Improve MultiPage Widget

@@ -2,6 +2,7 @@
 * Add available dimensions for PdfPageFormat
 * Add Document properties
 * Add Page.orientation to force landscape or portrait
+* Improve MultiPage Widget
 
 # 1.3.3
 * Fix a bug with the RichText Widget

@@ -33,6 +33,7 @@ part 'widgets/font.dart';
 part 'widgets/geometry.dart';
 part 'widgets/grid_view.dart';
 part 'widgets/image.dart';
+part 'widgets/multi_page.dart';
 part 'widgets/placeholders.dart';
 part 'widgets/stack.dart';
 part 'widgets/table.dart';

@@ -190,160 +190,3 @@ class Page {
     }
   }
 }
-
-class MultiPage extends Page {
-  const MultiPage(
-      {PdfPageFormat pageFormat = PdfPageFormat.a4,
-      BuildListCallback build,
-      this.crossAxisAlignment = CrossAxisAlignment.start,
-      this.header,
-      this.footer,
-      Theme theme,
-      PageOrientation orientation = PageOrientation.natural,
-      EdgeInsets margin})
-      : _buildList = build,
-        super(
-            pageFormat: pageFormat,
-            margin: margin,
-            theme: theme,
-            orientation: orientation);
-
-  final BuildListCallback _buildList;
-
-  final CrossAxisAlignment crossAxisAlignment;
-
-  final BuildCallback header;
-
-  final BuildCallback footer;
-
-  void paintChild(
-      Context context, Widget child, double x, double y, double pageHeight) {
-    if (mustRotate) {
-      final EdgeInsets _margin = margin;
-      context.canvas
-        ..saveContext()
-        ..setTransform(Matrix4.identity()
-          ..rotateZ(-math.pi / 2)
-          ..translate(x - pageHeight + _margin.top - _margin.left,
-              y + _margin.left - _margin.bottom));
-      child.paint(context);
-      context.canvas.restoreContext();
-    } else {
-      child.box = PdfRect(x, y, child.box.width, child.box.height);
-      child.paint(context);
-    }
-  }
-
-  @override
-  void generate(Document document) {
-    if (_buildList == null) {
-      return;
-    }
-
-    final EdgeInsets _margin = margin;
-    final bool _mustRotate = mustRotate;
-    final double pageHeight =
-        _mustRotate ? pageFormat.width : pageFormat.height;
-    final double pageHeightMargin =
-        _mustRotate ? _margin.horizontal : _margin.vertical;
-    final BoxConstraints constraints = BoxConstraints(
-        maxWidth: _mustRotate
-            ? (pageFormat.height - _margin.vertical)
-            : (pageFormat.width - _margin.horizontal));
-    final Theme calculatedTheme = theme ?? document.theme ?? Theme.base();
-    final Map<Type, Inherited> inherited = <Type, Inherited>{};
-    inherited[calculatedTheme.runtimeType] = calculatedTheme;
-    Context context;
-    double offsetEnd;
-    double offsetStart;
-    int index = 0;
-    final Context baseContext =
-        Context(document: document.document, inherited: inherited);
-    final List<Widget> children = _buildList(baseContext);
-    WidgetContext widgetContext;
-
-    while (index < children.length) {
-      final Widget child = children[index];
-
-      if (context == null) {
-        final PdfPage pdfPage =
-            PdfPage(document.document, pageFormat: pageFormat);
-        final PdfGraphics canvas = pdfPage.getGraphics();
-        context = baseContext.copyWith(page: pdfPage, canvas: canvas);
-        assert(() {
-          if (Document.debug) {
-            debugPaint(context);
-          }
-          return true;
-        }());
-        offsetStart = pageHeight -
-            (_mustRotate ? pageHeightMargin - margin.bottom : _margin.top);
-        offsetEnd =
-            _mustRotate ? pageHeightMargin - _margin.left : _margin.bottom;
-        if (header != null) {
-          final Widget headerWidget = header(context);
-          if (headerWidget != null) {
-            headerWidget.layout(context, constraints, parentUsesSize: false);
-            paintChild(context, headerWidget, _margin.left,
-                offsetStart - headerWidget.box.height, pageFormat.height);
-            offsetStart -= headerWidget.box.height;
-          }
-        }
-
-        if (footer != null) {
-          final Widget footerWidget = footer(context);
-          if (footerWidget != null) {
-            footerWidget.layout(context, constraints, parentUsesSize: false);
-            paintChild(context, footerWidget, _margin.left, _margin.bottom,
-                pageFormat.height);
-            offsetEnd += footerWidget.box.height;
-          }
-        }
-      }
-
-      if (widgetContext != null && child is SpanningWidget) {
-        child.restoreContext(widgetContext);
-        widgetContext = null;
-      }
-
-      child.layout(context, constraints, parentUsesSize: false);
-
-      if (offsetStart - child.box.height < offsetEnd) {
-        if (child.box.height <= pageHeight - pageHeightMargin &&
-            !(child is SpanningWidget)) {
-          context = null;
-          continue;
-        }
-
-        if (!(child is SpanningWidget)) {
-          throw Exception(
-              'Widget won\'t fit into the page as its height (${child.box.height}) '
-              'exceed a page height (${pageHeight - pageHeightMargin}). '
-              'You probably need a SpanningWidget or use a single page layout');
-        }
-
-        final SpanningWidget span = child;
-
-        child.layout(
-            context, constraints.copyWith(maxHeight: offsetStart - offsetEnd),
-            parentUsesSize: false);
-        paintChild(context, child, _margin.left, offsetStart - child.box.height,
-            pageFormat.height);
-
-        if (span.canSpan) {
-          widgetContext = span.saveContext();
-        } else {
-          index++;
-        }
-
-        context = null;
-        continue;
-      }
-
-      paintChild(context, child, _margin.left, offsetStart - child.box.height,
-          pageFormat.height);
-      offsetStart -= child.box.height;
-      index++;
-    }
-  }
-}

+/*
+ * 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;
+
+class WidgetContext {}
+
+abstract class SpanningWidget extends Widget {
+  bool get canSpan => false;
+
+  @protected
+  WidgetContext saveContext();
+
+  @protected
+  void restoreContext(WidgetContext context);
+}
+
+class NewPage extends Widget {
+  @override
+  void layout(Context context, BoxConstraints constraints,
+      {bool parentUsesSize = false}) {
+    box = PdfRect.zero;
+  }
+}
+
+class MultiPage extends Page {
+  const MultiPage(
+      {PdfPageFormat pageFormat = PdfPageFormat.a4,
+      BuildListCallback build,
+      this.crossAxisAlignment = CrossAxisAlignment.start,
+      this.header,
+      this.footer,
+      Theme theme,
+      PageOrientation orientation = PageOrientation.natural,
+      EdgeInsets margin})
+      : _buildList = build,
+        super(
+            pageFormat: pageFormat,
+            margin: margin,
+            theme: theme,
+            orientation: orientation);
+
+  final BuildListCallback _buildList;
+
+  final CrossAxisAlignment crossAxisAlignment;
+
+  final BuildCallback header;
+
+  final BuildCallback footer;
+
+  void _paintChild(
+      Context context, Widget child, double x, double y, double pageHeight) {
+    if (mustRotate) {
+      final EdgeInsets _margin = margin;
+      context.canvas
+        ..saveContext()
+        ..setTransform(Matrix4.identity()
+          ..rotateZ(-math.pi / 2)
+          ..translate(x - pageHeight + _margin.top - _margin.left,
+              y + _margin.left - _margin.bottom));
+      child.paint(context);
+      context.canvas.restoreContext();
+    } else {
+      child.box = PdfRect(x, y, child.box.width, child.box.height);
+      child.paint(context);
+    }
+  }
+
+  @override
+  void generate(Document document) {
+    if (_buildList == null) {
+      return;
+    }
+
+    final EdgeInsets _margin = margin;
+    final bool _mustRotate = mustRotate;
+    final double pageHeight =
+        _mustRotate ? pageFormat.width : pageFormat.height;
+    final double pageHeightMargin =
+        _mustRotate ? _margin.horizontal : _margin.vertical;
+    final BoxConstraints constraints = BoxConstraints(
+        maxWidth: _mustRotate
+            ? (pageFormat.height - _margin.vertical)
+            : (pageFormat.width - _margin.horizontal));
+    final Theme calculatedTheme = theme ?? document.theme ?? Theme.base();
+    final Map<Type, Inherited> inherited = <Type, Inherited>{};
+    inherited[calculatedTheme.runtimeType] = calculatedTheme;
+    Context context;
+    double offsetEnd;
+    double offsetStart;
+    int index = 0;
+    int sameCount = 0;
+    final Context baseContext =
+        Context(document: document.document, inherited: inherited);
+    final List<Widget> children = _buildList(baseContext);
+    WidgetContext widgetContext;
+
+    while (index < children.length) {
+      final Widget child = children[index];
+
+      assert(() {
+        // Detect too big widgets
+        if (sameCount++ > 20) {
+          throw Exception(
+              'This widget created more than 20 pages. This may be an issue in the widget or the document.');
+        }
+        return true;
+      }());
+
+      // Create a new page if we don't already have one
+      if (context == null || child is NewPage) {
+        final PdfPage pdfPage =
+            PdfPage(document.document, pageFormat: pageFormat);
+        context =
+            baseContext.copyWith(page: pdfPage, canvas: pdfPage.getGraphics());
+
+        assert(() {
+          if (Document.debug) {
+            debugPaint(context);
+          }
+          return true;
+        }());
+
+        offsetStart = pageHeight -
+            (_mustRotate ? pageHeightMargin - margin.bottom : _margin.top);
+        offsetEnd =
+            _mustRotate ? pageHeightMargin - _margin.left : _margin.bottom;
+
+        if (header != null) {
+          final Widget headerWidget = header(context);
+          if (headerWidget != null) {
+            headerWidget.layout(context, constraints, parentUsesSize: false);
+            _paintChild(context, headerWidget, _margin.left,
+                offsetStart - headerWidget.box.height, pageFormat.height);
+            offsetStart -= headerWidget.box.height;
+          }
+        }
+
+        if (footer != null) {
+          final Widget footerWidget = footer(context);
+          if (footerWidget != null) {
+            footerWidget.layout(context, constraints, parentUsesSize: false);
+            _paintChild(context, footerWidget, _margin.left, _margin.bottom,
+                pageFormat.height);
+            offsetEnd += footerWidget.box.height;
+          }
+        }
+      }
+
+      // If we are processing a multi-page widget, we restore its context
+      if (widgetContext != null && child is SpanningWidget) {
+        child.restoreContext(widgetContext);
+        widgetContext = null;
+      }
+
+      child.layout(context, constraints, parentUsesSize: false);
+
+      // What to do if the widget is too big for the page?
+      if (offsetStart - child.box.height < offsetEnd) {
+        // If it is not a multi=page widget and its height
+        // is smaller than a full new page, we schedule a new page creation
+        if (child.box.height <= pageHeight - pageHeightMargin &&
+            !(child is SpanningWidget)) {
+          context = null;
+          continue;
+        }
+
+        // Else we crash if the widget is too big and cannot be splitted
+        if (!(child is SpanningWidget)) {
+          throw Exception(
+              'Widget won\'t fit into the page as its height (${child.box.height}) '
+              'exceed a page height (${pageHeight - pageHeightMargin}). '
+              'You probably need a SpanningWidget or use a single page layout');
+        }
+
+        final SpanningWidget span = child;
+
+        child.layout(
+            context, constraints.copyWith(maxHeight: offsetStart - offsetEnd),
+            parentUsesSize: false);
+        _paintChild(context, child, _margin.left,
+            offsetStart - child.box.height, pageFormat.height);
+
+        // Has it finished spanning?
+        if (span.canSpan) {
+          widgetContext = span.saveContext();
+        } else {
+          sameCount = 0;
+          index++;
+        }
+
+        // Schedule a new page
+        context = null;
+        continue;
+      }
+
+      _paintChild(context, child, _margin.left, offsetStart - child.box.height,
+          pageFormat.height);
+      offsetStart -= child.box.height;
+      sameCount = 0;
+      index++;
+    }
+  }
+}

@@ -92,18 +92,6 @@ abstract class Widget {
   }
 }
 
-class WidgetContext {}
-
-abstract class SpanningWidget extends Widget {
-  bool get canSpan => false;
-
-  @protected
-  WidgetContext saveContext();
-
-  @protected
-  void restoreContext(WidgetContext context);
-}
-
 abstract class StatelessWidget extends Widget {
   StatelessWidget() : super();