signal/slot在底層會使用三種方式傳遞消息。參見QObject::connect()方法：
bool QObject::connect ( const QObject * sender, const char * signal, const QObject * receiver, const char * method, Qt::ConnectionType type = Qt::AutoCompatConnection )
最後一個參數是就是傳遞消息的方式了，有四個取值：
Qt::DirectConnection
When emitted, the signal is immediately delivered to the slot.
假設當前有4個slot連接到QPushButton::clicked(bool)，當按鈕被按下時，QT就把這4個slot按連接的時間順序調用一遍。顯然這種方式不能跨線程（傳遞消息）。
Qt::QueuedConnection
When emitted, the signal is queued until the event loop is able to deliver it to the slot.
假設當前有4個slot連接到QPushButton::clicked(bool)，當按鈕被按下時，QT就把這個signal包裝成一個 QEvent，放到消息隊列裏。QApplication::exec()或者線程的
QThread::exec()會從消息隊列裏取消息，然後調用 signal關聯的幾個slot。這種方式既可以在線程內傳遞消息，也可以跨線程傳遞消息。
Qt::BlockingQueuedConnection
Same as QueuedConnection, except that the current thread blocks until the slot has been delivered. This connection type should only be used for receivers in a different
thread. Note that misuse of this type can lead to dead locks in your application.
與Qt::QueuedConnection類似，但是會阻塞等到關聯的slot都被執行。這裏出現了阻塞這個詞，說明它是專門用來多線程間傳遞消息的。
Qt::AutoConnection
If the signal is emitted from the thread in which the receiving object lives, the slot is invoked directly, as with Qt::DirectConnection; otherwise the signal is queued, as with
Qt::QueuedConnection.
這種連接類型根據signal和slot是否在同一個線程裏自動選擇Qt::DirectConnection或Qt::QueuedConnection
這樣看來，第一種類型的效率肯定比第二種高，畢竟第二種方式需要將消息存儲到隊列，而且可能會涉及到大對象的複製（考慮sig_produced(BigObject bo)，bo需要複製到隊
列裏）。