Add Stack Widget

@@ -3,6 +3,7 @@
 * Improve font bounds calculation
 * Add RichText Widget
 * Fix MultiPage max height
+* Add Stack Widget
 
 # 1.3.2
 * Update Readme

@@ -34,6 +34,7 @@ part 'widgets/geometry.dart';
 part 'widgets/grid_view.dart';
 part 'widgets/image.dart';
 part 'widgets/placeholders.dart';
+part 'widgets/stack.dart';
 part 'widgets/table.dart';
 part 'widgets/text.dart';
 part 'widgets/theme.dart';

@@ -32,6 +32,13 @@ class BoxConstraints {
         minHeight = height != null ? height : 0.0,
         maxHeight = height != null ? height : double.infinity;
 
+  /// Creates box constraints that is respected only by the given size.
+  BoxConstraints.tight(PdfPoint size)
+      : minWidth = size.x,
+        maxWidth = size.x,
+        minHeight = size.y,
+        maxHeight = size.y;
+
   /// Creates box constraints that expand to fill another box constraints.
   const BoxConstraints.expand({double width, double height})
       : minWidth = width != null ? width : double.infinity,

+/*
+ * 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.
+ */
+
+part of widget;
+
+/// How to size the non-positioned children of a [Stack].
+enum StackFit { loose, expand, passthrough }
+
+/// Whether overflowing children should be clipped, or their overflow be
+/// visible.
+enum Overflow { visible, clip }
+
+/// A widget that controls where a child of a [Stack] is positioned.
+class Positioned extends SingleChildWidget {
+  Positioned({
+    this.left,
+    this.top,
+    this.right,
+    this.bottom,
+    @required Widget child,
+  }) : super(child: child);
+
+  final double left;
+
+  final double top;
+
+  final double right;
+
+  final double bottom;
+
+  double get width => box?.width;
+
+  double get height => box?.height;
+}
+
+/// A widget that positions its children relative to the edges of its box.
+class Stack extends MultiChildWidget {
+  Stack({
+    this.alignment = Alignment.topLeft,
+    this.fit = StackFit.loose,
+    this.overflow = Overflow.clip,
+    List<Widget> children = const <Widget>[],
+  }) : super(children: children);
+
+  /// How to align the non-positioned and partially-positioned children in the
+  /// stack.
+  final Alignment alignment;
+
+  /// How to size the non-positioned children in the stack.
+  final StackFit fit;
+
+  /// Whether overflowing children should be clipped.
+  final Overflow overflow;
+
+  @override
+  void layout(Context context, BoxConstraints constraints,
+      {bool parentUsesSize = false}) {
+    final int childCount = children.length;
+
+    bool hasNonPositionedChildren = false;
+
+    if (childCount == 0) {
+      box = PdfRect.fromPoints(PdfPoint.zero, constraints.biggest);
+      return;
+    }
+
+    double width = constraints.minWidth;
+    double height = constraints.minHeight;
+
+    BoxConstraints nonPositionedConstraints;
+    assert(fit != null);
+    switch (fit) {
+      case StackFit.loose:
+        nonPositionedConstraints = constraints.loosen();
+        break;
+      case StackFit.expand:
+        nonPositionedConstraints = BoxConstraints.tight(constraints.biggest);
+        break;
+      case StackFit.passthrough:
+        nonPositionedConstraints = constraints;
+        break;
+    }
+    assert(nonPositionedConstraints != null);
+
+    for (Widget child in children) {
+      if (!(child is Positioned)) {
+        hasNonPositionedChildren = true;
+
+        child.layout(context, nonPositionedConstraints, parentUsesSize: true);
+
+        final PdfRect childSize = child.box;
+        width = math.max(width, childSize.width);
+        height = math.max(height, childSize.height);
+      }
+    }
+
+    if (hasNonPositionedChildren) {
+      box = PdfRect.fromPoints(PdfPoint.zero, PdfPoint(width, height));
+      assert(box.width == constraints.constrainWidth(width));
+      assert(box.height == constraints.constrainHeight(height));
+    } else {
+      box = PdfRect.fromPoints(PdfPoint.zero, constraints.biggest);
+    }
+
+    for (Widget child in children) {
+      if (!(child is Positioned)) {
+        child.box = PdfRect.fromPoints(
+            alignment.inscribe(child.box.size, box).offset, child.box.size);
+      } else {
+        final Positioned positioned = child;
+        BoxConstraints childConstraints = const BoxConstraints();
+
+        if (positioned.left != null && positioned.right != null)
+          childConstraints = childConstraints.tighten(
+              width: box.width - positioned.right - positioned.left);
+        else if (positioned.width != null)
+          childConstraints = childConstraints.tighten(width: positioned.width);
+
+        if (positioned.top != null && positioned.bottom != null)
+          childConstraints = childConstraints.tighten(
+              height: box.height - positioned.bottom - positioned.top);
+        else if (positioned.height != null)
+          childConstraints =
+              childConstraints.tighten(height: positioned.height);
+
+        positioned.layout(context, childConstraints, parentUsesSize: true);
+
+        double x;
+        if (positioned.left != null) {
+          x = positioned.left;
+        } else if (positioned.right != null) {
+          x = box.width - positioned.right - positioned.width;
+        } else {
+          x = alignment.inscribe(positioned.box.size, box).x;
+        }
+
+        double y;
+        if (positioned.bottom != null) {
+          y = positioned.bottom;
+        } else if (positioned.top != null) {
+          y = box.height - positioned.top - positioned.height;
+        } else {
+          y = alignment.inscribe(positioned.box.size, box).y;
+        }
+
+        positioned.box =
+            PdfRect.fromPoints(PdfPoint(x, y), positioned.box.size);
+      }
+    }
+  }
+
+  @override
+  void paint(Context context) {
+    super.paint(context);
+
+    final Matrix4 mat = Matrix4.identity();
+    mat.translate(box.x, box.y);
+    context.canvas
+      ..saveContext()
+      ..setTransform(mat);
+    if (overflow == Overflow.clip) {
+      context.canvas
+        ..drawRect(0, 0, box.width, box.height)
+        ..clipPath();
+    }
+    for (Widget child in children) {
+      child.paint(context);
+    }
+    context.canvas.restoreContext();
+  }
+}

@@ -133,32 +133,48 @@ void main() {
                 <String>['York Steak House', 'Outi Vuorinen', 'Finland'],
                 <String>['Weathervane', 'Else Jeremiassen', 'Iceland'],
               ]),
-              CustomPaint(
-                  size: const PdfPoint(50, 50),
-                  painter: (PdfGraphics canvas, PdfPoint size) {
-                    canvas
-                      ..setColor(PdfColor.indigo)
-                      ..drawRRect(0, 0, size.x, size.y, 10, 10)
-                      ..fillPath();
-                  }),
-              RichText(
-                text: TextSpan(
-                  text: 'Hello ',
-                  style: Theme.of(context).defaultTextStyle,
-                  children: <TextSpan>[
-                    TextSpan(
-                        text: 'bold',
-                        style: Theme.of(context)
-                            .defaultTextStyleBold
-                            .copyWith(fontSize: 20, color: PdfColor.blue)),
-                    const TextSpan(
-                      text: ' world!',
-                    ),
-                  ],
-                ),
-              )
             ]));
 
+    pdf.addPage(Page(
+        pageFormat: const PdfPageFormat(400.0, 200.0),
+        margin: const EdgeInsets.all(10.0),
+        build: (Context context) => Stack(overflow: Overflow.visible,
+                // fit: StackFit.expand,
+                // alignment: Alignment.bottomRight,
+                children: <Widget>[
+                  Positioned(
+                      right: 10,
+                      top: 10,
+                      child: CustomPaint(
+                          size: const PdfPoint(50, 50),
+                          painter: (PdfGraphics canvas, PdfPoint size) {
+                            canvas
+                              ..setColor(PdfColor.indigo)
+                              ..drawRRect(0, 0, size.x, size.y, 10, 10)
+                              ..fillPath();
+                          })),
+                  Positioned(
+                      left: 10,
+                      bottom: 10,
+                      child: RichText(
+                        text: TextSpan(
+                          text: 'Hello ',
+                          style: Theme.of(context).defaultTextStyle,
+                          children: <TextSpan>[
+                            TextSpan(
+                                text: 'bold',
+                                style: Theme.of(context)
+                                    .defaultTextStyleBold
+                                    .copyWith(
+                                        fontSize: 20, color: PdfColor.blue)),
+                            const TextSpan(
+                              text: ' world!',
+                            ),
+                          ],
+                        ),
+                      ))
+                ])));
+
     final File file = File('widgets.pdf');
     file.writeAsBytesSync(pdf.document.save());
   });