* introduce a NullEngine

* return an instance of the NullEngine on engine loading failure; this way applets don't have to test for engine existence and can just assume something gets returned
* DataEngine::isValid() in case someone really cares to know whether they have a non-functional engine (e.g. the NullEngine)

svn path=/trunk/KDE/kdebase/workspace/lib/plasma/; revision=670900

dataengine.cpp

@@ -34,7 +34,8 @@ class DataEngine::Private
     public:
         Private(DataEngine* e)
             : engine(e),
-              limit(0)
+              limit(0),
+              valid(true)
         {
             updateTimer = new QTimer(engine);
             updateTimer->setSingleShot(true);
@@ -97,6 +98,7 @@ class DataEngine::Private
         QTimer* updateTimer;
         QString icon;
         uint limit;
+        bool valid;
 };
 
 
@@ -111,6 +113,7 @@ DataEngine::DataEngine(QObject* parent)
 
 DataEngine::~DataEngine()
 {
+    //kDebug() << objectName() << ": bye bye birdy! " << endl;
     delete d;
 }
 
@@ -244,6 +247,16 @@ bool DataEngine::isUsed() const
     return d->ref != 0;
 }
 
+bool DataEngine::isValid() const
+{
+    return d->valid;
+}
+
+bool DataEngine::setValid(bool valid)
+{
+    d->valid = valid;
+}
+
 void DataEngine::setIcon(const QString& icon)
 {
     d->icon = icon;

dataengine.h

@@ -128,6 +128,11 @@ class PLASMA_EXPORT DataEngine : public QObject
          **/
         bool isUsed() const;
 
+        /**
+         * Returns true if this engine is valid, otherwise returns false
+         **/
+        bool isValid() const;
+
         /**
          * Sets the icon for this data engine
          **/
@@ -211,6 +216,15 @@ class PLASMA_EXPORT DataEngine : public QObject
          **/
         void clearAllDataSources();
 
+        /**
+         * Sets whether or not this engine is valid, e.g. can be used.
+         * In practice, only the internal fall-back engine, the NullEngine
+         * should have need for this.
+         *
+         * @param valid whether or not the engine is valid
+         **/
+        bool setValid(bool valid);
+
     private Q_SLOTS:
         void checkForUpdates();
 

dataenginemanager.cpp

@@ -25,14 +25,37 @@
 namespace Plasma
 {
 
+class NullEngine : public DataEngine
+{
+    public:
+        NullEngine(QObject* parent = 0)
+            : DataEngine(parent)
+        {
+            setValid(false);
+        }
+};
+
 class DataEngineManager::Private
 {
     public:
         Private()
+            : null(0)
         {}
 
-        Plasma::DataEngine::Dict m_engines;
+        ~Private()
+        {
+            delete null;
+        }
 
+        DataEngine* nullEngine()
+        {
+            if (!null) {
+                null = new NullEngine;
+            }
+        }
+
+        DataEngine::Dict m_engines;
+        DataEngine* null;
 };
 
 class DataEngineManagerSingleton
@@ -70,7 +93,7 @@ Plasma::DataEngine* DataEngineManager::dataEngine(const QString& name) const
         return *it;
     }
 
-    return 0;
+    return d->nullEngine();
 }
 
 Plasma::DataEngine* DataEngineManager::loadDataEngine(const QString& name)
@@ -95,7 +118,7 @@ Plasma::DataEngine* DataEngineManager::loadDataEngine(const QString& name)
     engine = KService::createInstance<Plasma::DataEngine>(offers.first(), 0);
     if (!engine) {
         kDebug() << "Couldn't load engine \"" << name << "\"!" << endl;
-        return 0;
+        engine = d->nullEngine();
     }
 
     engine->ref();

dataenginemanager.h

@@ -48,7 +48,8 @@ class PLASMA_EXPORT DataEngineManager
 
         /**
          * Returns a data engine object if one is loaded and available.
-         * Otherwise, returns 0.
+         * On failure, the fallback NullEngine (which does nothing and
+         * !isValid()) is returned.
          *
          * @param name the name of the engine
          */
@@ -61,7 +62,7 @@ class PLASMA_EXPORT DataEngineManager
          * value cached. Call unloadDataEngine when finished with the engine.
          *
          * @param name the name of the engine
-         * @return the data engine that was loaded, or 0 on failure.
+         * @return the data engine that was loaded, or the NullEngine on failure.
          */
         Plasma::DataEngine* loadDataEngine(const QString& name);