This enum describes the types of connection that can be used between signals and slots. In particular, it determines whether a particular signal is delivered to a slot immediately or queued for delivery at a later time.
| Constant |
Value |
Description |
| Qt::AutoConnection |
0 |
(Default) If the receiver lives in the thread that emits the signal, Qt::DirectConnection is used. Otherwise, Qt::QueuedConnection is used. The connection type is determined when the signal is emitted. |
| Qt::DirectConnection |
1 |
The slot is invoked immediately when the signal is emitted. The slot is executed in the signalling thread. |
| Qt::QueuedConnection |
2 |
The slot is invoked when control returns to the event loop of the receiver's thread. The slot is executed in the receiver's thread. |
| Qt::BlockingQueuedConnection |
3 |
Same as Qt::QueuedConnection, except that the signalling thread blocks until the slot returns. This connection must not be used if the receiver lives in the signalling thread, or else the application will deadlock. |
| Qt::UniqueConnection |
0x80 |
This is a flag that can be combined with any one of the above connection types, using a bitwise OR. When Qt::UniqueConnection is set, QObject::connect() will fail if the connection already exists (i.e. if the same signal is already connected to the same slot for the same pair of objects). This flag was introduced in Qt 4.6. |
With queued connections, the parameters must be of types that are known to Qt's meta-object system, because Qt needs to copy the arguments to store them in an event behind the scenes. If you try to use a queued connection and get the error message:
QObject::connect: Cannot queue arguments of type 'MyType'
Call qRegisterMetaType() to register the data type before you establish the connection.
Qt::UniqueConnection特别说明
如果信号槽以相同的方式连接了N次，那么当信号emit时，槽函数会被调用N次。
除了第一次调用，后续的调用都需要这个选项，如下面的代码还是会调用槽函数两次：
connect(this, SIGNAL(signal2()), this, SLOT(slot1()), Qt::UniqueConnection); connect(this, SIGNAL(signal2()), this, SLOT(slot1()));