I find it on the website. Only the following passage:
The QTGUI component also includes a number of widgets that can be used to perform live updates of variables through standard QT input widgets. Most of the widgets are implemented directly in Python through PyQT. However, GNU Radio is introducing more widgets, written and therefore available in C++ that also produce messsages. The Python-based widgets only act as variables and so as they are changed, any block using those widgets to set paramters has the callback (i.e., set_value()) function's called.
Entry: An edit box that allows a user to directly set a new value for the parameter.
And I only found a related file "qtgui_entry.xml" in the program.