Re: [Pyobjc-dev] "out" parameters and return values

[Mani G. <ma...@tu...>]

Interestingly, while developing my simplified implementation, I hit a snag.

There's one protocol function to be implemented, a very simple one:
'preferredSyncModeForEntityName:'
(http://developer.apple.com/documentation/Cocoa/Reference/ISyncSessionDriverDataSource_Protocol/Reference/Reference.html#//apple_ref/occ/intfm/ISyncSessionDriverDataSource/preferredSyncModeForEntityName:
).

It returns an ISyncSessionDriverMode (
http://developer.apple.com/documentation/Cocoa/Reference/SyncServicesFramework/Miscellaneous/SyncServices_Constants/Reference/reference.html#//apple_ref/doc/c_ref/ISyncSessionDriverMode
) which is just an enum value (int).

For some reason, it doesn't like my return value, even though I'm
simply returning one of the enum values
(ISyncSessionDriverModeRefresh). Here is the error:

(ValueError: NSInvalidArgumentException - ISyncSessionDriverDataSource
<MySyncSource: 0x1f803a0> responded poorly to
preferredSyncModeForEntityName: with entity
com.apple.contacts.Contact)

Any ideas? It's all very straightforward, yet it fails. Source code is attached.

Note: this test app syncs with your Address Book data (only first
names/last names) and displays the raw data in the window. I've
configured the sync settings (see SyncDriverTest.plist) so that it can
only pull data. You can change SyncDriverTest.plist so that it
retrieves any other data type recognized by sync services, but if you
do make sure the entity is listed under "PullOnlyEntities" to protect
your data.

Cheers,
Mani

On Thu, Feb 19, 2009 at 10:28 AM, Mani Ghasemlou <ma...@tu...> wrote:
> Ok, I'll put together some simplified code and post my results as soon
> as I can (might be a couple days). In the meantime, here is the
> protocol in question: ISyncSessionDriverDataSource
> (http://developer.apple.com/documentation/Cocoa/Reference/ISyncSessionDriverDataSource_Protocol/Reference/Reference.html#//apple_ref/occ/intf/ISyncSessionDriverDataSource)
>
> Kind regards,
> Mani
>
> On Thu, Feb 19, 2009 at 2:11 AM, Ronald Oussoren <ron...@ma...> wrote:
>>
>> On 19 Feb, 2009, at 5:16, Mani Ghasemlou wrote:
>>
>>> Thanks so much. Then, there must be something wrong with my code,
>>> because I keep getting a crash when hitting this code. This however
>>> rules out that my overall approach is wrong.
>>
>> Can you reproduce your crash in a smaller program? That would allow us to
>> look at the code and could help pintpoint the issue (either something in
>> your code or a bug in PyObjC).
>>
>> Is the method your writing overriding an existing method, or implementing an
>> informal protocol?  PyObjC cannot deduce the right signature for methods
>> with output arguments without that information. An easy way to check if
>> that's your problem is to add some  decoration[1] to your code that
>> explicitly sets the signature:
>>
>>   def returnIntWithError_(self, outError):
>>        pass # body goes here
>>   returnIntWithError_ = objc.selector(returnIntWithError_,
>> signature='@@:o^@')
>>
>> Ronald
>>
>> [1] the reason for this strange formulation is that I wanted to write "a
>> decorator", but noticed that the required decorator isn't actually part of
>> PyObjC yet.
>>
>>>
>>>
>>> Kind regards,
>>> Mani
>>>
>>> On Wed, Feb 18, 2009 at 4:17 PM, Ronald Oussoren <ron...@ma...>
>>> wrote:
>>>>
>>>> Hi,
>>>>
>>>> You're approach is correct.
>>>>
>>>>>> def returnIntWithError_(self, outError):
>>>>>> myError = NSError.errorWithDomain_code_userInfo_('myDomain', '1',
>>>>>> objc.nil)
>>>>>> myReturnVal = 1
>>>>>> return myReturnVal, myError
>>>>>
>>>>
>>>> As an optimization you could check if "outError is objc.NULL", if it is
>>>> the
>>>> caller isn't interested in the error value and you could return
>>>> (myReturnVal, objc.NULL) instead of (myReturnVal, myError).   That's not
>>>> really an issue here, but could help when it is hard to calculate the
>>>> value
>>>> of the output argument.
>>>>
>>>> BTW. Use "None" instead of "objc.nil", both have the same value and
>>>> "objc.nil" is basicly only present to make it easier to translate ObjC
>>>> code
>>>> into Python. The same is true for objc.YES and objc.NO, those are aliases
>>>> for True and False.
>>>>
>>>> Ronald
>>>>
>>>>
>>>> On 17 Feb, 2009, at 20:51, Mani Ghasemlou wrote:
>>>>
>>>>> Just wanted to raise this question again, as I still haven't found a
>>>>> solution.
>>>>>
>>>>> I've posted the same question on Cocoa-dev, also with no response:
>>>>> http://www.cocoabuilder.com/archive/message/cocoa/2009/2/2/229316
>>>>>
>>>>> Would greatly appreciate any insights!
>>>>>
>>>>> Kind regards,
>>>>> Mani
>>>>>
>>>>> On Mon, Feb 2, 2009 at 9:34 AM, Mani Ghasemlou <ma...@tu...> wrote:
>>>>>>
>>>>>> Hi all,
>>>>>>
>>>>>> From what I understand of the PyObjc documentation, "out" pointer
>>>>>> variables (such as "NSError **outError") are actually appended to the
>>>>>> list of return values for a bridge function.
>>>>>>
>>>>>> For example:
>>>>>>
>>>>>> int returnIntWithError: NSError **outError
>>>>>>
>>>>>> from Python would be invoked as:
>>>>>>
>>>>>> returned_int, returned_error = returnIntWithError_(None)
>>>>>>
>>>>>> I hope that my understanding of the above is correct.
>>>>>>
>>>>>> Now, my real question is to extend the above concept to the Python
>>>>>> side, when we want to implement an informal protocol.  Let's say the
>>>>>> protocol says that I have to implement the returnIntWithError:
>>>>>> function as described above. How do I implement this function? Is the
>>>>>> code below enough?
>>>>>>
>>>>>> def returnIntWithError_(self, outError):
>>>>>> myError = NSError.errorWithDomain_code_userInfo_('myDomain', '1',
>>>>>> objc.nil)
>>>>>> myReturnVal = 1
>>>>>> return myReturnVal, myError
>>>>>>
>>>>>> Note that I am completely ignoring the "outError" parameter. Does this
>>>>>> look kosher?
>>>>>>
>>>>>> Thanks in advance for any help!
>>>>>>
>>>>>> Cheers,
>>>>>> Mani
>>>>>>
>>>>>
>>>>>
>>>>>
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>>>>
>>>>
>>
>>
>