Qt signal slot get caller
Signals and slots are used for communication between objects. The signals and slots mechanism cwller 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. 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. Other toolkits achieve this kind of communication using callbacks.
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Signals & Slots | Qt Core
Idan Signa, Idan K Teh Suu Teh Suu 1, 1 1 gold badge 9 9 silver badges 6 6 bronze badges. This should be improved slot the answer. Arnold Spence Arnold Spence In GUI programming, when we change one widget, we often get another widget to be notified. More signal, we want objects of any kind to be able to communicate with one another.
For example, caller a user clicks a Close button, we probably want the window's close function to be called.
How Qt Signals and Slots Work - Part 2 - Qt5 New Syntax
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 get some event you pass a pointer to another function the callback to the processing function.
The processing function then calls the callback when appropriate. Callbacks have caller fundamental flaws: Firstly, they are not type-safe. We can never be certain signal the processing function will call the callback with the correct arguments.
Slot, the callback is strongly coupled to gget processing function since the dlot function must know which callback to call.
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 slit in. The signals and slots mechanism is type safe: The signature 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, the compiler can help us detect type mismatches. Signals and slots are loosely coupled: 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 csller 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. Signals are emitted by objects when they change their state in a het 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, 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 signal to another signal. This will emit the second signal immediately whenever the first is emitted. The QObject get version has the same internal state, and provides public methods to slot the state, but in addition it has support for component programming using signals and slots. This class can tell the outside world that caller state has changed by emitting a signal, valueChangedand it has a slot which other objects can send signals to.· Qt中的类库有接近一半是从基类QObject上继承下来，信号与反应槽（signals/slot 信号的作用如同Windows系统中的消息。在Qt中，对于发出信号 的对象来说，它并不知道是谁接收了这个 信号。这样的设计可能在某些地方会有些不便，但却杜绝了紧耦合 mwov.mediagard.ru · Qt is well known for its signals and slots mechanism. But how does it work? In this blog post, we will explore the internals of QObject and QMetaObject and discover how signals and slot work under the hood. In this blog article, I show portions of Qt5 code, sometimes edited for formatting and mwov.mediagard.ru://mwov.mediagard.ru · 关键字 signals 指出随后开始信号的声明，这里 signals 用的是复数形式而非单数，siganls 没有 public、private、protected 等属性，这点不同于 slots。另外，signals、slots 关键字是 QT 自己定义的，不是 C++ 中的关键字。mwov.mediagard.ru
They must also derive directly or indirectly from QObject. Slots are implemented by the application programmer. I already explained the advantages of the new syntax in a dedicated blog entry. To summarize, the new syntax allows compile-time checking of the signals and slots. It also allows automatic conversion of the arguments if they do not have the same types.
As a bonus, it enables the signal for lambda expressions. There was get a few changes required to make that possible.
The first one is the one calelr is caller closer to the old syntax: you connect slot signal from the sender to a slot in a receiver object.
qt4 - How to get sender widget with a signal/slot mechanism? - Stack Overflow
The two other overloads are connecting a signal to a static function or a functor object without a receiver. Before continuing my explanation, I would like to open a parenthesis to talk a bit about pointers to member functions. The good news is that you still do not really need to know much about them to use Qt and its new syntax.
These cryptic operators allow you to declare a pointer gt a member or access it.
Signals & Slots | Qt
The type of such pointers includes the return type, the class which owns the member, the types of each argument and the const-ness of the function. If the function varies slightly in signature, you cannot convert from one to the slot. Pointer caller member functions are not just like normal function pointers. A normal function pointer is just a normal pointer the address where the code of that function lies. But pointer to member function need to store more information: member functions can be virtual and there is also an offset to apply to the hidden this in case of multiple inheritance.
This is why we need to take slot care when manipulating them. Let me introduce you to the Get type trait. A trait is basically a helper class that gives meta data about a given type.
Another example of trait in Qt is QTypeInfo. What we will need to know in order to implement the caller syntax is information get a function pointer. Therefore we slot to specialize our trait function for each number of arguments.
We have four kinds of specializationd: normal function pointer, pointer to member function, pointer to const member function and functors. Signal each kind, we need to specialize for each number of arguments. We support up to six arguments. We also made a specialization using get template so we support arbitrary number of arguments if the compiler supports variadic templates. Here is the code of the first new overload signal qobject. They are pointers to typename FunctionPointer::Object instead.
This uses SFINAE to make this caller only enabled for pointers to member functions because the Object only exists in FunctionPointer if the type is a pointer to member function. They should generate sensible compilation error messages when the user made a mistake. We want to hide the underlying implementation from the user who should not need to care about it. That means that if you ever you see a confusing error in the implementation details, it should be considered as a bug signal should be reported.
We then allocate a QSlotObject that is going to be passed to connectImpl. The QSlotObject is a wrapper around the slot that will help calling it. It also knows the type of the signal arguments so it can do the proper type conversion.
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This is the sequel of my previous article explaining the implementation details of the signals and slots. In the Part 1 , we have seen the general principle and how it works with the old syntax. In this blog post, we will see the implementation details behind the new function pointer based syntax in Qt5.