传递消息的方式有四个取值

传递消息的方式有四个取值：

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. 假设当前有4个slot连接到QPushButton::clicked(bool)，当按钮被按下时，QT就把这个signal包装成一个 QEvent，放到消息队列里。QApplication::exec()或者线程的QThread::exec()会从消息队列里取消息，然后调用 signal关联的几个slot。这种方式既可以在线程内传递消息，也可以跨线程传递消息。

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. 与Qt::QueuedConnection类似，但是会阻塞等到关联的slot都被执行。这里出现了阻塞这个词，说明它是专门用来多线程间传递消息的。

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. 这种连接类型根据signal和slot是否在同一个线程里自动选择Qt::DirectConnection或Qt::QueuedConnection