Rubymotion Dispatch exiting early

Question

I'm executing a fairly vanilla dispatch queue in Rubymotion, however it is apparently exiting early. It never gets past the initWithContentsOfURL call. However, removing the Dispatch::Queue wrapper and putting the calls in the main thread works.

The application in the simulator exits with no stack trace or indication of what went wrong. Am I mis-using the dispatch queue?

def foo
  Dispatch::Queue.concurrent.async do
    error_ptr = Pointer.new(:object)
    data = NSData.alloc.initWithContentsOfURL(
      NSURL.URLWithString(url), options:NSDataReadingUncached, error:error_ptr)
    unless data
      p error_ptr[0]
      return
    end
    json = NSJSONSerialization.JSONObjectWithData(data, options:0, error:error_ptr)
    unless json
      presentError error_ptr[0]
      return
    end
    Dispatch::Queue.main.sync { print_results(json) }
  end
end

def print_results(json)
  p "#{json}"
end

Answer 1

I think I tracked down the issue. It was because I was declaring url in the method

def foo
url = "www.google.com"
  Dispatch
    take action on url
  end
end

By moving the url declaration in to the Dispatch thread, it works. I think it was a matter of a method local variable going out of scope before the task had time to execute.

def foo
  Dispatch
    url = "www.google.com"
    take action on url
  end
end

Answer 2

Right now it seems that RubyMotion does not properly retain the local variables outside the dispatch block, so is probably getting an EXEC_BAD_ACCESS and crashing. The following fails:

foo = "some value"
Dispatch::Queue.concurrent.async do
    puts foo
end

However the following two will work:

@foo = "some value"
Dispatch::Queue.concurrent.async do
    puts @foo
end

and also:

foo = "some value"
foo.retain
Dispatch::Queue.concurrent.async do
    puts foo
    foo.release
end

Answer 3

In my case, declaring url in method was fine in RubyMotion 2.5, but my app was crashing with EXC_BAD_ACCESS and sometimes with other error messages because of unicode characters in the app name and all over my code. Fixing encoding header and changing the app name solved my problem. Surprisingly, the app was working fine and crashed only when I called initWithContentsOfURL.