# Utility methods for threading

`NSObject` has a number of methods for calling selectors in different contexts, such as after a delay or on another thread.

These methods can be called from Python (of course), but you need to be a bit careful because the implementation of these methods won’t catch exceptions for you and therefore an uncaught exception in your python code can cause the runloop to stop due to an exception, which might terminate your program.

To make matters worse, due to a bug in Python 2.5 this will cause a hard crash (segmentation fault) when this happens with `performSelectorOnMainThread:withObject:waitUntilDone:`.

Because of this PyObjC 2.1 provides a category on `NSObject` that implements safe alternatives to the stock `NSObject` methods.

## Safe replacements for stock `NSObject` methods

These safe replacements have the same signature as the stock `NSObject` methods, but will log exceptions instead of letting them escape into the rest of your program.

- `pyobjc_performSelector_onThread_withObject_waitUntilDone_`

  This is the safe alternative for `performSelector_onThread_withObject_waitUntilDone_`.

  (Introduced in macOS 10.5)

- `pyobjc_performSelector_onThread_withObject_waitUntilDone_modes_`

  This is the safe alternative for `performSelector_onThread_withObject_waitUntilDone_modes_`.

  (Introduced in macOS 10.5)

- `pyobjc_performSelector_withObject_afterDelay_`

  This is the safe alternative for `performSelector_withObject_afterDelay_`.

- `pyobjc_performSelector_withObject_afterDelay_inModes_`

  This is the safe alternative for `performSelector_withObject_afterDelay_inModes_`.

- `pyobjc_performSelectorInBackground_withObject_`

  This is the safe alternative for `performSelectorInBackground_withObject_`.

  (Introduced in macOS 10.5)

- `pyobjc_performSelectorOnMainThread_withObject_waitUntilDone_`

  This is the safe alternative for `performSelectorOnMainThread_withObject_waitUntilDone_`.

- `pyobjc_performSelectorOnMainThread_withObject_waitUntilDone_modes_`

  This is the safe alternative for `performSelectorOnMainThread_withObject_waitUntilDone_modes_`.

## Enhanced methods

The stock methods are quite useful, but at times it is useful to get the result back from the other thread. The methods below will call the selector on another thread and will return the result from that call in the current thread. If the call on the “other” thread raises an exception this exception will be reraised in the current thread.

Note: these methods are synchronous, that is, they block the current thread until the call on the “other” thread is done.

As an example:

>      1  class MyClass (NSObject):
>      2
>      3     def divideByZero_(self, arg):
>      4          return arg/0
>      5
>      6     def doit(self):
>      7         try:
>      8             result = self.performSelectorOnMainThread_withObject_(
>      9                  'divideByZero:', 55)
>     10             print result
>     11
>     12         except:
>     13             print "Division failed"

The available methods are:

- `pyobjc_performSelector_onThread_withObject_(selector, thread, arg)`

  (Introduced in macOS 10.5)

- `pyobjc_performSelector_onThread_withObject_modes_(selector, thread, arg, modes)`

  (Introduced in macOS 10.5)

- `pyobjc_performSelectorOnMainThread_withObject_(selector, arg)`

- `pyobjc_performSelectorOnMainThread_withObject_modes_(selector, arg, modes)`