信号和槽 Signals and Slots

学习QT,不了解信号和槽是不行的。这篇文章在CSDN(http://dev.csdn.net/htmls/13/13471.html)有翻译,不过似乎是老版的,所以个别地方不一样。里面的一些词汇,以及有几个地方原文不易理解,我特地注释了一下。重要句子我也用横线划出。

 

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.

 

Introduction

In GUI programming, when we change one widget, we often want another widget to be notified. More generally, we want objects 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.

Older toolkits achieve this kind of communication using callbacks. A callback is a pointer to a function, so if you want a processing function to notify you about some event you pass a pointer to another function (the callback) to the processing function.(如果你希望一个处理函数通知你某些事件发生了,你可以传递一个指向其他函数(回调)的指针给处理函数). The processing function then calls the callback when appropriate. Callbacks have two fundamental flaws(缺点): 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(紧耦合) to the processing function since the processing function must know which callback to call.

 

 

Signals and 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 widgets 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 your own slots so that you can handle the signals that you are interested in.

qt

图1

The signals and slots mechanism is type safe: The signature(署名,签名,信号) of a signal must match the signature of the receiving slot. (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(兼容的), the compiler can help us detect type mismatches. Signals and slots are loosely coupled(松耦合): A class which emits a signal neither knows nor cares which slots receive the signal. Qt’s signals and slots mechanism ensures that if you 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 communicate. It does not know or care whether anything is receiving the signals it emits. This is true information encapsulation(封装), and ensures that the object can be used as a software component.

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.

You can connect as many signals as you want to a single slot, and a signal can be connected to as many slots as you need. It is even possible to connect a signal directly to another signal. (This will emit the second signal immediately whenever the first is emitted.)见图1.

Together, signals and slots make up a powerful component programming mechanism.

 

A Small Example

A minimal C++ class declaration might read:

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 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:

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 #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 signal, 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:

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 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(摘要), we create two Counter objects and connect the first object’s valueChanged() signal to the second object’s setValue() slot using QObject::connect():

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     Counter a, b;
     QObject::connect(&a, SIGNAL(valueChanged(int)),
                      &b, SLOT(setValue(int)));
 
     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 value != m_value. This prevents infinite looping in the case of cyclic connections (e.g., if b.valueChanged() were connected to a.setValue()).

A signal is emitted for every connection you make; if you duplicate a connection, two signals will be emitted. You can always break a connection using QObject::disconnect().

This example illustrates that objects can work together without needing to know any information about each other. To enable this, the objects only need to be connected together, and this can be achieved with some simple QObject::connect() function calls, or with uic‘s automatic connections feature.

 

Building the Example

The C++ preprocessor changes or removes the signals, slots, and emit keywords so that the compiler is presented with standard C++.

By running the moc on class definitions that contain signals or slots, a C++ source file is produced which should be compiled and linked with the other object files for the application. If you use qmake, the makefile rules to automatically invoke moc will be added to your project’s makefile.

 

Signals

Signals are emitted by an object when its internal state has changed in some way that might be interesting to the object’s client or owner. Only the class that defines a signal and its subclasses can emit the signal.

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 an arbitrary order, when the signal is emitted.

Signals are automatically generated by the moc 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.

You 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 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.

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.

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.

 

Meta-Object Information

The meta-object(元对象) compiler (moc) parses the class declaration in a C++ file and generates C++ code that initializes the meta-object. The meta-object contains the names of all the signal and slot members, as well as pointers to these functions.

The meta-object contains additional information such as the object’s class name. You can also check if an object inherits a specific class, for example:

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     if (widget->inherits("QAbstractButton")) {
         QAbstractButton *button = static_cast<QAbstractButton *>(widget);
         button->toggle();
     }

The meta-object information is also used by qobject_cast<T>(), which is similar to QObject::inherits() but is less error-prone:

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     if (QAbstractButton *button = qobject_cast<QAbstractButton *>(widget))
         button->toggle();

See Meta-Object System for more information.

 

更多内容请见原文:

http://doc.trolltech.com/4.5/signalsandslots.html

发布者

Tanky Woo

Tanky Woo,[个人主页:https://tankywoo.com] / [新博客:https://blog.tankywoo.com]

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