Qt之信號槽（signals and slots）

signals and slots

1. 目的：Signals and slots are used for communication between objects.
2. 信號槽相對回調函數優點：Callbacks have two fundamental flaws: Firstly, they are not type-safe. We can never be certain that the processing function will call the callback with the correct arguments. Secondly, the callback is strongly coupled to the processing function since the processing function must know which callback to call.
3. 使用信號槽：
   1. 繼承QObject類，聲明Q_OBJECT宏。 All classes that contain signals or slots must mention Q_OBJECT at the top of their declaration. They must also derive (directly or indirectly) from QObject.
   2. 連接信號槽 connected objects together with QObject::connect()or with uic’s automatic connections feature. disconnect()
   3. 連接可以多對多的關係 By default, for every connection you make, a signal is emitted; two signals are emitted for duplicate connections. You can break all of these connections with a single disconnect() call. If you pass the Qt::UniqueConnection type, the connection will only be made if it is not a duplicate. If there is already a duplicate (exact same signal to the exact same slot on the same objects), the connection will fail and connect will return false
4. Signals
   1. 信號發送後立會立即執行槽函數，使用Qt::QueuedConnection，槽函數會延遲執行。 When a signal is emitted, the slots connected to it are usually executed immediately, just like a normal function call. When this happens, the signals and slots mechanism is totally independent of any GUI event loop. Execution of the code following the emit statement will occur once all slots have returned. The situation is slightly different when using queued connections; in such a case, the code following the emit keyword will continue immediately, and the slots will be executed later.
   2. 不使用具體類型的信號會更有複用性。 A note about arguments: Our experience shows that signals and slots are more reusable if they do not use special types. If QScrollBar::valueChanged() were to use a special type such as the hypothetical QScrollBar::Range, it could only be connected to slots designed specifically for QScrollBar. Connecting different input widgets together would be impossible.
5. Slots
   1. 一句話，信號槽也是快的一筆。 Compared to callbacks, signals and slots are slightly slower because of the increased flexibility they provide, although the difference for real applications is insignificant. In general, emitting a signal that is connected to some slots, is approximately ten times slower than calling the receivers directly, with non-virtual function calls. This is the overhead required to locate the connection object, to safely iterate over all connections (i.e. checking that subsequent receivers have not been destroyed during the emission), and to marshall any parameters in a generic fashion. While ten non-virtual function calls may sound like a lot, it’s much less overhead than any new or delete operation, for example. As soon as you perform a string, vector or list operation that behind the scene requires new or delete, the signals and slots overhead is only responsible for a very small proportion of the complete function call costs.
   2. The same is true whenever you do a system call in a slot; or indirectly call more than ten functions. On an i586-500, you can emit around 2,000,000 signals per second connected to one receiver, or around 1,200,000 per second connected to two receivers. The simplicity and flexibility of the signals and slots mechanism is well worth the overhead, which your users won’t even notice.
   3. 其他庫編譯時使用信號和槽時 Note that other libraries that define variables called signals or slots may cause compiler warnings and errors when compiled alongside a Qt-based application. To solve this problem, #undef the offending preprocessor symbol