Accessing Barometer Measurements

mikael

... but of course you have the pressure value there. Here’s a modification that gives you a simple get_pressure function (a bit of a hack because of the global variable use).

from objc_util import ObjCInstance, ObjCClass, ObjCBlock, c_void_p

pressure = None

def get_pressure():
  
  def handler(_cmd, _data, _error):
    global pressure
    pressure = ObjCInstance(_data).pressure()

  handler_block = ObjCBlock(handler, restype=None, argtypes=[c_void_p, c_void_p, c_void_p])

  CMAltimeter = ObjCClass('CMAltimeter')
  NSOperationQueue = ObjCClass('NSOperationQueue')
  if not CMAltimeter.isRelativeAltitudeAvailable():
    print('This device has no barometer.')
    return
  altimeter = CMAltimeter.new()
  main_q = NSOperationQueue.mainQueue()
  altimeter.startRelativeAltitudeUpdatesToQueue_withHandler_(main_q, handler_block)
  #print('Started altitude updates.')
  try:
    while pressure is None:
      pass
  finally:
    altimeter.stopRelativeAltitudeUpdates()
    #print('Updates stopped.')
    return pressure

if __name__ == '__main__':
  print(get_pressure())

victordomingos

What kind of value (units) is this last version of the script printing out?

JonB

timestamp (an NSTimeInterval in seconds since the device has been booted)
relativeAltitude (an NSNumber in meters)
pressure (an NSNumber in kilopascal)

DaveClark

I want to use the NSNumber for pressure in a calculation. How do I convert it to something I can use with mathematical functions?

mikael

@DaveClark, change return pressure on line 28 to return pressure.CGFloatValue().

DaveClark

That did it and now I know I need to read the documents more. Thanks you for your help

DaveClark

It was working after I change line 28 to your suggestion. But today without changing anything(I think)

line 28 was return pressure

line 28 now return pressure.CGFloatValue()

Now it is showing an error message complaining about line 28. It says:

AttributeError
no method found for
CGFloatValue

again, I am at a loss

DaveClark

OK. I changed the

return pressure.CGFloatValue()

to

return pressure.floatValue()

and it works again. I still don't understand how it worked earlier.

Thanks for pointing me in the right direction

mikael

@DaveClark, did you update to 11.2.5?

zrzka

Side note, just something to learn about double, float, NSNumber, ...

CGFloat is kind of unfortunate type name. In the C / ObjC world, it's double (C) on 64-bit platforms and float (C) on the rest.

On the other side, Python doesn't have double. It has "just" float and almost all platforms map Python float to IEEE-754 double precision -> double (C).

What I would like to say, you should get your value via [NSNumber doubleValue] instead of floatValue these days.

Not saying that it affects your case, but it's good to know. For example 1.3 is actually stored as 1.2999999523162841796875 (float) and as 1.30000000000000004440892098501 (double), etc.

You can learn more at Floating Point Arithmetic: Issues and Limitations or if you'd like to explore how is float (C) represented just check this nice converter. You can see what is actually stored, what's the error b/o conversion, ... Or just Google for "IEE 754 converter" to find more of them.

DaveClark

@mikael said:

@DaveClark, did you update to 11.2.5?

Yes I did update to 11.2.5

@zrzka Thanks for the good info

mikael

As a general note, I just updated as well. CGFloatValue worked before the update, and works no longer.

Lesson learned: Only use stuff mentioned in the docs, not something that you discover in REPL.