Lambda Rules

Rules and arrangements using lambdas scheme for database access

• All database access must be applied in separate thread (TrackCollections thread). So all database access must be wrapped into lambda.

• It is strictly prohibited to access GUI from lambda. GUI access must be be applied only from main thread.

• If you need to access GUI from lambda it can be done by emitting signal (ConnectionType must be Qt::BlockingQueuedConnection -- same as QueuedConnection, except the current thread (thread where lambda executes -- TrackCollection) blocks until the slot returns. This connection type should only be used where the emitter and receiver are in different threads. Note: Violating this rule can cause your application to deadlock. Beware of emitting Qt::BlockingQueuedConnection signal from main thread!):
• All access to GUI moves to separate private slot (slotChangeUI).
• Signal must be created (changeUI).
• changeUI connects to slotChangeUI with last parameter Qt::BlockingQueuedConnection.

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Example.

Please, take a look at DlgMissing::onShow(). Here we must disable button after calling m_pMissingTableModel->select();

So we created: private slots: void slotActivateButtons(bool enable);

connect it: connect(this, SIGNAL(activateButtons(bool)), this, SLOT(slotActivateButtons(bool)), Qt::BlockingQueuedConnection);

and call it from the lambda inside DlgMissing::onShow():

    // tro's lambda idea
    m_pTrackCollection->callAsync(
                [this] (void) {
        m_pMissingTableModel->select();
        // no buttons can be selected
        emit(activateButtons(false));
    });