Convert Flex to a SpanningWidget

@@ -11,6 +11,7 @@
 - Add more font drawing options
 - Add Opacity Widget
 - Fix Text height with TrueType fonts
+- Convert Flex to a SpanningWidget
 
 ## 1.4.1
 

@@ -56,7 +56,28 @@ enum VerticalDirection {
 
 typedef _ChildSizingFunction = double Function(Widget child, double extent);
 
-class Flex extends MultiChildWidget {
+class _FlexContext extends WidgetContext {
+  int firstChild = 0;
+  int lastChild = 0;
+
+  @override
+  void apply(WidgetContext other) {
+    if (other is _FlexContext) {
+      firstChild = other.firstChild;
+      lastChild = other.lastChild;
+    }
+  }
+
+  @override
+  WidgetContext clone() {
+    return _FlexContext()..apply(this);
+  }
+
+  @override
+  String toString() => '$runtimeType first:$firstChild last:$lastChild';
+}
+
+class Flex extends MultiChildWidget implements SpanningWidget {
   Flex({
     @required this.direction,
     this.mainAxisAlignment = MainAxisAlignment.start,
@@ -80,6 +101,8 @@ class Flex extends MultiChildWidget {
 
   final VerticalDirection verticalDirection;
 
+  final _FlexContext _context = _FlexContext();
+
   double _getIntrinsicSize(
       {Axis sizingDirection,
       double
@@ -209,7 +232,6 @@ class Flex extends MultiChildWidget {
       {bool parentUsesSize = false}) {
     // Determine used flex factor, size inflexible items, calculate free space.
     int totalFlex = 0;
-    final int totalChildren = children.length;
     Widget lastFlexChild;
     assert(constraints != null);
     final double maxMainSize = direction == Axis.horizontal
@@ -219,8 +241,9 @@ class Flex extends MultiChildWidget {
 
     double crossSize = 0;
     double allocatedSize = 0; // Sum of the sizes of the non-flexible children.
+    int index = _context.firstChild;
 
-    for (Widget child in children) {
+    for (Widget child in children.sublist(_context.firstChild)) {
       final int flex = child is Flexible ? child.flex : 0;
       final FlexFit fit = child is Flexible ? child.fit : FlexFit.loose;
       if (flex > 0) {
@@ -266,9 +289,16 @@ class Flex extends MultiChildWidget {
         assert(child.box != null);
         allocatedSize += _getMainSize(child);
         crossSize = math.max(crossSize, _getCrossSize(child));
+        if (direction == Axis.vertical &&
+            allocatedSize > constraints.maxHeight) {
+          break;
+        }
       }
       lastFlexChild = child;
+      index++;
     }
+    _context.lastChild = index;
+    final int totalChildren = _context.lastChild - _context.firstChild;
 
     // Distribute free space to flexible children, and determine baseline.
     final double freeSpace =
@@ -409,7 +439,9 @@ class Flex extends MultiChildWidget {
             direction == Axis.vertical);
     double childMainPosition =
         flipMainAxis ? actualSize - leadingSpace : leadingSpace;
-    for (Widget child in children) {
+
+    for (Widget child
+        in children.sublist(_context.firstChild, _context.lastChild)) {
       double childCrossPosition;
       switch (crossAxisAlignment) {
         case CrossAxisAlignment.start:
@@ -458,11 +490,31 @@ class Flex extends MultiChildWidget {
     context.canvas
       ..saveContext()
       ..setTransform(mat);
-    for (Widget child in children) {
+
+    for (Widget child
+        in children.sublist(_context.firstChild, _context.lastChild)) {
       child.paint(context);
     }
     context.canvas.restoreContext();
   }
+
+  @override
+  bool get canSpan => direction == Axis.vertical;
+
+  @override
+  bool get hasMoreWidgets => true;
+
+  @override
+  void restoreContext(WidgetContext context) {
+    if (context is _FlexContext) {
+      _context.firstChild = context.lastChild;
+    }
+  }
+
+  @override
+  WidgetContext saveContext() {
+    return _context;
+  }
 }
 
 class Row extends Flex {

@@ -328,6 +328,9 @@ class GridView extends MultiChildWidget implements SpanningWidget {
   bool get canSpan => true;
 
   @override
+  bool get hasMoreWidgets => true;
+
+  @override
   void restoreContext(WidgetContext context) {
     if (context is _GridViewContext) {
       _context.firstChild = context.lastChild;

@@ -25,7 +25,9 @@ abstract class WidgetContext {
 }
 
 abstract class SpanningWidget extends Widget {
-  bool get canSpan => false;
+  bool get canSpan;
+
+  bool get hasMoreWidgets;
 
   /// Get unmodified mutable context object
   @protected
@@ -170,6 +172,10 @@ class MultiPage extends Page {
 
     while (index < children.length) {
       final Widget child = children[index];
+      bool canSpan = false;
+      if (child is SpanningWidget) {
+        canSpan = child.canSpan;
+      }
 
       assert(() {
         // Detect too big widgets
@@ -226,7 +232,7 @@ class MultiPage extends Page {
       }
 
       // If we are processing a multi-page widget, we restore its context
-      if (widgetContext != null && child is SpanningWidget) {
+      if (widgetContext != null && canSpan && child is SpanningWidget) {
         child.restoreContext(widgetContext);
         widgetContext = null;
       }
@@ -238,14 +244,13 @@ class MultiPage extends 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)) {
+        if (child.box.height <= pageHeight - pageHeightMargin && !canSpan) {
           context = null;
           continue;
         }
 
         // Else we crash if the widget is too big and cannot be splitted
-        if (!(child is SpanningWidget)) {
+        if (!canSpan) {
           throw Exception(
               'Widget won\'t fit into the page as its height (${child.box.height}) '
               'exceed a page height (${pageHeight - pageHeightMargin}). '
@@ -270,7 +275,7 @@ class MultiPage extends Page {
         );
 
         // Has it finished spanning?
-        if (!span.canSpan) {
+        if (!span.hasMoreWidgets) {
           sameCount = 0;
           index++;
         }
@@ -286,8 +291,9 @@ class MultiPage extends Page {
           x: _margin.left,
           y: offsetStart - child.box.height,
           constraints: constraints,
-          widgetContext:
-              child is SpanningWidget ? child.saveContext().clone() : null,
+          widgetContext: child is SpanningWidget && canSpan
+              ? child.saveContext().clone()
+              : null,
         ),
       );
 
@@ -315,7 +321,7 @@ class MultiPage extends Page {
 
       for (_MultiPageWidget widget in page.widgets) {
         final Widget child = widget.child;
-        if (child is SpanningWidget) {
+        if (child is SpanningWidget && child.canSpan) {
           final WidgetContext context = child.saveContext();
           context.apply(widget.widgetContext);
         }

@@ -224,6 +224,9 @@ class Table extends Widget implements SpanningWidget {
   @override
   bool get canSpan => true;
 
+  @override
+  bool get hasMoreWidgets => true;
+
   /// The rows of the table.
   final List<TableRow> children;
 

@@ -109,7 +109,10 @@ class Wrap extends MultiChildWidget implements SpanningWidget {
   bool get textDirection => false;
 
   @override
-  bool get canSpan => _context.lastChild < children.length;
+  bool get canSpan => true;
+
+  @override
+  bool get hasMoreWidgets => _context.lastChild < children.length;
 
   final _WrapContext _context = _WrapContext();