https://doc.qt.io/qt-5/signalsandslots.html
signals & slots
signals and slots are used for communication between objects. the signals and slots mechanism is a central feature of Qt and probably the part that differs most from the features provided by other frameworks.
signals and slots are made possible by Qt’s meta-object system.
introduction
in GUI programming, when we change one widget, we often want another widget to be notified. more generally, we want object of any kind to be able to communicate with one another. for example, if a user clicks a Close button, we probably want the window’s close() function to be called.
other toolkits achieve this kind of communication using callbacks.
a callback is a pointer to a function, so if u want a processing function to notify u about some event u pass a pointer to another function (the callback) to the processing function.
the processing function then calls the callback when appropriate.
while successful frameworks using this method do exist, callbacks cna be unintuitive and may suffer from problems in ensuring the type-correctness of callback arguments.
signals & slots
in qt, we have an alternative to the callback technique: we use signals and slots.
a signal is emitted when a particular event occurs. qt’s widges have many predefined signals,
but we can always subclass widgets to add our own signals to them.
a slot is a function that is called in response to a particular signal.
Qt’s widgets have many pre-defined slots, but it is common practice to subclass widgets and add our own slots so that u can handle the signals that u are interested in.
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the signals and slots mechanism is type safe:
the signature of a signa must match the signature of the receiving slots. 两个签名要匹配
(in fact a slot may have a shorter signature than the signal it receives because it can ignore extra arguments.)
since the signatures are compatible, the compiler can help us detect type mismatches when using the function pointer-based syntax. the string-based SIGNAL and SLOT syntax will detect type mismathches at runtime.
signals and slots are loosely coupled: 解耦的
a class which emits a signal neither knows nor cares which slots slots receive the signal.
Qt’s signals and slots mechanism ensures that if u connect a signal to a slot, the slot will be called with the signal’s parameters at the right time. signals and slots can take any number of arguments of any type. they are completely type safe.
all classes that inherit from QObject or one of its subclasses (e.g, QWidget) can contain signals and slots.
signals are emitted by objects when they change their state in a way that may be interesting to other objects.
this is all the object does to commnicate.
it does not know or care whether anything is receiving the signals it emits.
this is true information encapsulation, 信息的封装
and ensures that the object can be used as a software component. 组件
slots can be used for receiving signals, but they are also normal member functions.
just as an object does not know if anything receives its signals, a slot does not know if it has any
signals connected to it.
this ensures that truly independent components can be created with Qt.
u can connect as many signals as u want to a single slot, and a single can be connected to as many slots as u need. it is even possible to coonect a singal directly to another signals. (this will emit the second signal immediately whenever the first is emitted).
together, signals and slots make up a poweful component programming mechanism.
signals
signals are emitted by an object when its internal state has changed in some way that might be interesting to the obejct’s client or owner.
signals are public access functions and can be emitted from anywhere, but we recommend to only emit them from the class that defines the signal and its subclasses.
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.
if several slots are connected to one signal, the slots will be executed one after the other, in the order they have been connected, when the signal is emitted.
signals are automatically generated by the moc(Meta-Object Compiler ) and must not be implemented in the .cpp file. they can never have return types (i.e. use void).
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.
Slots
a slot is called when a signal connected to it is emitted. slots are normal C++ functions and can be called normally; their only special feature is that signals can be connected to them.
since slots are normal member functions, they follow the normal c++ rules when called directly.
however, as slots, they can be invoked by any component, regardless of its access level, via a signal-slot connection. this means that a signal emitted from an instance of an arbitrary class can cause a private slot to be invoked in an instance of an unrelated class.
u can also define slots to be virtual, which we have found quite useful in practice.
compared to callbacks, signals and slots are slightly slower because of the increased flexibility the provide, although the difference for real applications is insignificant. in general, emitting a singal 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 u 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.
the same is true whenever u do a system call in a slot;
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.
A Small Example
A minimal C++ class declaration might read:
class Counter
{
public:
Counter() { m_value = 0; }
int value() const { return m_value; }
void setValue(int value);
private:
int m_value;
};
A small QObject-based class might read:
#include <QObject>
class Counter : public QObject
{
Q_OBJECT
public:
Counter() { m_value = 0; }
int value() const { return m_value; }
public slots:
void setValue(int value);
signals:
void valueChanged(int newValue);
private:
int m_value;
};
the QObject-based version has the same internal state, and provides public methods to access the state, but in addition it has support for component programming using signals and slots. this class can tell the outside world that its state has changed by emitting a singal, valueChanged(), and it has a slot which other objects can send signals to.
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.
slots are implemented by the application programmer. here is a possible implementation of the Counter::setValue() slot:
void Counter::setValue(int value)
{
if (value != m_value) {
m_value = value;
emit valueChanged(value);
}
}
the emit line emits the signal valueChanged() from the object, with the new value as argument.
in the following code snippet, we create two Counter objects and conect the first object’s valueChanged() signal to the second object’s setValue() slot using QObject::connect():
Counter a, b;
QObject::connect(&a, &Counter::valueChanged,
&b, &Counter::setValue);
a.setValue(12); // a.value() == 12, b.value() == 12
b.setValue(48); // a.value() == 12, b.value() == 48
Calling a.setValue(12) makes a emit a valueChanged(12) signal, which b will receive in its setValue() slot, i.e.
b.setValue(12) is called.
then b emits the same valueChanged() signal, but since no slot has been connected to b’s valueChanged() signal, the signal is ignored.
note that the setValue() function sets the value and emits the signal only if vlaue!=m_value.
this prevent infinite looping in the case of cyclic connections (e.g., if b.valueChanged() were connected to a.setValue()).
a real example
the following is an example of the header of a simple widget class without member functions.
the purpose is to show how u can utilize signals and slots in your own applications.
#ifndef LCDNUMBER_H
#define LCDNUMBER_H
#include <QFrame>
class LcdNumber : public QFrame
{
Q_OBJECT
LcdNumber inherits QObject, which has most of the signal-slot knowledge, via QFrame and QWidget.
it is somewhat similar to the built-in QLCDNumber widget.
the Q_OBJECT macro is expanded by the preprocessor to declare several member funtions that are implemented by the moc; moc上面提到过，自己找找
if u get compiler errors along the lines of “undefined reference to vtable for LcdNumber”, u have probably forgotten to run the moc or to include the moc output in the link command.
构造函数和信号：
public:
LcdNumber(QWidget *parent = nullptr);
signals:
void overflow();
after the class constructor and public memebers, we declare the class signals. the LcdNumber class emits a signal, overflow(), when it is asked to show an impossible value.
if u do not care about overflow, or u know that overflow cannot occur, u can ignore the overflow() signal, i.e. do not connect it to any slot.
if on the other hand u want to call two different error funtions when the number overflows, simply connect the signal to two different slots. Qt will call both (in the order they were connected).
public slots:
void display(int num);
void display(double num);
void display(const QString &str);
void setHexMode();
void setDecMode();
void setOctMode();
void setBinMode();
void setSmallDecimalPoint(bool point);
};
#endif
a slot is a receiving function used to get information about safe changes in other widgets. LcdNumber uses it, as the code above indicates, to set the displayed number. since display() is part of the class’s interface with the rest of the program, the slot is public.
notet the display() is overhead; Qt will select the appropriate version when u connect a signal to the slot. with callbacks, u would have to finid five different names and keep track of the types yourself.
signals and slots with Default Arguments
the signatures of signals and slots may contain arguments, and the arguments can have default values. consider QObject::destroyed():
void destroyed(QObject* = nullptr);
when a QObject is deleted, it emits this QObject::destroyed() signal.
we want to catch this signal, wherever we might have a dangling reference to the deleted QObject, so we can clean it up.
a suitable slot signature might be:
void objectDestroyed(QObject* obj = nullptr);
to connect the signal to the slot, we use QObject::connect(). there are several ways to connect signal and slots.
the first is to use function pointers:
connect(sender, &QObject::destroyed, this, &MyObject::objectDestroyed);
the other way to connect a signal to a slot is to use QObject::connect() and the SIGNAL and SLOT macros.
the rule about whether to include arguments or not in the SIGNAL() and SLOT() macors, if the arguments have default values, is that the signature passed to the SIGNAL() macro must not have fewer arguments than the signature passed to the SLOT() macro.
All of these would work:
connect(sender, SIGNAL(destroyed(QObject*)), this, SLOT(objectDestroyed(Qbject*)));
connect(sender, SIGNAL(destroyed(QObject*)), this, SLOT(objectDestroyed()));
connect(sender, SIGNAL(destroyed()), this, SLOT(objectDestroyed()));
But this one won’t work:
connect(sender, SIGNAL(destroyed()), this, SLOT(objectDestroyed(QObject*)));
…because the slot will be expecting a QObject that the signal will not send. This connection will report a runtime error.
note that signal and slot arguments are not checked by the compiler when using this QObject::connect() overload.
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