),
+            ),
+          ),
+          // Add a vertical separator
+          Container(width: 5 * px, color: PdfColors.lightBlue),
+          // Show "Hello World!" centered and rotated on the second half of the page
+          Flexible(
+            fit: FlexFit.tight,
+            child: Center(
+              child: Transform.rotateBox(
+                angle: .2,
+                child: Text(
+                  'Hello World!',
+                  style: TextStyle(
+                    font: Font.helveticaBold(),
+                    fontSize: 50 * px,
+                  ),
+                ),
+              ),
+            ),
+          ),
+        ],
+      ),
+    ),
+  );
+  // END
+
+  // Save the file
+  File('social_preview.pdf').writeAsBytesSync(pdf.save());
+
+  // Convert to png
+  Process.runSync('pdftocairo',
+      ['social_preview.pdf', '-png', '-r', '72', 'social_preview.png']);
+}
+
+class SyntaxHighlighterStyle {
+  const SyntaxHighlighterStyle(
+      {this.baseStyle,
+      this.numberStyle,
+      this.commentStyle,
+      this.keywordStyle,
+      this.stringStyle,
+      this.punctuationStyle,
+      this.classStyle,
+      this.constantStyle});
+
+  final TextStyle baseStyle;
+  final TextStyle numberStyle;
+  final TextStyle commentStyle;
+  final TextStyle keywordStyle;
+  final TextStyle stringStyle;
+  final TextStyle punctuationStyle;
+  final TextStyle classStyle;
+  final TextStyle constantStyle;
+
+  factory SyntaxHighlighterStyle.dark() => SyntaxHighlighterStyle(
+        baseStyle: TextStyle(
+          font: Font.courierBold(),
+          color: PdfColors.white,
+          fontSize: 10 * px,
+        ),
+        numberStyle: TextStyle(color: PdfColors.purple300),
+        commentStyle: TextStyle(color: PdfColors.green600),
+        keywordStyle: TextStyle(color: PdfColors.blue600),
+        stringStyle: TextStyle(color: PdfColors.orange400),
+        punctuationStyle: TextStyle(color: PdfColors.pink),
+        classStyle: TextStyle(color: PdfColors.cyan),
+        constantStyle: TextStyle(color: PdfColors.pink),
+      );
+}
+
+class DartSyntaxHighlighter {
+  DartSyntaxHighlighter(this._style) {
+    _spans = <_HighlightSpan>[];
+  }
+
+  final SyntaxHighlighterStyle _style;
+
+  static const List<String> _keywords = <String>[
+    'abstract',
+    'as',
+    'assert',
+    'async',
+    'await',
+    'break',
+    'case',
+    'catch',
+    'class',
+    'const',
+    'continue',
+    'default',
+    'deferred',
+    'do',
+    'dynamic',
+    'else',
+    'enum',
+    'export',
+    'external',
+    'extends',
+    'factory',
+    'false',
+    'final',
+    'finally',
+    'for',
+    'get',
+    'if',
+    'implements',
+    'import',
+    'in',
+    'is',
+    'library',
+    'new',
+    'null',
+    'operator',
+    'part',
+    'rethrow',
+    'return',
+    'set',
+    'static',
+    'super',
+    'switch',
+    'sync',
+    'this',
+    'throw',
+    'true',
+    'try',
+    'typedef',
+    'var',
+    'void',
+    'while',
+    'with',
+    'yield'
+  ];
+
+  static const List<String> _builtInTypes = <String>[
+    'int',
+    'double',
+    'num',
+    'bool'
+  ];
+
+  String _src;
+  StringScanner _scanner;
+
+  List<_HighlightSpan> _spans;
+
+  TextSpan format(String source) {
+    _src = source;
+
+    _scanner = StringScanner(_src);
+
+    if (_generateSpans()) {
+      // Successfully parsed the code
+      final List<TextSpan> formattedText = <TextSpan>[];
+      int currentPosition = 0;
+
+      for (_HighlightSpan span in _spans) {
+        if (currentPosition != span.start)
+          formattedText
+              .add(TextSpan(text: _src.substring(currentPosition, span.start)));
+
+        formattedText.add(TextSpan(
+            style: span.textStyle(_style), text: span.textForSpan(_src)));
+
+        currentPosition = span.end;
+      }
+
+      if (currentPosition != _src.length)
+        formattedText
+            .add(TextSpan(text: _src.substring(currentPosition, _src.length)));
+      _spans.clear();
+      return TextSpan(style: _style.baseStyle, children: formattedText);
+    } else {
+      // Parsing failed, return with only basic formatting
+      return TextSpan(style: _style.baseStyle, text: source);
+    }
+  }
+
+  bool _generateSpans() {
+    int lastLoopPosition = _scanner.position;
+
+    while (!_scanner.isDone) {
+      // Skip White space
+      _scanner.scan(RegExp(r'\s+'));
+
+      // Block comments
+      if (_scanner.scan(RegExp(r'/\*(.|\n)*\*/'))) {
+        _spans.add(_HighlightSpan(_HighlightType.comment,
+            _scanner.lastMatch.start, _scanner.lastMatch.end));
+        continue;
+      }
+
+      // Line comments
+      if (_scanner.scan('//')) {
+        final int startComment = _scanner.lastMatch.start;
+
+        bool eof = false;
+        int endComment;
+        if (_scanner.scan(RegExp(r'.*\n'))) {
+          endComment = _scanner.lastMatch.end - 1;
+        } else {
+          eof = true;
+          endComment = _src.length;
+        }
+
+        _spans.add(
+            _HighlightSpan(_HighlightType.comment, startComment, endComment));
+
+        if (eof) {
+          break;
+        }
+
+        continue;
+      }
+
+      // Raw r"String"
+      if (_scanner.scan(RegExp(r'r".*"'))) {
+        _spans.add(_HighlightSpan(_HighlightType.string,
+            _scanner.lastMatch.start, _scanner.lastMatch.end));
+        continue;
+      }
+
+      // Raw r'String'
+      if (_scanner.scan(RegExp(r"r'.*'"))) {
+        _spans.add(_HighlightSpan(_HighlightType.string,
+            _scanner.lastMatch.start, _scanner.lastMatch.end));
+        continue;
+      }
+
+      // Multiline """String"""
+      if (_scanner.scan(RegExp(r'"""(?:[^"\\]|\\(.|\n))*"""'))) {
+        _spans.add(_HighlightSpan(_HighlightType.string,
+            _scanner.lastMatch.start, _scanner.lastMatch.end));
+        continue;
+      }
+
+      // Multiline '''String'''
+      if (_scanner.scan(RegExp(r"'''(?:[^'\\]|\\(.|\n))*'''"))) {
+        _spans.add(_HighlightSpan(_HighlightType.string,
+            _scanner.lastMatch.start, _scanner.lastMatch.end));
+        continue;
+      }
+
+      // "String"
+      if (_scanner.scan(RegExp(r'"(?:[^"\\]|\\.)*"'))) {
+        _spans.add(_HighlightSpan(_HighlightType.string,
+            _scanner.lastMatch.start, _scanner.lastMatch.end));
+        continue;
+      }
+
+      // 'String'
+      if (_scanner.scan(RegExp(r"'(?:[^'\\]|\\.)*'"))) {
+        _spans.add(_HighlightSpan(_HighlightType.string,
+            _scanner.lastMatch.start, _scanner.lastMatch.end));
+        continue;
+      }
+
+      // Double
+      if (_scanner.scan(RegExp(r'\d+\.\d+'))) {
+        _spans.add(_HighlightSpan(_HighlightType.number,
+            _scanner.lastMatch.start, _scanner.lastMatch.end));
+        continue;
+      }
+
+      // Integer
+      if (_scanner.scan(RegExp(r'\d+'))) {
+        _spans.add(_HighlightSpan(_HighlightType.number,
+            _scanner.lastMatch.start, _scanner.lastMatch.end));
+        continue;
+      }
+
+      // Punctuation
+      if (_scanner.scan(RegExp(r'[\[\]{}().!=<>&\|\?\+\-\*/%\^~;:,]'))) {
+        _spans.add(_HighlightSpan(_HighlightType.punctuation,
+            _scanner.lastMatch.start, _scanner.lastMatch.end));
+        continue;
+      }
+
+      // Meta data
+      if (_scanner.scan(RegExp(r'@\w+'))) {
+        _spans.add(_HighlightSpan(_HighlightType.keyword,
+            _scanner.lastMatch.start, _scanner.lastMatch.end));
+        continue;
+      }
+
+      // Words
+      if (_scanner.scan(RegExp(r'\w+'))) {
+        _HighlightType type;
+
+        String word = _scanner.lastMatch[0];
+        if (word.startsWith('_')) {
+          word = word.substring(1);
+        }
+
+        if (_keywords.contains(word))
+          type = _HighlightType.keyword;
+        else if (_builtInTypes.contains(word))
+          type = _HighlightType.keyword;
+        else if (_firstLetterIsUpperCase(word))
+          type = _HighlightType.klass;
+        else if (word.length >= 2 &&
+            word.startsWith('k') &&
+            _firstLetterIsUpperCase(word.substring(1)))
+          type = _HighlightType.constant;
+
+        if (type != null) {
+          _spans.add(_HighlightSpan(
+              type, _scanner.lastMatch.start, _scanner.lastMatch.end));
+        }
+      }
+
+      // Check if this loop did anything
+      if (lastLoopPosition == _scanner.position) {
+        // Failed to parse this file, abort gracefully
+        return false;
+      }
+      lastLoopPosition = _scanner.position;
+    }
+
+    _simplify();
+    return true;
+  }
+
+  void _simplify() {
+    for (int i = _spans.length - 2; i >= 0; i -= 1) {
+      if (_spans[i].type == _spans[i + 1].type &&
+          _spans[i].end == _spans[i + 1].start) {
+        _spans[i] =
+            _HighlightSpan(_spans[i].type, _spans[i].start, _spans[i + 1].end);
+        _spans.removeAt(i + 1);
+      }
+    }
+  }
+
+  bool _firstLetterIsUpperCase(String str) {
+    if (str.isNotEmpty) {
+      final String first = str.substring(0, 1);
+      return first == first.toUpperCase();
+    }
+    return false;
+  }
+}
+
+enum _HighlightType {
+  number,
+  comment,
+  keyword,
+  string,
+  punctuation,
+  klass,
+  constant
+}
+
+class _HighlightSpan {
+  _HighlightSpan(this.type, this.start, this.end);
+  final _HighlightType type;
+  final int start;
+  final int end;
+
+  String textForSpan(String src) {
+    return src.substring(start, end);
+  }
+
+  TextStyle textStyle(SyntaxHighlighterStyle style) {
+    if (type == _HighlightType.number)
+      return style.numberStyle;
+    else if (type == _HighlightType.comment)
+      return style.commentStyle;
+    else if (type == _HighlightType.keyword)
+      return style.keywordStyle;
+    else if (type == _HighlightType.string)
+      return style.stringStyle;
+    else if (type == _HighlightType.punctuation)
+      return style.punctuationStyle;
+    else if (type == _HighlightType.klass)
+      return style.classStyle;
+    else if (type == _HighlightType.constant)
+      return style.constantStyle;
+    else
+      return style.baseStyle;
+  }
+}