QGAL: parallel anchors sometimes have to fix constraints

For an out-of-order parallel anchor, i.e. when the second child is
backwards in relation to the parallel anchor, we need to fix the
constraints that used to have the second child.

The parallel anchor will replace its child, but we must change the
multiplier signal, so the constraint remain valid.

This commit also changes the order of restoration steps, since now we
depend on correct vertex information to fix back the constraints, we do
it before the restoring of pair vertices.

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

1 files changed, 35 insertions, 16 deletions

diff --git a/src/gui/graphicsview/qgraphicsanchorlayout_p.cpp b/src/gui/graphicsview/qgraphicsanchorlayout_p.cpp
index 4ee58d1..9114fc3 100644
--- a/src/gui/graphicsview/qgraphicsanchorlayout_p.cpp
+++ b/src/gui/graphicsview/qgraphicsanchorlayout_p.cpp
@@ -633,16 +633,25 @@ AnchorData *QGraphicsAnchorLayoutPrivate::addAnchorMaybeParallel(AnchorData *new
             AnchorData *child = children[i];
             QList<QSimplexConstraint *> *childConstraints = childrenConstraints[i];
 
+            // We need to fix the second child constraints if the parallel group will have the
+            // opposite direction of the second child anchor. For the point of view of external
+            // entities, this anchor was reversed. So if at some point we say that the parallel
+            // has a value of 20, this mean that the second child (when reversed) will be
+            // assigned -20.
+            const bool needsReverse = i == 1 && !parallel->secondForward();
+
             if (!child->isCenterAnchor)
                 continue;
 
             parallel->isCenterAnchor = true;
 
-            for (int i = 0; i < constraints.count(); ++i) {
-                QSimplexConstraint *c = constraints[i];
+            for (int j = 0; j < constraints.count(); ++j) {
+                QSimplexConstraint *c = constraints[j];
                 if (c->variables.contains(child)) {
                     childConstraints->append(c);
                     qreal v = c->variables.take(child);
+                    if (needsReverse)
+                        v *= -1;
                     c->variables.insert(parallel, v);
                 }
             }
@@ -1165,9 +1174,15 @@ void QGraphicsAnchorLayoutPrivate::restoreSimplifiedConstraints(ParallelAnchorDa
         c->variables.insert(parallel->firstEdge, v);
     }
 
+    // When restoring, we might have to revert constraints back. See comments on
+    // addAnchorMaybeParallel().
+    const bool needsReverse = !parallel->secondForward();
+
     for (int i = 0; i < parallel->m_secondConstraints.count(); ++i) {
         QSimplexConstraint *c = parallel->m_secondConstraints.at(i);
         qreal v = c->variables[parallel];
+        if (needsReverse)
+            v *= -1;
         c->variables.remove(parallel);
         c->variables.insert(parallel->secondEdge, v);
     }
@@ -1209,7 +1224,24 @@ void QGraphicsAnchorLayoutPrivate::restoreVertices(Orientation orientation)
     Graph<AnchorVertex, AnchorData> &g = graph[orientation];
     QList<AnchorVertexPair *> &toRestore = simplifiedVertices[orientation];
 
-    // We will restore the vertices in the inverse order of creation, this way we ensure that
+    // Since we keep a list of parallel anchors and vertices that were created during vertex
+    // simplification, we can now iterate on those lists instead of traversing the graph
+    // recursively.
+
+    // First, restore the constraints changed when we created parallel anchors. Note that this
+    // works at this point because the constraints doesn't depend on vertex information and at
+    // this point it's always safe to identify whether the second child is forward or backwards.
+    // In the next step, we'll change the anchors vertices so that would not be possible anymore.
+    QList<AnchorData *> &parallelAnchors = anchorsFromSimplifiedVertices[orientation];
+
+    for (int i = parallelAnchors.count() - 1; i >= 0; --i) {
+        ParallelAnchorData *parallel = static_cast<ParallelAnchorData *>(parallelAnchors.at(i));
+        restoreSimplifiedConstraints(parallel);
+        delete parallel;
+    }
+    parallelAnchors.clear();
+
+    // Then, we will restore the vertices in the inverse order of creation, this way we ensure that
     // the vertex being restored was not wrapped by another simplification.
     for (int i = toRestore.count() - 1; i >= 0; --i) {
         AnchorVertexPair *pair = toRestore.at(i);
@@ -1254,19 +1286,6 @@ void QGraphicsAnchorLayoutPrivate::restoreVertices(Orientation orientation)
         delete pair;
     }
     toRestore.clear();
-
-    // The restoration process for vertex simplification also restored the effect of the
-    // parallel anchors created during vertex simplification, so we just need to restore
-    // the constraints in case of parallels that contain center anchors. For the same
-    // reason as above, order matters here.
-    QList<AnchorData *> &parallelAnchors = anchorsFromSimplifiedVertices[orientation];
-
-    for (int i = parallelAnchors.count() - 1; i >= 0; --i) {
-        ParallelAnchorData *parallel = static_cast<ParallelAnchorData *>(parallelAnchors.at(i));
-        restoreSimplifiedConstraints(parallel);
-        delete parallel;
-    }
-    parallelAnchors.clear();
 }
 
 QGraphicsAnchorLayoutPrivate::Orientation