Add GridPaper widget

@@ -4,6 +4,7 @@
 
 - A borderRadius can only be given for a uniform Border
 - Add LayoutWidgetBuilder
+- Add GridPaper widget
 
 ## 1.13.0
 

@@ -37,6 +37,7 @@ export 'widgets/flex.dart';
 export 'widgets/font.dart';
 export 'widgets/forms.dart';
 export 'widgets/geometry.dart';
+export 'widgets/grid_paper.dart';
 export 'widgets/grid_view.dart';
 export 'widgets/icon.dart';
 export 'widgets/image.dart';

+/*
+ * Copyright (C) 2017, David PHAM-VAN <dev.nfet.net@gmail.com>
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import '../pdf.dart';
+import 'box_border.dart';
+import 'geometry.dart';
+
+import 'widget.dart';
+
+/// A widget that draws a rectilinear grid of lines.
+/// The grid is drawn over the [child] widget.
+class GridPaper extends SingleChildWidget {
+  /// Creates a widget that draws a rectilinear grid lines.
+  GridPaper({
+    PdfColor color = lineColor,
+    double interval = 100,
+    int divisions = 5,
+    int subdivisions = 2,
+    Widget child,
+  })  : assert(divisions > 0,
+            'The "divisions" property must be greater than zero. If there were no divisions, the grid paper would not paint anything.'),
+        assert(subdivisions > 0,
+            'The "subdivisions" property must be greater than zero. If there were no subdivisions, the grid paper would not paint anything.'),
+        horizontalColor = color,
+        verticalColor = color,
+        horizontalInterval = interval,
+        verticalInterval = interval,
+        horizontalDivisions = divisions,
+        verticalDivisions = divisions,
+        horizontalSubdivisions = subdivisions,
+        verticalSubdivisions = subdivisions,
+        margin = EdgeInsets.zero,
+        horizontalOffset = 0,
+        verticalOffset = 0,
+        border = const Border(),
+        scale = 1,
+        opacity = 0.5,
+        super(child: child);
+
+  GridPaper.millimeter({
+    PdfColor color = lineColor,
+    Widget child,
+  })  : horizontalColor = color,
+        verticalColor = color,
+        horizontalInterval = 5 * PdfPageFormat.cm,
+        verticalInterval = 5 * PdfPageFormat.cm,
+        horizontalDivisions = 5,
+        verticalDivisions = 5,
+        horizontalSubdivisions = 10,
+        verticalSubdivisions = 10,
+        margin = EdgeInsets.zero,
+        horizontalOffset = 0,
+        verticalOffset = 0,
+        border = const Border(),
+        scale = 1,
+        opacity = 0.5,
+        super(child: child);
+
+  GridPaper.seyes({
+    this.margin = const EdgeInsets.only(
+      top: 20 * PdfPageFormat.mm,
+      bottom: 10 * PdfPageFormat.mm,
+      left: 36 * PdfPageFormat.mm,
+      right: 0,
+    ),
+    Widget child,
+  })  : horizontalColor = const PdfColor.fromInt(0xffc8c8de),
+        verticalColor = const PdfColor.fromInt(0xffc8c8de),
+        horizontalInterval = 8 * PdfPageFormat.mm,
+        verticalInterval = 8 * PdfPageFormat.mm,
+        horizontalDivisions = 1,
+        verticalDivisions = 4,
+        horizontalSubdivisions = 1,
+        verticalSubdivisions = 1,
+        horizontalOffset = 0,
+        verticalOffset = 1,
+        border = const Border(
+            left: BorderSide(
+          color: PdfColor.fromInt(0xfff6bbcf),
+        )),
+        scale = 1,
+        opacity = 1,
+        super(child: child);
+
+  GridPaper.collegeRuled({
+    this.margin = const EdgeInsets.only(
+      top: 1 * PdfPageFormat.inch,
+      bottom: 0.6 * PdfPageFormat.inch,
+      left: 1.25 * PdfPageFormat.inch,
+      right: 0,
+    ),
+    Widget child,
+  })  : horizontalColor = lineColor,
+        verticalColor = lineColor,
+        horizontalInterval = double.infinity,
+        verticalInterval = 9 / 32 * PdfPageFormat.inch,
+        horizontalDivisions = 1,
+        verticalDivisions = 1,
+        horizontalSubdivisions = 1,
+        verticalSubdivisions = 1,
+        horizontalOffset = 0,
+        verticalOffset = 1,
+        border = const Border(
+            left: BorderSide(
+          color: PdfColors.red,
+        )),
+        scale = 1,
+        opacity = 1,
+        super(child: child);
+
+  GridPaper.quad({
+    PdfColor color = lineColor,
+    Widget child,
+  })  : horizontalColor = color,
+        verticalColor = color,
+        horizontalInterval = PdfPageFormat.inch,
+        verticalInterval = PdfPageFormat.inch,
+        horizontalDivisions = 4,
+        verticalDivisions = 4,
+        horizontalSubdivisions = 1,
+        verticalSubdivisions = 1,
+        margin = EdgeInsets.zero,
+        horizontalOffset = 0,
+        verticalOffset = 0,
+        border = const Border(),
+        scale = 1,
+        opacity = 0.5,
+        super(child: child);
+
+  GridPaper.engineering({
+    PdfColor color = lineColor,
+    Widget child,
+  })  : horizontalColor = color,
+        verticalColor = color,
+        horizontalInterval = PdfPageFormat.inch,
+        verticalInterval = PdfPageFormat.inch,
+        horizontalDivisions = 5,
+        verticalDivisions = 5,
+        horizontalSubdivisions = 2,
+        verticalSubdivisions = 2,
+        margin = EdgeInsets.zero,
+        horizontalOffset = 0,
+        verticalOffset = 0,
+        border = const Border(),
+        scale = 1,
+        opacity = 0.5,
+        super(child: child);
+
+  static const lineColor = PdfColor.fromInt(0xffc3e8f3);
+
+  /// The color to draw the horizontal lines in the grid.
+  final PdfColor horizontalColor;
+
+  /// The color to draw the vertical lines in the grid.
+  final PdfColor verticalColor;
+
+  /// The distance between the primary horizontal lines in the grid, in logical pixels.
+  final double horizontalInterval;
+
+  /// The distance between the primary vertical lines in the grid, in logical pixels.
+  final double verticalInterval;
+
+  /// The number of major horizontal divisions within each primary grid cell.
+  final int horizontalDivisions;
+
+  /// The number of major vertical divisions within each primary grid cell.
+  final int verticalDivisions;
+
+  /// The number of minor horizontal divisions within each major division, including the
+  /// major division itself.
+  final int horizontalSubdivisions;
+
+  /// The number of minor vertical divisions within each major division, including the
+  /// major division itself.
+  final int verticalSubdivisions;
+
+  /// The margin to apply to the horizontal and vertical lines
+  final EdgeInsets margin;
+
+  final int horizontalOffset;
+
+  final int verticalOffset;
+
+  final BoxBorder border;
+
+  final double scale;
+
+  final double opacity;
+
+  @override
+  void layout(Context context, BoxConstraints constraints,
+      {bool parentUsesSize = false}) {
+    box = PdfRect.fromPoints(PdfPoint.zero, constraints.biggest);
+    if (child != null) {
+      if (constraints.hasBoundedWidth && constraints.hasBoundedHeight) {
+        final childConstraints = BoxConstraints(
+          maxWidth: constraints.maxWidth - margin.horizontal,
+          maxHeight: constraints.maxHeight - margin.vertical,
+        );
+        child.layout(context, childConstraints, parentUsesSize: false);
+      } else {
+        child.layout(context, constraints, parentUsesSize: false);
+      }
+
+      assert(child.box != null);
+      child.box = PdfRect.fromPoints(
+          PdfPoint(margin.left, box.top - margin.top - child.box.height),
+          child.box.size);
+    }
+  }
+
+  @override
+  void paint(Context context) {
+    super.paint(context);
+    paintChild(context);
+
+    context.canvas.saveContext();
+    context.canvas.setGraphicState(PdfGraphicState(opacity: 0.5));
+    context.canvas.setStrokeColor(horizontalColor);
+    final l = scale;
+    final m = l / 2;
+    final s = m / 2;
+
+    final allHorizontalDivisions =
+        (horizontalDivisions * horizontalSubdivisions).toDouble();
+    var n = horizontalOffset;
+    for (var x = box.left + margin.left;
+        x <= box.right - margin.right;
+        x += horizontalInterval / allHorizontalDivisions) {
+      context.canvas
+        ..setLineWidth((n % (horizontalSubdivisions * horizontalDivisions) == 0)
+            ? l
+            : (n % horizontalSubdivisions == 0)
+                ? m
+                : s)
+        ..drawLine(x, box.top, x, box.bottom)
+        ..strokePath();
+      n++;
+    }
+
+    context.canvas.setStrokeColor(verticalColor);
+    final allVerticalDivisions =
+        (verticalDivisions * verticalSubdivisions).toDouble();
+    n = verticalOffset;
+    for (var y = box.top - margin.top;
+        y >= box.bottom + margin.bottom;
+        y -= verticalInterval / allVerticalDivisions) {
+      context.canvas
+        ..setLineWidth((n % (verticalSubdivisions * verticalDivisions) == 0)
+            ? l
+            : (n % verticalSubdivisions == 0)
+                ? m
+                : s)
+        ..drawLine(box.left, y, box.right, y)
+        ..strokePath();
+      n++;
+    }
+
+    if (border.left.style != BorderStyle.none) {
+      context.canvas
+        ..setStrokeColor(border.left.color)
+        ..setLineWidth(border.left.width)
+        ..drawLine(
+            box.left + margin.left, box.top, box.left + margin.left, box.bottom)
+        ..strokePath();
+    }
+    if (border.right.style != BorderStyle.none) {
+      context.canvas
+        ..setStrokeColor(border.right.color)
+        ..setLineWidth(border.right.width)
+        ..drawLine(box.right - margin.right, box.top, box.right - margin.right,
+            box.bottom)
+        ..strokePath();
+    }
+    if (border.top.style != BorderStyle.none) {
+      context.canvas
+        ..setStrokeColor(border.top.color)
+        ..setLineWidth(border.top.width)
+        ..drawLine(
+            box.left, box.top - margin.top, box.right, box.top - margin.top)
+        ..strokePath();
+    }
+    if (border.bottom.style != BorderStyle.none) {
+      context.canvas
+        ..setStrokeColor(border.bottom.color)
+        ..setLineWidth(border.bottom.width)
+        ..drawLine(box.left, box.bottom + margin.bottom, box.right,
+            box.bottom + margin.bottom)
+        ..strokePath();
+    }
+  }
+}