libdeng2|Widgets: Added IndirectRule

Indirect rules are useful when others need to depend one a rule that
may be replaced at any time.

doomsday/libdeng2/include/de/IndirectRule

@@ -0,0 +1 @@
+#include "widgets/indirectrule.h"

doomsday/libdeng2/include/de/widgets/indirectrule.h

@@ -0,0 +1,59 @@
+/** @file indirectrule.h  Indirect rule.
+ *
+ * @authors Copyright (c) 2013 Jaakko Keränen <jaakko.keranen@iki.fi>
+ *
+ * @par License
+ * GPL: http://www.gnu.org/licenses/gpl.html
+ *
+ * <small>This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by the
+ * Free Software Foundation; either version 2 of the License, or (at your
+ * option) any later version. This program is distributed in the hope that it
+ * will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty
+ * of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General
+ * Public License for more details. You should have received a copy of the GNU
+ * General Public License along with this program; if not, see:
+ * http://www.gnu.org/licenses</small> 
+ */
+
+#ifndef LIBDENG2_INDIRECTRULE_H
+#define LIBDENG2_INDIRECTRULE_H
+
+#include "../Rule"
+
+namespace de {
+
+/**
+ * Rule that gets its value indirectly from another rule. The value of an
+ * indirect rule cannot be set directly.
+ *
+ * Indirect rules are useful when others need to depend one a rule that may
+ * change dynamically. Anyone relying on the indirect rule will be duly
+ * notified of changes in the source of the indirect rule, without having to
+ * change anything in the existing rule relationships.
+ */
+class IndirectRule : public Rule
+{
+public:
+    IndirectRule();
+    ~IndirectRule();
+
+    /**
+     * Sets the source rule whose value this indirect rule will reflect.
+     *
+     * @param rule  Source rule.
+     */
+    void setSource(Rule const &rule);
+
+    void update();
+
+    Rule const &source() const;
+    String description() const;
+
+private:
+    Rule const *_source;
+};
+
+} // namespace de
+
+#endif // LIBDENG2_INDIRECTRULE_H

doomsday/libdeng2/include/de/widgets/rules.h

@@ -20,6 +20,7 @@
 #include "../Rule"
 #include "../ConstantRule"
 #include "../DelegateRule"
-#include "../ScalarRule"
+#include "../IndirectRule"
 #include "../OperatorRule"
+#include "../ScalarRule"
 #include "../RuleRectangle"

doomsday/libdeng2/src/widgets/indirectrule.cpp

@@ -0,0 +1,61 @@
+/** @file indirectule.cpp  Indirect rule.
+ *
+ * @authors Copyright (c) 2013 Jaakko Keränen <jaakko.keranen@iki.fi>
+ *
+ * @par License
+ * GPL: http://www.gnu.org/licenses/gpl.html
+ *
+ * <small>This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by the
+ * Free Software Foundation; either version 2 of the License, or (at your
+ * option) any later version. This program is distributed in the hope that it
+ * will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty
+ * of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General
+ * Public License for more details. You should have received a copy of the GNU
+ * General Public License along with this program; if not, see:
+ * http://www.gnu.org/licenses</small> 
+ */
+
+#include "de/IndirectRule"
+
+namespace de {
+
+IndirectRule::IndirectRule() : _source(0)
+{}
+
+IndirectRule::~IndirectRule()
+{
+    independentOf(_source);
+}
+
+void IndirectRule::setSource(Rule const &rule)
+{
+    if(_source)
+    {
+        independentOf(_source);
+    }
+    dependsOn(_source = &rule);
+
+    invalidate();
+}
+
+void IndirectRule::update()
+{
+    if(_source)
+    {
+        setValue(_source->value());
+    }
+}
+
+Rule const &IndirectRule::source() const
+{
+    DENG2_ASSERT(_source != 0);
+    return *_source;
+}
+
+String IndirectRule::description() const
+{
+    return String("Indirect => ") + source().description();
+}
+
+} // namespace de

doomsday/libdeng2/widgets.pri

@@ -4,6 +4,7 @@ HEADERS += \
     include/de/AnimationVector \
     include/de/DelegateRule \
     include/de/ConstantRule \
+    include/de/IndirectRule \
     include/de/OperatorRule \
     include/de/RuleBank \
     include/de/RuleRectangle \
@@ -17,6 +18,7 @@ HEADERS += \
     include/de/widgets/animationvector.h \
     include/de/widgets/constantrule.h \
     include/de/widgets/delegaterule.h \
+    include/de/widgets/indirectrule.h \
     include/de/widgets/operatorrule.h \
     include/de/widgets/rulerectangle.h \
     include/de/widgets/rootwidget.h \
@@ -31,6 +33,7 @@ SOURCES += \
     src/widgets/animation.cpp \
     src/widgets/constantrule.cpp \
     src/widgets/delegaterule.cpp \
+    src/widgets/indirectrule.cpp \
     src/widgets/operatorrule.cpp \
     src/widgets/rulerectangle.cpp \
     src/widgets/rootwidget.cpp \