qt connect Qt::ConnectionType介绍

signal/slot在底层会使用三种方式传递消息。参见QObject::connect()方法：
bool QObject::connect ( const QObject * sender, const char * signal, const QObject * receiver, const char * method, Qt::ConnectionType type = Qt::AutoCompatConnection )
最后一个参数是就是传递消息的方式了，有四个取值：

Qt::DirectConnection
When emitted, the signal is immediately delivered to the slot.
假设当前有4个slot连接到QPushButton::clicked(bool)，当按钮被按下时，QT就把这4个slot按连接的时间顺序调用一遍。显然这种方式不能跨线程（传递消息）。

Qt::QueuedConnection
When emitted, the signal is queued until the event loop is able to deliver it to the slot.
假设当前