QT | some summaries of signals and slots

Preface ：

Signals and slots are used for communication between two objects , The signal and slot mechanism is Qt Core features of . When using signals and slots , A signal can be associated to multiple slots , Multiple signals can also be associated to the same slot , One signal can also be correlated to another signal . If a signal is associated with multiple slots , These slots will perform... One by one , The execution order is the same as the association order .

I met one before

Interview questions

：

• If the same signal and the same slot function are connected multiple times （connect）, Will it cause a crash ？

• answer
  Will not , If the same signal and the same slot function are connected multiple times , So after the signal is sent , The slot function will be executed multiple times .

Signal and slot details ：

• Declaring a signal requires the use of signals keyword , stay signals Cannot add... Before public、private、protected And so on . The signal defaults to public Of , It can be launched anywhere (emit), However, it is suggested that only the class and its subclasses that define the signal should be used to transmit the modified signal .

• Signals can only be declared , There is no need to achieve . and , The signal has no return value , Can only be void Type of .

• Only QObject Class and its subclasses can use the signal and slot mechanism . Using signals and slots must also add... At the beginning of the class declaration Q_OBJECT macro .

• Slots are ordinary C++ function , It can be used like a normal function . Declare that the slot will use slots keyword , A slot can be private、public perhaps protected Type of , It can also be declared as a virtual function .

• The type of the parameter in the slot function should correspond to the type of the signal parameter , And not more than the parameters of the signal .

connect Detailed explanation ：

• The first parameter is the object transmitting the signal .

• The second parameter is the signal to be transmitted .

• The third parameter is the object receiving the signal .

• The fourth parameter is the slot to be executed .

• Fifth parameter type For the way of association .

Qt::AutoConnection:

  Automatic association , The default value is . If receiver The signal is transmitted in the same thread , Then the type represents Qt::DirectConnection; otherwise , This type represents Qt::QueuedConnection. When the signal is transmitted, decide which type to represent .

Qt::DirectConnection:

  Directly related . Call the slot function immediately after the signal is transmitted , Only after the slot function is executed and returned , The code after transmitting the signal will execute .

Qt::QueuedConnection:

  Queue Association . When control returns receiver Execute the slot function after the event loop of the thread , Whether the slot function is executed or not , The code behind the signal will be executed immediately .

Qt::BlockingQueuedConnection:

  Blocking queue associations . similar Qt::QueuedConnection, however , The semaphore thread keeps blocking , Know that the slot function returns . When receiver This type cannot be used when it exists in a semaphore thread , Otherwise the program will deadlock .

Qt::UniqueConnection:

  The only connection . It's a sign , Several other connection types can be combined , Use bitwise OR operation . At this time, the same signal and slot between two objects can only have a unique association . This type is mainly used to prevent repeated associations .

• Qt5 The previous form ：
   connect(dlg, SIGNAL(funcSignal(int)), this, SLOT(funcSlot(int)));

• Qt5 A form of joining ：
   connect(dlg, &MyDialog::funcSignal, this, &Widget::funcSlot);

Use Qt5 One advantage of this form of is that it can be checked at compile time , Misspelled signal or slot 、 Errors such as the number of slot function parameters is more than the number of signal parameters will be found during compilation .

in addition , This form also

Support C++11 Medium Lambda expression

, The code to be executed after signal transmission can be written directly during association , for example ：

connect(dlg, &MyDialog::funcSignal, [=](int value){ui->label->setText(QString::number(value))});

Automatic correlation of signals and slots ：

Slot function ：on_" Of components objectName"_" Signal name ".

• Slot functions named in this way can be directly associated with signals , Don't use connect() function .

• connectSlotsByName() Function is used to support automatic correlation between signals and slots , because setupUi() Function is called automatically connectSlotsByName() function , Therefore, the definition of components to be automatically associated should be placed in setupUi() Before function call , And must use setObjectName() Specify their objectName, Only in this way can automatic association be used normally .

Disconnect  disconnect：

• Disconnect all associations with multiple signals of an object ：disconnect(myObject, 0, 0, 0);  Equivalent to myObject->disconnect();

• Disconnect all associations with a specified signal ：disconnect(myObject, SIGNAL(mySignal()), 0, 0); Equivalent to myObject->disconnect(SIGNAL(mySignal()));

• Disconnect from a specified receiver All associations of ：disconnect(myObject, 0, myReceiver, 0);  Equivalent to myObject->disconnect(myReceiver);

• Disconnect a specified signal from the slot ：disconnect(myObject, SIGNAL(mySignal()), myReceiver, SLOT(mySlot()));  Equivalent to myObject->disconnect(SIGNAL(mySignal()), myReceiver, SLOT(mySlot())); Is equivalent to the disconnect(myConnection);//myConnection Yes, when associating connect() The return value of .

disconnect You can also use overloaded forms based on function pointers , And Qt5 Those added later do not need SIGNAL() and SLOT() Macro form .

Advanced applications of signal slots ：

Qt Provides  

QObject::sender()

  Function to

Returns a pointer to the object that sent the signal

.

Sometimes there are multiple signals associated to the same slot , At this point, if you want to perform special processing on a signal in the slot function , You can go through QObject::sender() To get the object pointer of the signal , To determine whether it is an object that we need to deal with specially .

If multiple signals are connected to the same slot , In this slot, each signal needs to be processed differently , have access to

QSignalMapper

class .QSignalMapper It's called a signal mapper , Sure

Realize the mapping of the same signals of multiple identical components

, Add a string or numeric parameter to it , Then launch it .

QSignalMapper Use examples ：

QSignalMapper *signalMapper = new QSignalMapper(this);
for (i = 0; i < 5; i++) 
{
 QPushButton *button = new QPushButton(this);
 connect(button, SIGNAL(clicked()), signalMapper, SLOT(map()));
 signalMapper->setMapping(button, i);
}

connect(signalMapper, SIGNAL(mapped(int)), this, SLOT(handle(int)));

Then it can be written like this in the slot ：
switch(i) //i Which is it button It happened. clicked The signal
{
 // Custom actions
}