Lambda Rules

Rules and arrangements using lambdas scheme for database access

General

• Beware of recursive lambda calls.
• All database access must be applied in separate thread (TrackCollections thread). So all database access must be wrapped into lambda (via m_pTrackCollection->callAsync/callSync).

Accessing GUI

• 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::QueuedConnection.
• All access to GUI moves to separate private slot (slotChangeUI).
• Signal must be created (changeUI).
• changeUI connects to slotChangeUI with last parameter Qt::QueuedConnection.
• OR You can use helper class MainExecuter, which is constructed like TrackCollection and has its methods callSync/callAsync. Using MainExecuter you must wrap UI access to lambda.

Capture parameters

• Understand the difference between Asynchronous and Synchronous calls.
• If you do callAsync -- capture parameters by value.
• If you do callSync -- capture parameters by reference (no need to capture pointers by reference).
• Try to order parameters as it is function parameters: last parameters are results or alternating variables.

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::QueuedConnection);

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));
    });