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POQThread: Making parallel execution with GUI-Feedback blocking?
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  1. POQThread: Making parallel execution with GUI-Feedback blocking?
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    copied!<h1>Scenario</h1> <p>Lets say, I have a procedure called <code>parallelRun</code>. It would take a list of workers, each having a <code>getWorkAmount():int</code>, a <code>run()</code> method, a <code>finished()</code> signal and a <code>cancel()</code> slot:</p> <pre><code>void parallelRun( std::vector&lt; Worker* &gt; workers ); </code></pre> <p>Its implementation should:</p> <h2>1. Open a <code>QPogressDialog</code>:</h2> <pre><code>unsigned int totalWorkAmount = 0; for( auto it = workers.begin(); it != workers.end(); ++it ) { totalWorkAmount += ( **it ).getWorkAmount(); } LoadUI ui( 0, totalWorkAmount, this ); </code></pre> <p>with</p> <pre><code>class LoadUI : public QObject { Q_OBJECT public: LoadUI( int min, int max, QWidget* modalParent ) : totalProgres( 0 ) , progressDlg( "Working", "Abort", min, max, modalParent ) { connect( &amp;progressDlg, SIGNAL( canceled() ), this, SLOT( cancel() ) ); progressDlg.setWindowModality( Qt::WindowModal ); progressDlg.show(); } bool wasCanceled() const { return progressDlg.wasCanceled(); } public slots: void progress( int amount ) { totalProgres += amount; progressDlg.setValue( totalProgres ); progressDlg.update(); QApplication::processEvents(); } signals: void canceled(); private slots: void cancel() { emit canceled(); } private: int totalProgres; QProgressDialog progressDlg; } </code></pre> <h2>2. Create one thread for each worker</h2> <pre><code>std::vector&lt; std::unique_ptr&lt; QThread &gt; &gt; threads; for( auto it = workers.begin(); it != workers.end(); ++it ) { std::unique_ptr&lt; QThread &gt; thread( new QThread() ); Worker* const worker = *it; worker-&gt;moveToThread( thread.get() ); QObject::connect( worker, SIGNAL( finished() ), thread.get(), SLOT( quit() ) ); QObject::connect( &amp;ui, SIGNAL( canceled() ), worker, SLOT( cancel() ) ); QObject::connect( *it, SIGNAL( progressed( int ) ), &amp;ui, SLOT( progress( int ) ) ); thread-&gt;start( priority ); threads.push_back( std::move( thread ) ); } </code></pre> <h2>3. Run them simultaneously</h2> <pre><code>for( auto it = workers.begin(); it != workers.end(); ++it ) { QMetaObject::invokeMethod( *it, "run", Qt::QueuedConnection ); } </code></pre> <p><code>load()</code> is run when the user clicks an UI-button.</p> <h1>Problem</h1> <p>How am I supposed to extend this code, if I want to make <code>parallelRun</code> block until all workers are finished, without freezing the <code>QProgressDialog</code>?</p> <h2>Deliberations</h2> <h3>Using a barrier</h3> <p>I tried adding the following code at the end of the <code>parallelRun</code> routine:</p> <pre><code>QApplication::processEvents(); for( auto it = threads.begin(); it != threads.end(); ++it ) { ( **it ).wait(); } </code></pre> <p>The impact of this few lines of extra-code is, that <code>LoadUI::progress</code> is <em>never entered</em>, since the GUI-thread is <em>asleep</em> and therefore it's event loop isn't processed: In Qt, signals are delivered to slots by posting them to the event loop of the thread, associated to the object the slot belongs to. This is why the <code>progressed</code> signal of a worker is never delivered.</p> <p>I think, the appropriate solution would be to run <code>QApplication::processEvents()</code> <em>within the GUI-thread</em> anytime a <code>progressed</code> signal is emitted by a worker. On the other hand, I guess this cannot be done, since the GUI-thread is asleep.</p> <h3>Another possible solution</h3> <p>Another possibility would be to use an <em>active waiting</em>-like solution:</p> <pre><code>for( auto it = threads.begin(); it != threads.end(); ++it ) { while( ( **it ).isRunning() ) { QApplication::processEvents(); } } for( auto it = threads.begin(); it != threads.end(); ++it ) { ( **it ).wait(); } </code></pre> <p>This also requires adding the following line of code right after <code>thread-&gt;start( priority );</code>:</p> <pre><code>while( !thread-&gt;isRunning() ); </code></pre> <p>I don't think that this is a nice solution, but at least it works. How can this be done without the drawbacks of active waiting?</p> <p>Thanks in advance!</p>
 

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