refs #476 Facility to ask worker to abandon its task and finish early.

src/blackmisc/worker.h

@@ -155,14 +155,25 @@ namespace BlackMisc
         }
 
         //! Blocks until the task is finished.
-        //! \threadsafe
+        //! \threadsafe Will deadlock if called by the worker thread.
         void waitForFinished() Q_DECL_NOEXCEPT;
 
+        //! Notify the task that its result is no longer needed, so it can finish early.
+        //! \threadsafe
+        void abandon() Q_DECL_NOEXCEPT;
+
+        //! Convenience to call abandon() followed by waitForFinished().
+        void abandonAndWait() Q_DECL_NOEXCEPT;
+
     signals:
         //! Emitted when the task is finished.
         void finished();
 
     protected:
+        //! For the task to check whether it can finish early.
+        //! \threadsafe
+        bool isAbandoned() const;
+
         //! Mark the task as finished.
         void setFinished()
         {
@@ -186,6 +197,9 @@ namespace BlackMisc
         }
 
     private:
+        virtual void quit() Q_DECL_NOEXCEPT {}
+        virtual void quitAndWait() Q_DECL_NOEXCEPT { waitForFinished(); }
+
         bool m_finished = false;
         mutable QMutex m_finishedMutex { QMutex::Recursive };
     };
@@ -276,11 +290,11 @@ namespace BlackMisc
         //! Stops the thread the next time around its event loop.
         //! The thread and the worker will then be deleted.
         //! \threadsafe
-        void quit() Q_DECL_NOEXCEPT;
+        virtual void quit() Q_DECL_NOEXCEPT final override;
 
         //! Calls quit() and blocks until the thread is finished.
         //! \threadsafe Will deadlock if called by the worker thread.
-        void quitAndWait() Q_DECL_NOEXCEPT;
+        virtual void quitAndWait() Q_DECL_NOEXCEPT final override;
 
     protected slots:
         //! Called when the thread is started.
@@ -294,6 +308,8 @@ namespace BlackMisc
         void ps_finish();
 
     private:
+        using CWorkerBase::setFinished;
+
         QObject *m_owner = nullptr;
         QString m_name;
     };