信号与槽的连接
| Constant | Value | Description |
| Qt::DirectConnection | 1 | When emitted, the signal is immediately delivered to the slot. |
| Qt::QueuedConnection | 2 | When emitted, the signal is queued until the event loop is able to deliver it to the slot. |
| Qt::BlockingQueuedConnection | 4 | 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. |
| Qt::AutoConnection | 0 | 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. |
Qt支持三种类型的信号-槽连接：
1，直接连接，当signal发射时，slot立即调用。此slot在发射signal的那个线程中被执行(不一定是接收对象生存的那个线程)
2，队列连接，当控制权回到对象属于的那个线程的事件循环时，slot被调用。此slot在接收对象生存的那个线程中被执行
3，自动连接(缺省)，假如信号发射与接收者在同一个线程中，其行为如直接连接，否则，其行为如队列连接。
连接类型可能通过以向connect()传递参数来指定。注意的是，当发送者与接收者生存在不同的线程中，而事件循环正运行于接收者的线程中，使用直接连接是不安全的。同样的道理，调用生存在不同的线程中的对象的函数也是不是安全的。QObject::connect()本身是线程安全的。
