const QByteArray & type paras, will cause lift cycle problems. using Value-pass instead.
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QString, QByteArray supports ref-count and auto-delete , also, these classes support
least-copying, deep-copy will only take place, when the data is about to be changed.
so, in singnals and slots, using QString, QByteArray instead of const QString & and const QByteArray & ,
will not cause performance problem.