使用emit和调用信号，就像它是Qt中的常规函数 [英] Using emit vs calling a signal as if it's a regular function in Qt

问题描述

假设我有这个信号：

signals:
    void progressNotification(int progress);

我最近才了解到Qt中的emit关键字。到现在为止，我只是像普通函数一样调用它们来执行信号。而不是：

I only recently learned about the emit keyword in Qt. Until now, I used to execute signals by just calling them like a regular function. So instead of:

emit progressNotification(1000 * seconds);

我会写：

progressNotification(1000 * seconds);

像这样调用他们似乎工作，所有连接的插槽都会执行，关键字导致不同的行为，还是只是语法糖？

Calling them like that seemed to work, and all the connected slots would execute, so does using the emit keyword cause a different behaviour, or is it just syntactic sugar?

推荐答案

emit 只是语法糖。如果你看看发出信号的函数的预处理输出，你会看到 emit 刚刚走了。

emit is just syntactic sugar. If you look at the pre-processed output of function that emits a signal, you'll see emit is just gone.

magic发生在信号发射函数的生成代码中，您可以通过检查moc生成的C ++代码来查看。

The "magic" happens in the generated code for the signal emitting function, which you can look at by inspecting the C++ code generated by moc.

例如 foo 没有参数的信号生成此成员函数：

For example a foo signal with no parameters generates this member function:

void W::foo()
{
    QMetaObject::activate(this, &staticMetaObject, 0, 0);
}

而代码 emit foo code>被预处理为简单的 foo（）;

emit 在 Qt / qobjectdefs.h 中定义（在源的开源风格中），如下所示：

emit is defined in Qt/qobjectdefs.h (in the open-source flavor of the source anyway), like this:

#ifndef QT_NO_EMIT
# define emit
#endif

（定义保护是允许你通过 no_keywords QMake配置选项使用Qt与其他具有冲突名称的框架。）

(The define guard is to allow you to use Qt with other frameworks that have colliding names via the no_keywords QMake config option.)

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