Convert Wrap to SpanningWidget

@@ -55,9 +55,11 @@ class MultiPage extends Page {
       this.header,
       this.footer,
       Theme theme,
+      this.maxPages = 20,
       PageOrientation orientation = PageOrientation.natural,
       EdgeInsets margin})
       : _buildList = build,
+        assert(maxPages != null && maxPages > 0),
         super(
             pageFormat: pageFormat,
             margin: margin,
@@ -74,6 +76,8 @@ class MultiPage extends Page {
 
   final List<_MultiPageInstance> _pages = <_MultiPageInstance>[];
 
+  final int maxPages;
+
   void _paintChild(
       Context context, Widget child, double x, double y, double pageHeight) {
     if (mustRotate) {
@@ -126,9 +130,9 @@ class MultiPage extends Page {
 
       assert(() {
         // Detect too big widgets
-        if (sameCount++ > 20) {
+        if (sameCount++ > maxPages) {
           throw Exception(
-              'This widget created more than 20 pages. This may be an issue in the widget or the document.');
+              'This widget created more than $maxPages pages. This may be an issue in the widget or the document.');
         }
         return true;
       }());
@@ -188,7 +192,7 @@ class MultiPage extends Page {
 
       // 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
+        // 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)) {

@@ -37,8 +37,16 @@ class _RunMetrics {
   final int childCount;
 }
 
+class _WrapContext extends WidgetContext {
+  int firstChild = 0;
+  int lastChild = 0;
+
+  @override
+  String toString() => 'WrapContext first:$firstChild last:$lastChild';
+}
+
 /// A widget that displays its children in multiple horizontal or vertical runs.
-class Wrap extends MultiChildWidget {
+class Wrap extends MultiChildWidget implements SpanningWidget {
   /// Creates a wrap layout.
 
   Wrap({
@@ -83,7 +91,10 @@ class Wrap extends MultiChildWidget {
 
   bool get textDirection => false;
 
-  bool _hasVisualOverflow = false;
+  @override
+  bool get canSpan => _context.lastChild < children.length;
+
+  final _WrapContext _context = _WrapContext();
 
   bool get _debugHasNecessaryDirections {
     assert(direction != null);
@@ -192,9 +203,8 @@ class Wrap extends MultiChildWidget {
   void layout(Context context, BoxConstraints constraints,
       {bool parentUsesSize = false}) {
     assert(_debugHasNecessaryDirections);
-    _hasVisualOverflow = false;
 
-    if (children.isEmpty) {
+    if (children.isEmpty || _context.firstChild >= children.length) {
       box = PdfRect.fromPoints(PdfPoint.zero, constraints.smallest);
       return;
     }
@@ -234,7 +244,7 @@ class Wrap extends MultiChildWidget {
     double runCrossAxisExtent = 0.0;
     int childCount = 0;
 
-    for (Widget child in children) {
+    for (Widget child in children.sublist(_context.firstChild)) {
       child.layout(context, childConstraints, parentUsesSize: true);
 
       final double childMainAxisExtent = _getMainAxisExtent(child);
@@ -297,9 +307,6 @@ class Wrap extends MultiChildWidget {
         break;
     }
 
-    _hasVisualOverflow = containerMainAxisExtent < mainAxisExtent ||
-        containerCrossAxisExtent < crossAxisExtent;
-
     final double crossAxisFreeSpace =
         math.max(0.0, containerCrossAxisExtent - crossAxisExtent);
     double runLeadingSpace = 0.0;
@@ -333,7 +340,7 @@ class Wrap extends MultiChildWidget {
         ? containerCrossAxisExtent - runLeadingSpace
         : runLeadingSpace;
 
-    int currentWidget = 0;
+    _context.lastChild = _context.firstChild;
     for (int i = 0; i < runCount; ++i) {
       final _RunMetrics metrics = runMetrics[i];
       final double runMainAxisExtent = metrics.mainAxisExtent;
@@ -377,6 +384,12 @@ class Wrap extends MultiChildWidget {
         crossAxisOffset -= runCrossAxisExtent;
       }
 
+      if (crossAxisOffset < -.01 ||
+          crossAxisOffset + runCrossAxisExtent > containerCrossAxisExtent) {
+        break;
+      }
+
+      int currentWidget = _context.lastChild;
       for (Widget child in children.sublist(currentWidget)) {
         final int runIndex = childRunMetrics[child];
         if (runIndex != i) {
@@ -407,6 +420,8 @@ class Wrap extends MultiChildWidget {
       } else {
         crossAxisOffset += runCrossAxisExtent + runBetweenSpace;
       }
+
+      _context.lastChild = currentWidget;
     }
   }
 
@@ -416,19 +431,26 @@ class Wrap extends MultiChildWidget {
 
     context.canvas.saveContext();
 
-    if (_hasVisualOverflow) {
-      context.canvas
-        ..drawRect(box.left, box.bottom, box.width, box.height)
-        ..clipPath();
-    }
-
     final Matrix4 mat = Matrix4.identity();
     mat.translate(box.x, box.y);
     context.canvas.setTransform(mat);
-    for (Widget child in children) {
+    for (Widget child
+        in children.sublist(_context.firstChild, _context.lastChild)) {
       child.paint(context);
     }
 
     context.canvas.restoreContext();
   }
+
+  @override
+  void restoreContext(WidgetContext context) {
+    if (context is _WrapContext) {
+      _context.firstChild = context.lastChild;
+    }
+  }
+
+  @override
+  WidgetContext saveContext() {
+    return _context;
+  }
 }

@@ -254,6 +254,36 @@ void main() {
     );
   });
 
+  test('Wrap Widget Multipage', () {
+    final math.Random rnd = math.Random(42);
+    pdf.addPage(
+      MultiPage(
+        pageFormat: const PdfPageFormat(200, 200),
+        margin: const EdgeInsets.all(10),
+        build: (Context context) => <Widget>[
+          Wrap(
+            direction: Axis.vertical,
+            verticalDirection: VerticalDirection.up,
+            alignment: WrapAlignment.center,
+            runAlignment: WrapAlignment.center,
+            crossAxisAlignment: WrapCrossAlignment.center,
+            spacing: 10,
+            runSpacing: 10,
+            children: List<Widget>.generate(
+                17,
+                (int n) => Container(
+                      width: rnd.nextDouble() * 100,
+                      height: rnd.nextDouble() * 100,
+                      alignment: Alignment.center,
+                      color: PdfColors.blue800,
+                      child: Text('$n'),
+                    )),
+          )
+        ],
+      ),
+    );
+  });
+
   test('Wrap Widget Empty', () {
     pdf.addPage(Page(build: (Context context) => Wrap()));
   });