Re: [boost] Re: GUI Library Proposal for a Proposal

From: John Torjo (john.lists_at_[hidden])
Date: 2004-11-15 07:08:46

> 
> I *think* what you envisage is based around collecting data from the 
> user via dialogs. So in your view of things, there would be a C++ object 
> that holds the collected data, a set of validation routines, and a 
> reflection map that binds these to the UI. But you don't do anything to 
> the program state until the user clicks "OK" in your dialog, at which 
> point you suck the data out of your simple C++ object and act on it.
> 
> In contrast, other people (myself included) are considering binding 
> properties and member functions to the UI for similar purposes to why 
> you would expose them to script: to allow the user to drive them. So, 
> for a simple example, let's suppose you have some object that 
> corresponds to a file:
> 
> class file_backed_t
> {
> public:
>   std::string get_filename();
>   /// This actually renames the file on disk.
>   void set_filename( std::string const& );
> };
> 
> Now, when you bind the filename property to the UI, you'll automatically 
> do the rename as soon as the user changes the text in the edit box. 
> (Probably that isn't the desired behaviour in this case, but it serves 
> as a demonstration.) This usage pattern corresponds better to modeless 
> dialogs where you don't have an "OK" button.

Note: I support both modeless and modal dialogs (more to the point: 
saving data can happen automatically - click of "Ok", or programmatically).

Usually, you need more validation than a simple filename. The user could 
enter data into more controls, and once the user is done, you run the 
validation routine(s). This is the transaction commit/fallback pattern, 
which is much better the direct binding IMO.

Also, notice that when doing validation with save_dlg, when a validation 
routine is called, you have acess to more states of your data:
- the original value
- the old value
- the last known good value (the last value for which all validation 
code succeeded)
- the new value

When a validation routine is called, you can set an error, or for 
instance, simply set the value back to the
- old value
- last known good value
- or whatever you wish.

I would say this is very flexible. Hopefully in the near future I'll 
have the time to update the docs so that people will understand the 
save_dlg's full power.

Best,
John

-- 
John Torjo,    Contributing editor, C/C++ Users Journal
-- "Win32 GUI Generics" -- generics & GUI do mix, after all
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