Add Opacity Widget

@@ -9,6 +9,7 @@
 - Fix HSV and HSL Color constructors
 - Add PageTheme.copyWith
 - Add more font drawing options
+- Add Opacity Widget
 
 ## 1.4.1
 

@@ -40,22 +40,23 @@ part 'src/colors.dart';
 part 'src/compatibility.dart';
 part 'src/document.dart';
 part 'src/encryption.dart';
-part 'src/font_descriptor.dart';
 part 'src/exif.dart';
-part 'src/font_metrics.dart';
 part 'src/font.dart';
+part 'src/font_descriptor.dart';
+part 'src/font_metrics.dart';
 part 'src/formxobject.dart';
+part 'src/graphic_state.dart';
 part 'src/graphics.dart';
 part 'src/image.dart';
 part 'src/info.dart';
 part 'src/names.dart';
-part 'src/object_stream.dart';
 part 'src/object.dart';
+part 'src/object_stream.dart';
 part 'src/outline.dart';
 part 'src/output.dart';
+part 'src/page.dart';
 part 'src/page_format.dart';
 part 'src/page_list.dart';
-part 'src/page.dart';
 part 'src/point.dart';
 part 'src/polygon.dart';
 part 'src/rect.dart';

@@ -98,6 +98,9 @@ class PdfDocument {
   /// Object used to sign the document
   PdfSignature sign;
 
+  /// Graphics state, representing only opacity.
+  PdfGraphicStates _graphicStates;
+
   /// The PDF specification version
   final String version = '1.7';
 
@@ -149,6 +152,14 @@ class PdfDocument {
     return _outline;
   }
 
+  /// Graphic states for opacity and transfer modes
+  PdfGraphicStates get graphicStates {
+    _graphicStates ??= PdfGraphicStates(this);
+    return _graphicStates;
+  }
+
+  bool get hasGraphicStates => _graphicStates != null;
+
   /// This writes the document to an OutputStream.
   ///
   /// Note: You can call this as many times as you wish, as long as

+/*
+ * Copyright (C) 2017, David PHAM-VAN <dev.nfet.net@gmail.com>
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General  License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General
+ * License along with this library; if not, write to the Free Software
+ * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
+ */
+
+// ignore_for_file: omit_local_variable_types
+
+part of pdf;
+
+@immutable
+class PdfGraphicState {
+  const PdfGraphicState({this.opacity});
+
+  final double opacity;
+
+  @protected
+  PdfStream _output() {
+    final Map<String, PdfStream> params = <String, PdfStream>{};
+
+    if (opacity != null) {
+      params['/CA'] = PdfStream.num(opacity);
+      params['/ca'] = PdfStream.num(opacity);
+    }
+
+    return PdfStream.dictionary(params);
+  }
+
+  @override
+  bool operator ==(dynamic other) {
+    if (!other is PdfGraphicState) {
+      return false;
+    }
+    return other.opacity == opacity;
+  }
+
+  @override
+  int get hashCode => opacity.hashCode;
+}
+
+class PdfGraphicStates extends PdfObject {
+  PdfGraphicStates(PdfDocument pdfDocument) : super(pdfDocument);
+
+  final List<PdfGraphicState> _states = <PdfGraphicState>[];
+
+  static const String _prefix = '/a';
+
+  String stateName(PdfGraphicState state) {
+    int index = _states.indexOf(state);
+    if (index < 0) {
+      index = _states.length;
+      _states.add(state);
+    }
+    return '$_prefix$index';
+  }
+
+  @override
+  void _prepare() {
+    super._prepare();
+
+    for (int index = 0; index < _states.length; index++) {
+      params['$_prefix$index'] = _states[index]._output();
+    }
+  }
+}

@@ -320,6 +320,12 @@ class PdfGraphics {
     }
   }
 
+  /// Set the graphic state for drawing
+  void setGraphicState(PdfGraphicState state) {
+    final String name = page.pdfDocument.graphicStates.stateName(state);
+    buf.putString('$name gs\n');
+  }
+
   /// Set the transformation Matrix
   void setTransform(Matrix4 t) {
     final Float64List s = t.storage;

@@ -41,6 +41,18 @@ class PdfPage extends PdfObject {
   /// -1 indicates no thumbnail.
   PdfObject thumbnail;
 
+  /// Isolated transparency: If this flag is true, objects within the group
+  /// shall be composited against a fully transparent initial backdrop;
+  /// if false, they shall be composited against the group’s backdrop
+  bool isolatedTransparency = false;
+
+  /// Whether the transparency group is a knockout group.
+  /// If this flag is false, later objects within the group shall be composited
+  /// with earlier ones with which they overlap; if true, they shall be
+  /// composited with the group’s initial backdrop and shall overwrite any
+  /// earlier overlapping objects.
+  bool knockoutTransparency = false;
+
   /// This holds any Annotations contained within this page.
   List<PdfAnnot> annotations = <PdfAnnot>[];
 
@@ -115,6 +127,20 @@ class PdfPage extends PdfObject {
       resources['/XObject'] = PdfStream()..putObjectDictionary(xObjects);
     }
 
+    if (pdfDocument.hasGraphicStates) {
+      // Declare Transparency Group settings
+      params['/Group'] = PdfStream()
+        ..putDictionary(<String, PdfStream>{
+          '/Type': PdfStream.string('/Group'),
+          '/S': PdfStream.string('/Transparency'),
+          '/CS': PdfStream.string('/DeviceRGB'),
+          '/I': PdfStream()..putBool(isolatedTransparency),
+          '/K': PdfStream()..putBool(knockoutTransparency),
+        });
+
+      resources['/ExtGState'] = pdfDocument.graphicStates.ref();
+    }
+
     params['/Resources'] = PdfStream.dictionary(resources);
 
     // The thumbnail

@@ -740,3 +740,29 @@ class FullPage extends SingleChildWidget {
     context.canvas.restoreContext();
   }
 }
+
+class Opacity extends SingleChildWidget {
+  Opacity({
+    @required this.opacity,
+    Widget child,
+  })  : assert(opacity != null),
+        super(child: child);
+
+  final double opacity;
+
+  @override
+  void paint(Context context) {
+    super.paint(context);
+
+    if (child != null) {
+      final Matrix4 mat = Matrix4.identity();
+      mat.translate(box.x, box.y);
+      context.canvas
+        ..saveContext()
+        ..setTransform(mat)
+        ..setGraphicState(PdfGraphicState(opacity: opacity));
+      child.paint(context);
+      context.canvas.restoreContext();
+    }
+  }
+}