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       1                 :            : /***************************************************************************
       2                 :            :                qgsthreadingutils.h
       3                 :            :                      --------------------------------------
       4                 :            :                Date                 : 11.9.2018
       5                 :            :                Copyright            : (C) 2018 by Matthias Kuhn
       6                 :            :                email                : matthias@opengis.ch
       7                 :            :  ***************************************************************************
       8                 :            :  *                                                                         *
       9                 :            :  *   This program is free software; you can redistribute it and/or modify  *
      10                 :            :  *   it under the terms of the GNU General Public License as published by  *
      11                 :            :  *   the Free Software Foundation; either version 2 of the License, or     *
      12                 :            :  *   (at your option) any later version.                                   *
      13                 :            :  *                                                                         *
      14                 :            :  ***************************************************************************/
      15                 :            : 
      16                 :            : #ifndef QGSTHREADINGUTILS_H
      17                 :            : #define QGSTHREADINGUTILS_H
      18                 :            : 
      19                 :            : #define SIP_NO_FILE
      20                 :            : 
      21                 :            : #include "qgis_core.h"
      22                 :            : 
      23                 :            : #include "qgsfeedback.h"
      24                 :            : 
      25                 :            : #include <QThread>
      26                 :            : #include <QSemaphore>
      27                 :            : #include <memory>
      28                 :            : 
      29                 :            : /**
      30                 :            :  * \ingroup core
      31                 :            :  * \brief Provides threading utilities for QGIS.
      32                 :            :  *
      33                 :            :  * \since QGIS 3.4
      34                 :            :  */
      35                 :            : class CORE_EXPORT QgsThreadingUtils
      36                 :            : {
      37                 :            :   public:
      38                 :            : 
      39                 :            :     /**
      40                 :            :      * Guarantees that \a func is executed on the main thread. If this is called
      41                 :            :      * from another thread, the other thread will be blocked until the function
      42                 :            :      * has been executed.
      43                 :            :      * This is useful to quickly access information from objects that live on the
      44                 :            :      * main thread and copying this information into worker threads. Avoid running
      45                 :            :      * expensive code inside \a func.
      46                 :            :      * If a \a feedback is provided, it will observe if the feedback is canceled.
      47                 :            :      * In case the feedback is canceled before the main thread started to run the
      48                 :            :      * function, it will return without executing the function.
      49                 :            :      *
      50                 :            :      * \note Only works with Qt >= 5.10, earlier versions will execute the code
      51                 :            :      *       in the worker thread.
      52                 :            :      *
      53                 :            :      * \since QGIS 3.4
      54                 :            :      */
      55                 :            :     template <typename Func>
      56                 :         25 :     static bool runOnMainThread( const Func &func, QgsFeedback *feedback = nullptr )
      57                 :            :     {
      58                 :            :       // Make sure we only deal with the vector layer on the main thread where it lives.
      59                 :            :       // Anything else risks a crash.
      60                 :         25 :       if ( QThread::currentThread() == qApp->thread() )
      61                 :            :       {
      62                 :         25 :         func();
      63                 :         25 :         return true;
      64                 :            :       }
      65                 :            :       else
      66                 :            :       {
      67                 :          0 :         if ( feedback )
      68                 :            :         {
      69                 :            :           // This semaphore will block the worker thread until the main thread is ready.
      70                 :            :           // Ready means the event to execute the waitFunc has arrived in the event loop
      71                 :            :           // and is being executed.
      72                 :          0 :           QSemaphore semaphoreMainThreadReady( 1 );
      73                 :            : 
      74                 :            :           // This semaphore will block the main thread until the worker thread is ready.
      75                 :            :           // Once the main thread is executing the waitFunc, it will wait for this semaphore
      76                 :            :           // to be released. This way we can make sure that
      77                 :          0 :           QSemaphore semaphoreWorkerThreadReady( 1 );
      78                 :            : 
      79                 :            :           // Acquire both semaphores. We want the main thread and the current thread to be blocked
      80                 :            :           // until it's save to continue.
      81                 :          0 :           semaphoreMainThreadReady.acquire();
      82                 :          0 :           semaphoreWorkerThreadReady.acquire();
      83                 :            : 
      84                 :          0 :           std::function<void()> waitFunc = [&semaphoreMainThreadReady, &semaphoreWorkerThreadReady]()
      85                 :            :           {
      86                 :            :             // This function is executed on the main thread. As soon as it's executed
      87                 :            :             // it will tell the worker thread that the main thread is blocked by releasing
      88                 :            :             // the semaphore.
      89                 :          0 :             semaphoreMainThreadReady.release();
      90                 :            : 
      91                 :            :             // ... and wait for the worker thread to release its semaphore
      92                 :          0 :             semaphoreWorkerThreadReady.acquire();
      93                 :          0 :           };
      94                 :            : 
      95                 :          0 :           QMetaObject::invokeMethod( qApp, waitFunc, Qt::QueuedConnection );
      96                 :            : 
      97                 :            :           // while we are in the event queue for the main thread and not yet
      98                 :            :           // being executed, check all 100 ms if the feedback is canceled.
      99                 :          0 :           while ( !semaphoreMainThreadReady.tryAcquire( 1, 100 ) )
     100                 :            :           {
     101                 :          0 :             if ( feedback->isCanceled() )
     102                 :            :             {
     103                 :          0 :               semaphoreWorkerThreadReady.release();
     104                 :          0 :               return false;
     105                 :            :             }
     106                 :            :           }
     107                 :            : 
     108                 :            :           // finally, the main thread is blocked and we are (most likely) not canceled.
     109                 :            :           // let's do the real work!!
     110                 :          0 :           func();
     111                 :            : 
     112                 :            :           // work done -> tell the main thread he may continue
     113                 :          0 :           semaphoreWorkerThreadReady.release();
     114                 :          0 :           return true;
     115                 :          0 :         }
     116                 :          0 :         QMetaObject::invokeMethod( qApp, func, Qt::BlockingQueuedConnection );
     117                 :          0 :         return true;
     118                 :            :       }
     119                 :         25 :     }
     120                 :            : 
     121                 :            : };
     122                 :            : 
     123                 :            : 
     124                 :            : #endif