Add support for placing queued tasks on hold

python/core/qgstaskmanager.sip

@@ -15,6 +15,7 @@ class QgsTask : QObject
     enum TaskStatus
     {
       Queued, /*!< Task is queued and has not begun */
+      OnHold, /*!< Task is queued but on hold and will not be started */
       Running, /*!< Task is currently running */
       Complete, /*!< Task successfully completed */
       Terminated, /*!< Task was terminated or errored */
@@ -63,6 +64,16 @@ class QgsTask : QObject
     //! @see isCancelled()
     void cancel();
 
+    //! Called when the task is placed on hold. If the task in not queued
+    //! (ie it is running or has finished) then calling this has no effect.
+    //! @see unhold()
+    void hold();
+
+    //! Called when the task should be unheld and re-added to the queue. If the
+    //! task in not currently being held then calling this has no effect.
+    //! @see unhold()
+    void unhold();
+
     //! Sets the task's current progress. If task reports the CanReportProgress flag then
     //! the derived class should call this method whenever the task wants to update its
     //! progress. Calling will automatically emit the progressChanged signal.

src/core/qgstaskmanager.cpp

@@ -45,6 +45,24 @@ void QgsTask::cancel()
   mShouldTerminate = true;
 }
 
+void QgsTask::hold()
+{
+  if ( mStatus == Queued )
+  {
+    mStatus = OnHold;
+    emit statusChanged( OnHold );
+  }
+}
+
+void QgsTask::unhold()
+{
+  if ( mStatus == OnHold )
+  {
+    mStatus = Queued;
+    emit statusChanged( Queued );
+  }
+}
+
 void QgsTask::setProgress( double progress )
 {
   mProgress = progress;
@@ -107,9 +125,9 @@ long QgsTaskManager::addTask( QgsTask* task )
   connect( task, SIGNAL( progressChanged( double ) ), this, SLOT( taskProgressChanged( double ) ) );
   connect( task, SIGNAL( statusChanged( int ) ), this, SLOT( taskStatusChanged( int ) ) );
 
-  mTasks[ mNextTaskId ].future = QtConcurrent::run( task, &QgsTask::start );
-
   emit taskAdded( mNextTaskId );
+  processQueue();
+
   return mNextTaskId++;
 }
 
@@ -202,6 +220,7 @@ void QgsTaskManager::taskStatusChanged( int status )
     return;
 
   emit statusChanged( id, status );
+  processQueue();
 }
 
 bool QgsTaskManager::cleanupAndDeleteTask( QgsTask *task )
@@ -228,3 +247,15 @@ bool QgsTaskManager::cleanupAndDeleteTask( QgsTask *task )
   task->deleteLater();
   return true;
 }
+
+void QgsTaskManager::processQueue()
+{
+  for ( QMap< long, TaskInfo >::iterator it = mTasks.begin(); it != mTasks.end(); ++it )
+  {
+    QgsTask* task = it.value().task;
+    if ( task && task->status() == QgsTask::Queued )
+    {
+      mTasks[ it.key()].future = QtConcurrent::run( task, &QgsTask::start );
+    }
+  }
+}

src/core/qgstaskmanager.h

@@ -39,6 +39,7 @@ class CORE_EXPORT QgsTask : public QObject
     enum TaskStatus
     {
       Queued, /*!< Task is queued and has not begun */
+      OnHold, /*!< Task is queued but on hold and will not be started */
       Running, /*!< Task is currently running */
       Complete, /*!< Task successfully completed */
       Terminated, /*!< Task was terminated or errored */
@@ -87,6 +88,16 @@ class CORE_EXPORT QgsTask : public QObject
     //! @see isCancelled()
     void cancel();
 
+    //! Called when the task is placed on hold. If the task in not queued
+    //! (ie it is running or has finished) then calling this has no effect.
+    //! @see unhold()
+    void hold();
+
+    //! Called when the task should be unheld and re-added to the queue. If the
+    //! task in not currently being held then calling this has no effect.
+    //! @see unhold()
+    void unhold();
+
     //! Sets the task's current progress. If task reports the CanReportProgress flag then
     //! the derived class should call this method whenever the task wants to update its
     //! progress. Calling will automatically emit the progressChanged signal.
@@ -267,6 +278,10 @@ class CORE_EXPORT QgsTaskManager : public QObject
 
     bool cleanupAndDeleteTask( QgsTask* task );
 
+    //! Process the queue of outstanding jobs and starts up any
+    //! which are ready to go.
+    void processQueue();
+
 };
 
 #endif //QGSTASKMANAGER_H

tests/src/core/testqgstaskmanager.cpp

@@ -84,6 +84,7 @@ class TestQgsTaskManager : public QObject
     void taskId();
     void progressChanged();
     void statusChanged();
+    void holdTask();
 
   private:
 
@@ -334,6 +335,22 @@ void TestQgsTaskManager::statusChanged()
   QCOMPARE( static_cast< QgsTask::TaskStatus >( spy.last().at( 1 ).toInt() ), QgsTask::Complete );
 }
 
+void TestQgsTaskManager::holdTask()
+{
+  QgsTaskManager manager;
+  TestTask* task = new TestTask();
+  //hold task
+  task->hold();
+  manager.addTask( task );
+  //should not be started
+  QCOMPARE( task->status(), QgsTask::OnHold );
+
+  task->unhold();
+  // wait for task to spin up
+  while ( task->status() == QgsTask::Queued ) {}
+  QCOMPARE( task->status(), QgsTask::Running );
+}
+
 
 QTEST_MAIN( TestQgsTaskManager )
 #include "testqgstaskmanager.moc"