help about return type

ryubai

i created a new class and add a function to methods.
but it showed such error.
how can i return cgsize correctly?
thanks in advcance.

TypeError: invalid result type for callback function

def collectionView_layout_sizeForItemAtIndexPath_(_self, _cmd, _cv, _la, _ip):
return CGSize(340,300)

methods = [collectionView_layout_sizeForItemAtIndexPath_]

DataSource = create_objc_class('DataSource', methods=methods, protocols=['UICollectionViewDelegateFlowLayout', 'UICollectionViewDataSource', 'UICollectionViewDelegate'])

mcriley821

You need to specify the superclass that your custom class inherits from. This will let you override the method you have and infer the correct return type

ryubai

@mcriley821

it still can’t work.

superclass=NSObject is default when new class is created.

mcriley821

@ryubai
Yes it’s default is NSObject but if you’re trying to overwrite a class method you need to specify that class, or your controller won’t reference your function.

Is this the only bit of code you have? You must have a controller? You need to overwrite the controller itself.

mikael

@mcriley821, defining delegates based on NSObject is normal, and UICollectionViewDelegateFlowLayout protocol should be enough to define the return type for collectionview:layout:sizeForItemAtIndexPath:.

I agree there must be a lot of code that we are not seeing, e.g. covering the other protocols listed, but I get the error with this isolated piece of code:


import objc_util


def collectionView_layout_sizeForItemAtIndexPath_(_self, _cmd, _cv, _la, _ip):
    return objc_util.CGSize(349,300)

methods = [collectionView_layout_sizeForItemAtIndexPath_]

AnotherName = objc_util.create_objc_class(
    'AnotherName', 
    methods=methods, 
    protocols=[
        'UICollectionViewDelegateFlowLayout'
    ]
)

mikael

@ryubai, let’s see if we can wake the big bears (@JonB, @dgelessus).

JonB

@ryubai CGSize is not an object, it is a structure. You need to make your return type a CGSize Structure, not a c_void_p

JonB

I think there is a way to specify argtypes and restype when you create the class/method?

ryubai

@JonB

thank you all

when i overwrite a method requested a return type in a new class.
number and string can be returned directly.
cell object in tableview is returned in cell.ptr
but how to return a CGSize structure?
objc's CGSize does't work

cvp

@ryubai did you read this , it is to complex for me 😢

ryubai

anyone can help? please

mikael

@ryubai, I tried a few things:

confirming that defining another method of the same protocol that returns a float works fine
looking a the definition of CGSize in objc_util
fiddling with the retype of the function
using CGSizeMake function, but could not get it defined
checking if ctypes.Structure would have some additional capabilities

No help nowhere.

JonB

okay, the issue here is that ctypes does not allow complex return types, such as Unions or Structures for callbacks.

see
https://bugs.python.org/issue5710

However, @dgelessus actually came up with a solution.
https://github.com/beeware/rubicon-objc/pull/85/files

you need to download ctypes_patch.py and place in site-packages (or, you can use this executable link

Then, you just need to call

import ctypes_patch

...

ctypes_patch.make_callback_returnable(CGSize)

DataSource = create_objc_class('DataSource', methods=methods, protocols=['UICollectionViewDelegateFlowLayout', 'UICollectionViewDataSource', 'UICollectionViewDelegate'])

cvp

Sincerely, what could we do without @dgelessus and @JonB
Thanks to be here.

JonB

i updated the exec link, to download and install the patch into site-packages. click the link, then click run.

ryubai

@JonB @mikael @cvp and other dear friends

Sincerely,many thanks to you all!
i love to stay here.

ryubai

@JonB

run more than once will raise such errs,then how to solve?

ctypes_patch.make_callback_returnable(CGSize)
File "/private/var/mobile/Containers/Shared/AppGroup/02422888-CC52-45FC-BB53-72013D3F4AFA/Pythonista3/Documents/site-packages/ctypes_patch.py", line 128, in make_callback_returnable
raise ValueError("The ctype {} already has a getfunc")
ValueError: The ctype {} already has a getfunc

JonB

Probably you can just use a try/except around that, or explicitly check for the existence of a getfunc.

dgelessus

Sorry for the late reply - I forgot to check the forum recently... The version of ctypes_patch that @JonB linked to is the original version from the pull request that added it. There have been some updates to it that fix a few problems, including the "The ctype {} already has a getfunc" error that @ryubai posted above. I would recommend using the current version of ctypes_patch from the repo and not the old one from the pull request.

eliotradisa

This post is deleted!