The follow is my point.

The GUI should not conain and know the APP data( except for backup, or cache). we should make abstraction about the data can be displayed by GUI, the concept, such as Presentable Data.

The PD take charge on the data loading, updating, and interacting with GUI. So, The APP developer can work( dev and test ), even without the GUI be maked, by directly control the PD's interface.

How may kinds of PD? not many, single data(string, integer,image,etc), list/array, matrix, treee, stream, just so so.