[MacRuby-devel] How to retrieve an observer's context?

[Laurent Sansonetti]

Hi Edward,

Context arguments in Objective-C are generally void pointers, which  
makes them hard to use in Ruby. Also, since we run in Objective-C GC  
mode, objects could potentially be collected since contexts do not set  
up AFAIK write barriers.

I would recommend to set up an instance variable on your observer  
instead (and pass nil as the context).

   observer.instance_variable_set(:@context, your_context)

And later in your observer method, retrieve the ivar.

Laurent

On Apr 20, 2009, at 8:57 AM, Edward Hynes wrote:

> I'd like to use contexts when registering some observers, but have  
> been unable to retrieve them, getting a "can't convert C/Objective-C  
> value `0x2822731' of type `v' to Ruby object" instead.  The  
> following code, for example, will trigger the error
>
> class Subject
>  attr_accessor :abc
> end
>
> class Observer
>  def observeValueForKeyPath keyPath, ofObject:object, change:change,  
> context:context
>    context[0]
>  end
> end
>
> subject = Subject.new
> observer = Observer.new
>
> subject.addObserver observer, forKeyPath:'abc', options:0,  
> context:'a context'
> subject.setAbc 'some value'
>
> Is there a way to retrieve an observation context?  Or is there an  
> alternative way for observers to distinguish among multiple  
> registrations for a given object/keyPath?
>
> Thanks,
> Ed
>
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