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lesson2-Signal and Slot Mechanism_PHP Tutorial

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Release： 2016-07-12 09:05:42

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lesson2-signal and slot mechanism

1. Understanding of signals and slots
1. What are signals and slots?
In many GUI tools, each widget has a callback function to respond to the actions they can trigger. These callback functions are usually a pointer to a function. Pointers are a relatively troublesome concept. In Qt, signals and slots replace these messy function pointers, making program writing simpler.

When an object changes, the signal is emitted by the object. This is all the object has to do. It does not know who is receiving the signal on the other end. This process is the real information. Encapsulated, achieving absolute security.

Slots are used to receive signals. They are ordinary c member functions. They do not know whether there is a signal associated with them, and the object does not understand the communication mechanism.

In Qt, when clicking When a button is pressed, the button will send out a signal, and the signal will be passed to another object. The object that receives the signal will take corresponding actions. This is the mechanism of signals and slots

2. Signals and slots Association of slots
1), signals and slots through
QObject::connect(const QObject *sender, const char *signal, const QObject *receiver, const char *method, Qt::Connection type=Qt:: AutoCompatConnection) generates association
sender, signal sender
signal, signal to be sent
receiver, signal receiver
method, slot function
type, signal and slot association method

2) There are three types of type
Qt::DirectConnection. The signal is passed to the slot immediately after it is sent. Only after the slot function completes execution and returns, the code after sending the signal "emit" can be Execute
Qt::QueueConnection. After the signal is sent, it is queued until the event loop has the ability to pass it to the slot. Regardless of whether the slot function is executed or not, the code after sending the signal "emit
" is executed immediately.
Qt::AutoCompatConnection, if the signal and slot are in the same thread, it is equivalent to Qt::DirectConnection; if the signal and slot are not in the same thread, it is equivalent to
Qt::QueueConnection.

2. Examples of signals and slots
1. Signal
When the internal state of the object changes, the signal is emitted. Only one signal is defined Only classes and their subclasses can emit this signal. For example, the clicked signal is predefined in the QPushButton class. When the button is clicked, the clicked signal will be emitted.

In addition to the system predefined signals in Qt, you can also customize signals

The declaration of signals is in the header file, starting with the keyword signals
signals:
void signals();
void signals(int x);
There are two signals in this. The names are the same, but they carry different parameters, which is somewhat similar to the overloading of functions.
A signal is similar to an ordinary C function, but it has no entity, only an empty shell, and its return value is always void. Do not expect to return useful information from the signal. After the signal is declared, it is automatically generated by the moc tool, so it should not be placed in the .cpp file.
System predefined signals are generally automatically emitted, and we do not need to care. If you need to manually emit signals, you can write emit signals() like this;

2. Slots
Slots are ordinary C member functions , can be called normally, and many signals can be associated with slots. Since the slot is an ordinary member function, it also has permissions. Therefore, there are three types of slot functions, public slots, private slots, protect slots

The declaration of the slot function is also in the header file
private slots:
void mySlot1();
void mySlot1(int x);

Before the declaration of signal and slot functions, you must add a macro Q_OBJECT, only in this way Qt will recognize them

3. Meta-object tool
moc tool The class declaration in the C file will be initialized, and the C code used to initialize the meta-object will be automatically generated. The meta-object contains all signals and slots, and pointers to these names.
moc reads the c source file. If it finds a class declared with the Q_OBJECT macro, it will generate another c source file. The moc tool does not expand #define, so there cannot be macro definitions in signal and slot functions

4. Association
The association between signals and slots is through the function connect
btn = new QPushButton();
lab = new QLabel();
connect(btn, SIGNAL(clicked()), lab, SLOT(clear()))
The signal needs to use the macro SIGNAL(), and the slot function needs to use the macro SLOT();
The parameters of the signal and the slot must be consistent;
When the signal and the slot belong to the same object, the receiver of the signal can be omitted
connect(btn, SIGNAL(clicked()), SLOT(showMenu()) )

Problems that should be paid attention to with signals and slots
1), constructors cannot be within the area of signals and slots
2), function pointers cannot be used as parameters of signals and slots
3 ), signals and slots cannot have default parameters
4), signals and slots cannot have template class parameters
5), nested classes cannot be located in the signal and slot area, and there cannot be signals and slots