Re: Archiving GUI elements

[Richard Frith-Macdonald]

On Monday, October 1, 2001, at 02:48 AM, Adam Fedor wrote:

> Just a note for future reference. I'm designing Gorm inspectors using 
> Gorm itself, which means I'm using archiving to store the inspectors. 
> In the future, if you change the coding procedure for a GUI element, 
> please leave a path for reading in old archived files.
>
> It's probably possible to do this even temporarily (say with ifdefs), 
> since GUI is still in beta and Gorm is perhaps the only app that is 
> doing this (although GNUMail also archives images). Really, there are 
> clean ways to do this also (perhaps the best is by increasing the class 
> version).

For those of you who don't know ... the clean way to do this is  using 
the -versionForClassName: method
(see NSCoder documentation) and the +setVersion: method (see NSObject 
documentation).

So your code would look like this -

static const int        currentVersion = xxx;           // Current version

- (void) initialize
{
  if (self == [MyClass class])
    {
      [self setVersion: currentVersion];
    }
}

- (id) initWithCoder: (NSCoder*)aCoder
{
  int	version = [aCoder versionForClassName: NSStringFromClass([self 
class])];

  if (version < currentVersion)
    {
       ....
    }
  ....
  return self;
}


Typically, this sort of thing should be done to handle changes in 
archiving formats between
and within releases of the libraries.

When we make a release of the base library, we update the system 
version ... and this
makes it possible for us to write much more efficient code -

- (id) initWithCoder: (NSCoder*)aCoder
{
  int   systemVersion = [aCoder systemVersion];
  int   version;

  if (systemVersion <= lastVersionWithFormatChanges)
    {
      version = [aCoder versionForClassName: NSStringFromClass([self 
class])];
    }
  else
    {
      version = currentVersion;
    }

 if (version < currentVersion)
    {
       ....
    }
  ....
  return self;
}