[Rubycocoa-talk] key-value observing

[Brian M. <ma...@ex...>]

It seems as if RubyCocoa's key-value observing works differently in  
some ways than Objective-C's would. Is this intentional? An incomplete  
implementation? The real question: is it stable behavior an app can  
depend upon?

Here's the difference in the most important case (where you've  
declared the value with kvc_accessor):

- setting the variable directly (var= or setVar) behaves like Objc:
	observeValueForKeyPath... is called

- setting it with setValue_forKey does not:
	observeValueForKeyPath... is called twice (once, I assume,
	for willChangeValueForKey and once for didChangeValueForKey)
	In Objc, it'd be called once.
	
Details:

============= Here's Objective-C:

A Watcher object watches an Observed object like this:

- (id) initWatching: (id) observed
{
   [observed addObserver: self
              forKeyPath: @"value"
                 options: 0
                 context: NULL];
   return self;
}

- (void)observeValueForKeyPath:(NSString *)keyPath
                       ofObject:(id)object
                         change:(NSDictionary *)change
                        context:(void *)context
{
   NSLog(@"observed %@\n", keyPath);
}


Here a third object triggers observation:

   Observed *observed = [[Observed alloc] init];
   Watcher *watcher = [[Watcher alloc] initWatching: observed];
   NSLog(@"Setting directly.");
   observed.value = @"new value";
	// The result is "observed value"

   NSLog(@"Setting kvcly.");
   [observed setValue: @"newer value" forKey: @"value"];
	// The result is "observed value"

============ The Ruby code:

require 'osx/cocoa'

class Observed < OSX::NSObject
   if ARGV[0] == 'kvc'
     kvc_accessor :value
   else
     attr_accessor :value
   end
end

class Watcher < OSX::NSObject
   def initWatching(observed)
     init
     observed.addObserver_forKeyPath_options_context(self, 'value',
                                                     0, nil)
     self
   end

   def observeValueForKeyPath_ofObject_change_context(
                                          keyPath, object, change,  
context)
     puts "#{object} changed #{keyPath}"
   end
end


observed = Observed.alloc.init
watcher = Watcher.alloc.initWatching(observed)

puts "Setting directly"
observed.value = 5
	# Nothing if plain accessor; sees change if kvc_accessor.
puts "Setting via setValue method"
observed.setValue(5)
	# Nothing if plain accessor; sees change if kvc_accessor.
puts "setting kvcly"
observed.setValue_forKey(5, 'value')
	# Gets two messages (presumably willChange and hasChanged)

observed.removeObserver_forKeyPath(watcher, 'value')


-----
Brian Marick, independent consultant
Mostly on agile methods with a testing slant
www.exampler.com, www.exampler.com/blog, www.twitter.com/marick