fix NULL constraint on date/time widget with allow NULL

the issue was that widget wrapper was using parent QDateTimeEdit::dateTimeChanged signal. It was connected both internally to QgsDateTimeEdit::changed and in the form to detect changes. When QgsAttributeForm was recreating the updated feature it was calling QgsDateTimeEdit::value() before changed() could update the value (i.e. setting mIsNull to false).

fix #17790

Author: 3nids

python/gui/editorwidgets/qgsdatetimeedit.sip

@@ -65,6 +65,9 @@ Resets the widget to show no value (ie, an "unknown" state).
 .. versionadded:: 2.16
 %End
 
+  signals:
+    void dateTimeChanged( const QDateTime &date );
+
   protected:
     virtual void mousePressEvent( QMouseEvent *event );
 

src/gui/editorwidgets/qgsdatetimeedit.cpp

@@ -61,11 +61,6 @@ void QgsDateTimeEdit::clear()
     displayNull();
 
     changed( QDateTime() );
-
-    // avoid slot double activation
-    disconnect( this, &QDateTimeEdit::dateTimeChanged, this, &QgsDateTimeEdit::changed );
-    emit dateTimeChanged( QDateTime() );
-    connect( this, &QDateTimeEdit::dateTimeChanged, this, &QgsDateTimeEdit::changed );
   }
 }
 
@@ -174,6 +169,8 @@ void QgsDateTimeEdit::changed( const QDateTime &dateTime )
   }
 
   mClearAction->setVisible( mAllowNull && !mIsNull );
+
+  emit QgsDateTimeEdit::dateTimeChanged( dateTime );
 }
 
 void QgsDateTimeEdit::displayNull( bool updateCalendar )

src/gui/editorwidgets/qgsdatetimeedit.h

@@ -62,6 +62,9 @@ class GUI_EXPORT QgsDateTimeEdit : public QDateTimeEdit
      */
     void setEmpty();
 
+  signals:
+    void dateTimeChanged( const QDateTime &date );
+
   protected:
     void mousePressEvent( QMouseEvent *event ) override;
     void focusOutEvent( QFocusEvent *event ) override;

src/gui/editorwidgets/qgsdatetimeeditwrapper.cpp

@@ -93,7 +93,7 @@ void QgsDateTimeEditWrapper::initWidget( QWidget *editor )
 
   if ( mQgsDateTimeEdit )
   {
-    connect( mQgsDateTimeEdit, &QDateTimeEdit::dateTimeChanged, this, &QgsDateTimeEditWrapper::dateTimeChanged );
+    connect( mQgsDateTimeEdit, &QgsDateTimeEdit::dateTimeChanged, this, &QgsDateTimeEditWrapper::dateTimeChanged );
   }
   else
   {