Qt5 signal and slot mechanism (basic introduction to signal and slot)

QT5 Signal and slot mechanism

1.1 What is? Signals and slots

Signal slot mechanism and Windows The next message mechanism is similar to , The message mechanism is based on callback functions ,Qt Replace function pointers with signals and slots in , Make the program safer and simpler .

Signals and slots are used for communication between objects , The signal / The slot mechanism is Qt A core feature of .

In graphical user interface programming , We often notify one widget of a change to another via the signal slot .

• The signal

When an object changes its state , The signal is transmitted by the object (emit) get out , And the object is only responsible for sending signals , It doesn't know who's receiving the signal at the other end . In this way, the real information encapsulation is achieved , Ensure that the object is used as a real software component .

• Slot

For receiving signals , And slots are just ordinary object member functions . A slot doesn't know if there is any signal connected to itself . And the object doesn't know the specific communication mechanism .

• Signal to slot connection

All from QObject Or its subclass ( for example Qwidget ) Derived classes can contain both signals and slots . Because the signal is connected to the slot through QObject Of connect() Member functions .

connect(sender, SIGNAL(signal), receiver, SLOT(slot));

among sender And receiver Is a pointer to an object ,SIGNAL() And SLOT() Is a macro that converts signals to slots .

• characteristic

One signal can be connected to multiple slots

When the signal is transmitted , Will An indefinite sequence Call the slots one by one .

Multiple signals can be connected to the same slot

That is, no matter which signal is transmitted , Will call this slot .

Signals can be directly connected to each other

When transmitting the first signal , You can also send a second signal .

• What should be paid attention to

1. The signal and slot mechanism is the same as the call of ordinary functions , If not used properly , It is also possible to generate dead loops during program execution . therefore , When defining a slot function, be sure to avoid indirectly forming an infinite loop , That is, the same received signal is transmitted again in the slot . for example , In the example given above, if mySlot() Add a statement to the slot function emit mySignal() An endless cycle can be formed .

2. If a signal is associated with multiple slots , that , When this signal is transmitted , The order in which the associated slots are activated will be random .

3. Macro definitions cannot be used in signal and slot The parameters of the .

4. The number and type of parameters of signal and slot must be consistent .

1.2 Signal and slot sample program

1.2.1 Manual Association

1. The click signal of the button , Associated to the corresponding slot function .

connect(ui->pushButton_delete_server,SIGNAL(clicked()),this,SLOT(slot_delete_Server()));

Argument parsing ：

ui->pushButton_delete_server ： Signal source object

SIGNAL(clicked())： A signal from a signal source

This： The object at the receiving end

SLOT(slot_delete_Server())： Slot function at the receiving end

1.2.2 Automatic association

1.3 Custom signal slot

1.3.1 Define your own signals and slot functions

Custom signal and slot functions need to be defined in the class body class ！

Sample code ：

/* Inherit and TCP The server */ classtcp_server:publicQTcpServer { publicslots:// Custom slot function voidtcp_server_slot_RXdata(QStringstr,intlen); signals: /* Declare custom signals */ voidTcp_Server_Signal(QStringstr,intlen);// Display the received data in the text box };

Be careful ： Signals and slot functions can have new parameters , But there can't be a return value ！

1.3.2 Send custom signal

Sample code ：

voidtcp_server::tcp_server_slot_RXdata(QStringstr,intlen) { emitTcp_Server_Signal(str,len);// Sending signal }

emit Keywords are used to send signals ！

grammar ：

emit The signal ;

1.4 Signal communication between two classes

If we want to achieve B Such access A Class , The method of defining the signal and slot ！

step ：

1. stay A Class defines a slot function .

2. stay B Class defines a signal .

3. stay A Class to associate signals with slots ！

connect(B Object pointer to class ,SIGNAL(B Class signal )),A Object pointer to class ,SLOT(A Class ));

4. stay B Class member functions send defined signals

emit Signal function ;

5. B Class sends a signal ,A The slot function of class will get the corresponding ！！