Allow MultiPage to relayout individual pages with support for flex

 # Changelog
 
+## 1.9.0
+
+- Allow MultiPage to relayout individual pages with support for flex
+
 ## 1.8.1
 
 - Fix Wrap break condition

@@ -50,15 +50,11 @@ class NewPage extends Widget {
 class _MultiPageWidget {
   const _MultiPageWidget({
     @required this.child,
-    @required this.x,
-    @required this.y,
     @required this.constraints,
     @required this.widgetContext,
   });
 
   final Widget child;
-  final double x;
-  final double y;
   final BoxConstraints constraints;
   final WidgetContext widgetContext;
 }
@@ -80,18 +76,21 @@ class _MultiPageInstance {
 }
 
 class MultiPage extends Page {
-  MultiPage(
-      {PageTheme pageTheme,
-      PdfPageFormat pageFormat,
-      BuildListCallback build,
-      this.crossAxisAlignment = CrossAxisAlignment.start,
-      this.header,
-      this.footer,
-      ThemeData theme,
-      this.maxPages = 20,
-      PageOrientation orientation,
-      EdgeInsets margin})
-      : _buildList = build,
+  MultiPage({
+    PageTheme pageTheme,
+    PdfPageFormat pageFormat,
+    BuildListCallback build,
+    this.mainAxisAlignment = MainAxisAlignment.start,
+    this.crossAxisAlignment = CrossAxisAlignment.start,
+    this.header,
+    this.footer,
+    ThemeData theme,
+    this.maxPages = 20,
+    PageOrientation orientation,
+    EdgeInsets margin,
+  })  : _buildList = build,
+        assert(mainAxisAlignment != null),
+        assert(crossAxisAlignment != null),
         assert(maxPages != null && maxPages > 0),
         super(
             pageTheme: pageTheme,
@@ -108,6 +107,8 @@ class MultiPage extends Page {
 
   final BuildCallback footer;
 
+  final MainAxisAlignment mainAxisAlignment;
+
   final List<_MultiPageInstance> _pages = <_MultiPageInstance>[];
 
   final int maxPages;
@@ -202,7 +203,7 @@ class MultiPage extends Page {
         }());
 
         offsetStart = pageHeight -
-            (_mustRotate ? pageHeightMargin - margin.bottom : _margin.top);
+            (_mustRotate ? pageHeightMargin - _margin.bottom : _margin.top);
         offsetEnd =
             _mustRotate ? pageHeightMargin - _margin.left : _margin.bottom;
 
@@ -268,8 +269,6 @@ class MultiPage extends Page {
         _pages.last.widgets.add(
           _MultiPageWidget(
             child: child,
-            x: _margin.left,
-            y: offsetStart - child.box.height,
             constraints: localConstraints,
             widgetContext: widgetContext?.clone(),
           ),
@@ -289,8 +288,6 @@ class MultiPage extends Page {
       _pages.last.widgets.add(
         _MultiPageWidget(
           child: child,
-          x: _margin.left,
-          y: offsetStart - child.box.height,
           constraints: constraints,
           widgetContext: child is SpanningWidget && canSpan
               ? child.saveContext().clone()
@@ -307,8 +304,22 @@ class MultiPage extends Page {
   @override
   void postProcess(Document document) {
     final EdgeInsets _margin = margin;
+    final bool _mustRotate = mustRotate;
+    final double pageHeight =
+        _mustRotate ? pageFormat.width : pageFormat.height;
+    final double pageWidth = _mustRotate ? pageFormat.height : pageFormat.width;
+    final double pageHeightMargin =
+        _mustRotate ? _margin.horizontal : _margin.vertical;
+    final double pageWidthMargin =
+        _mustRotate ? _margin.vertical : _margin.horizontal;
+    final double availableWidth = pageWidth - pageWidthMargin;
 
     for (_MultiPageInstance page in _pages) {
+      double offsetStart = pageHeight -
+          (_mustRotate ? pageHeightMargin - _margin.bottom : _margin.top);
+      double offsetEnd =
+          _mustRotate ? pageHeightMargin - _margin.left : _margin.bottom;
+
       if (pageTheme.buildBackground != null) {
         final Widget child = pageTheme.buildBackground(page.context);
         if (child != null) {
@@ -320,16 +331,25 @@ class MultiPage extends Page {
         }
       }
 
+      int totalFlex = 0;
+      double allocatedSize = 0;
+      Widget lastFlexChild;
       for (_MultiPageWidget widget in page.widgets) {
         final Widget child = widget.child;
-        if (child is SpanningWidget && child.canSpan) {
-          final WidgetContext context = child.saveContext();
-          context.apply(widget.widgetContext);
+        final int flex = child is Flexible ? child.flex : 0;
+        if (flex > 0) {
+          totalFlex += flex;
+          lastFlexChild = child;
+        } else {
+          if (child is SpanningWidget && child.canSpan) {
+            final WidgetContext context = child.saveContext();
+            context.apply(widget.widgetContext);
+          }
+
+          child.layout(page.context, widget.constraints, parentUsesSize: false);
+          assert(child.box != null);
+          allocatedSize += child.box.height;
         }
-        child.layout(page.context, widget.constraints, parentUsesSize: false);
-        assert(child.box != null);
-        _paintChild(
-            page.context, widget.child, widget.x, widget.y, pageFormat.height);
       }
 
       if (header != null) {
@@ -338,6 +358,7 @@ class MultiPage extends Page {
           headerWidget.layout(page.context, page.constraints,
               parentUsesSize: false);
           assert(headerWidget.box != null);
+          offsetStart -= headerWidget.box.height;
           _paintChild(page.context, headerWidget, _margin.left,
               page.offsetStart - headerWidget.box.height, pageFormat.height);
         }
@@ -349,11 +370,114 @@ class MultiPage extends Page {
           footerWidget.layout(page.context, page.constraints,
               parentUsesSize: false);
           assert(footerWidget.box != null);
+          offsetEnd += footerWidget.box.height;
           _paintChild(page.context, footerWidget, _margin.left, _margin.bottom,
               pageFormat.height);
         }
       }
 
+      final double freeSpace =
+          math.max(0, offsetStart - offsetEnd - allocatedSize);
+
+      final double spacePerFlex =
+          totalFlex > 0 ? (freeSpace / totalFlex) : double.nan;
+      double allocatedFlexSpace = 0;
+
+      double leadingSpace = 0;
+      double betweenSpace = 0;
+
+      if (totalFlex == 0) {
+        final int totalChildren = page.widgets.length;
+
+        switch (mainAxisAlignment) {
+          case MainAxisAlignment.start:
+            leadingSpace = 0.0;
+            betweenSpace = 0.0;
+            break;
+          case MainAxisAlignment.end:
+            leadingSpace = freeSpace;
+            betweenSpace = 0.0;
+            break;
+          case MainAxisAlignment.center:
+            leadingSpace = freeSpace / 2.0;
+            betweenSpace = 0.0;
+            break;
+          case MainAxisAlignment.spaceBetween:
+            leadingSpace = 0.0;
+            betweenSpace =
+                totalChildren > 1 ? freeSpace / (totalChildren - 1) : 0.0;
+            break;
+          case MainAxisAlignment.spaceAround:
+            betweenSpace = totalChildren > 0 ? freeSpace / totalChildren : 0.0;
+            leadingSpace = betweenSpace / 2.0;
+            break;
+          case MainAxisAlignment.spaceEvenly:
+            betweenSpace =
+                totalChildren > 0 ? freeSpace / (totalChildren + 1) : 0.0;
+            leadingSpace = betweenSpace;
+            break;
+        }
+      }
+
+      for (_MultiPageWidget widget in page.widgets) {
+        final Widget child = widget.child;
+
+        final int flex = child is Flexible ? child.flex : 0;
+        final FlexFit fit = child is Flexible ? child.fit : FlexFit.loose;
+        if (flex > 0) {
+          assert(child is! SpanningWidget);
+          final double maxChildExtent = child == lastFlexChild
+              ? (freeSpace - allocatedFlexSpace)
+              : spacePerFlex * flex;
+          double minChildExtent;
+          switch (fit) {
+            case FlexFit.tight:
+              assert(maxChildExtent < double.infinity);
+              minChildExtent = maxChildExtent;
+              break;
+            case FlexFit.loose:
+              minChildExtent = 0.0;
+              break;
+          }
+          assert(minChildExtent != null);
+          final BoxConstraints innerConstraints = BoxConstraints(
+              minWidth: widget.constraints.maxWidth,
+              maxWidth: widget.constraints.maxWidth,
+              minHeight: minChildExtent,
+              maxHeight: maxChildExtent);
+
+          child.layout(page.context, innerConstraints, parentUsesSize: false);
+          assert(child.box != null);
+          final double childSize = child.box.height;
+          assert(childSize <= maxChildExtent);
+          allocatedSize += childSize;
+          allocatedFlexSpace += maxChildExtent;
+        }
+      }
+
+      double pos = offsetStart - leadingSpace;
+      for (_MultiPageWidget widget in page.widgets) {
+        pos -= widget.child.box.height;
+        double x;
+        switch (crossAxisAlignment) {
+          case CrossAxisAlignment.start:
+            x = 0;
+            break;
+          case CrossAxisAlignment.end:
+            x = availableWidth - widget.child.box.width;
+            break;
+          case CrossAxisAlignment.center:
+            x = availableWidth / 2 - widget.child.box.width / 2;
+            break;
+          case CrossAxisAlignment.stretch:
+            x = 0;
+            break;
+        }
+        _paintChild(page.context, widget.child, _margin.left + x, pos,
+            pageFormat.height);
+        pos -= betweenSpace;
+      }
+
       if (pageTheme.buildForeground != null) {
         final Widget child = pageTheme.buildForeground(page.context);
         if (child != null) {

@@ -4,7 +4,7 @@ description: A pdf producer for Dart. It can create pdf files for both web or fl
 homepage: https://github.com/DavBfr/dart_pdf/tree/master/pdf
 repository: https://github.com/DavBfr/dart_pdf
 issue_tracker: https://github.com/DavBfr/dart_pdf/issues
-version: 1.8.1
+version: 1.9.0
 
 environment:
   sdk: ">=2.3.0 <3.0.0"

@@ -93,6 +93,24 @@ void main() {
     );
   });
 
+  test('MultiPage Spacer', () {
+    pdf.addPage(
+      MultiPage(
+        mainAxisAlignment: MainAxisAlignment.center,
+        crossAxisAlignment: CrossAxisAlignment.center,
+        build: (Context context) => <Widget>[
+          for (int i = 0; i < 60; i++) Text('Begin $i'),
+          Spacer(), // Defaults to a flex of one.
+          Text('Middle'),
+          // Gives twice the space between Middle and End than Begin and Middle.
+          Spacer(flex: 2),
+          // Expanded(flex: 2, child: SizedBox.shrink()),
+          Text('End'),
+        ],
+      ),
+    );
+  });
+
   tearDownAll(() {
     final File file = File('widgets-flex.pdf');
     file.writeAsBytesSync(pdf.save());