Codebase: emiting a signal forcedly from outside is a bad practice

[asashnov]

kiwix-desktop 'master' (830e19b)

Found during working on #454

From src/zimview.cpp:

emit mp_tabBar->currentTitleChanged(str);
emit mp_tabBar->currentZimIdChanged(zimId);
emit mp_tabBar->webActionEnabledChanged(QWebEnginePage::Back, mp_webView->isWebActionEnabled(QWebEnginePage::Back));
emit mp_tabBar->webActionEnabledChanged(QWebEnginePage::Forward, mp_webView->isWebActionEnabled(QWebEnginePage::Forward));
emit mp_library->bookmarksChanged();

According to Qt documentation, this is a bad practice.
By signals, only the object itself should notify that its internal state has changed:

https://doc.qt.io/qt-5/signalsandslots.html

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. 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.

[kelson42]

@asashnov Fell free to propose an alternative if this helps to fix #454 or trivial to do.

[kelson42]

What could be an alternative approach?

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be now be reviewed manually. Thank you for your contributions.

[asashnov]

Good approach is suggested by Qt documentation:

Signals are emitted by an object when its internal state has changed

we recommend to only emit them from the class that defines the signal

It is even possible to connect a signal directly to another signal

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, and ensures that the object can be used as a software component.