問題描述

假設(shè)我有這個信號:

signals:
    void progressNotification(int progress);

我最近才知道 Qt 中的 emit 關(guān)鍵字.到現(xiàn)在為止，我曾經(jīng)通過像普通函數(shù)一樣調(diào)用它們來執(zhí)行信號.所以，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);

我會寫:

progressNotification(1000 * seconds);

像這樣調(diào)用它們似乎可行，并且所有連接的插槽都會執(zhí)行，所以使用emit 關(guān)鍵字會導(dǎo)致不同的行為，還是只是語法糖?

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?

推薦答案

emit 只是語法糖.如果您查看發(fā)出信號的函數(shù)的預(yù)處理輸出，您會看到 emit 剛剛消失.

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.

魔術(shù)"發(fā)生在信號發(fā)射函數(shù)的生成代碼中，您可以通過檢查由 moc 生成的 C++ 代碼來查看.

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.

例如一個沒有參數(shù)的 foo 信號生成這個成員函數(shù):

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

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

并且代碼emit foo();被預(yù)處理為簡單的foo();

And the code emit foo(); is pre-processed to simply foo();

emit 在 Qt/qobjectdefs.h 中定義(無論如何都是開源的)，像這樣:

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

(定義保護(hù)是允許您通過 no_keywords QMake 配置選項將 Qt 與具有沖突名稱的其他框架一起使用.)

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

這篇關(guān)于使用發(fā)射與調(diào)用信號，就好像它是 Qt 中的常規(guī)函數(shù)??一樣的文章就介紹到這了，希望我們推薦的答案對大家有所幫助，也希望大家多多支持html5模板網(wǎng)！