QGAL: Support for out-of-order parallel anchors

Now parallel anchors account for the fact they may be composed of
anchors with different directions. We arbitrarily set the direction
of the parallel anchor to be the same direction as the first child.

The methods refreshSizeHints and updateChildren were updated to
support this situation.

Signed-off-by: Eduardo M. Fleury <eduardo.fleury@openbossa.org>
Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@openbossa.org>

2 files changed, 45 insertions, 10 deletions

diff --git a/src/gui/graphicsview/qgraphicsanchorlayout_p.cpp b/src/gui/graphicsview/qgraphicsanchorlayout_p.cpp
index 7ad994c..95922ca 100644
--- a/src/gui/graphicsview/qgraphicsanchorlayout_p.cpp
+++ b/src/gui/graphicsview/qgraphicsanchorlayout_p.cpp
@@ -218,9 +218,20 @@ bool AnchorData::refreshSizeHints(const QLayoutStyleInfo *styleInfo)
 
 void ParallelAnchorData::updateChildrenSizes()
 {
-    firstEdge->sizeAtMinimum = secondEdge->sizeAtMinimum = sizeAtMinimum;
-    firstEdge->sizeAtPreferred = secondEdge->sizeAtPreferred = sizeAtPreferred;
-    firstEdge->sizeAtMaximum = secondEdge->sizeAtMaximum = sizeAtMaximum;
+    firstEdge->sizeAtMinimum = sizeAtMinimum;
+    firstEdge->sizeAtPreferred = sizeAtPreferred;
+    firstEdge->sizeAtMaximum = sizeAtMaximum;
+
+    const bool secondFwd = (secondEdge->from == from);
+    if (secondFwd) {
+        secondEdge->sizeAtMinimum = sizeAtMinimum;
+        secondEdge->sizeAtPreferred = sizeAtPreferred;
+        secondEdge->sizeAtMaximum = sizeAtMaximum;
+    } else {
+        secondEdge->sizeAtMinimum = -sizeAtMinimum;
+        secondEdge->sizeAtPreferred = -sizeAtPreferred;
+        secondEdge->sizeAtMaximum = -sizeAtMaximum;
+    }
 
     firstEdge->updateChildrenSizes();
     secondEdge->updateChildrenSizes();
@@ -239,8 +250,16 @@ bool ParallelAnchorData::refreshSizeHints_helper(const QLayoutStyleInfo *styleIn
         return false;
     }
 
-    minSize = qMax(firstEdge->minSize, secondEdge->minSize);
-    maxSize = qMin(firstEdge->maxSize, secondEdge->maxSize);
+    // Account for parallel anchors where the second edge is backwards.
+    // We rely on the fact that a forward anchor of sizes min, pref, max is equivalent
+    // to a backwards anchor of size (-max, -pref, -min)
+    const bool secondFwd = (secondEdge->from == from);
+    const qreal secondMin = secondFwd ? secondEdge->minSize : -secondEdge->maxSize;
+    const qreal secondPref = secondFwd ? secondEdge->prefSize : -secondEdge->prefSize;
+    const qreal secondMax = secondFwd ? secondEdge->maxSize : -secondEdge->minSize;
+
+    minSize = qMax(firstEdge->minSize, secondMin);
+    maxSize = qMin(firstEdge->maxSize, secondMax);
 
     // This condition means that the maximum size of one anchor being simplified is smaller than
     // the minimum size of the other anchor. The consequence is that there won't be a valid size
@@ -249,7 +268,22 @@ bool ParallelAnchorData::refreshSizeHints_helper(const QLayoutStyleInfo *styleIn
         return false;
     }
 
-    prefSize = qMax(firstEdge->prefSize, secondEdge->prefSize);
+    // The equivalent preferred Size of a parallel anchor is calculated as to
+    // reduce the deviation from the original preferred sizes _and_ to avoid shrinking
+    // items below their preferred sizes, unless strictly needed.
+
+    // ### This logic only holds if all anchors in the layout are "well-behaved" in the
+    // following terms:
+    //
+    // - There are no negative-sized anchors
+    // - All sequential anchors are composed of children in the same direction as the
+    //   sequential anchor itself
+    //
+    // With these assumptions we can grow a child knowing that no hidden items will
+    // have to shrink as the result of that.
+    // If any of these does not hold, we have a situation where the ParallelAnchor
+    // does not have enough information to calculate its equivalent prefSize.
+    prefSize = qMax(firstEdge->prefSize, secondPref);
     prefSize = qMin(prefSize, maxSize);
 
     // See comment in AnchorData::refreshSizeHints() about sizeAt* values
diff --git a/src/gui/graphicsview/qgraphicsanchorlayout_p.h b/src/gui/graphicsview/qgraphicsanchorlayout_p.h
index 1e11ee2..8d77b1a 100644
--- a/src/gui/graphicsview/qgraphicsanchorlayout_p.h
+++ b/src/gui/graphicsview/qgraphicsanchorlayout_p.h
@@ -256,10 +256,11 @@ struct ParallelAnchorData : public AnchorData
         type = AnchorData::Parallel;
         orientation = first->orientation;
 
-        // ### Those asserts force that both child anchors have the same direction,
-        // but can't we simplify a pair of anchors in opposite directions?
-        Q_ASSERT(first->from == second->from);
-        Q_ASSERT(first->to == second->to);
+        // This assert whether the child anchors share their vertices
+        Q_ASSERT(((first->from == second->from) && (first->to == second->to)) ||
+                 ((first->from == second->to) && (first->to == second->from)));
+
+        // We arbitrarily choose the direction of the first child as "our" direction
         from = first->from;
         to = first->to;
 #ifdef QT_DEBUG