Re: [xmlvm-users] Optional Methods in C-Backend

[Markus H. <ma...@ti...>]

Hi,

I have manually implemented optional method handling for 2 Methods from UITableViewDelegate. 

http://xmlvm-reviews.appspot.com/121001/

If you think that it is ok that way I would implement the other methods, too.

Markus

Am 06.05.2011 um 02:02 schrieb Arno Puder:

> 
> this is a loaded question. What you are effectively trying to do is to 
> introduce optional methods into the Java language. I know that there 
> once was a JSR for doing exactly that, but it never got adopted (and I 
> don't know the reason why it didn't get adopted).
> 
> I don't think such an annotation would help in this case. The C backend 
> knows nothing about Objective-C. In fact, the parts that are in 
> Objective-C are manually written (i.e., not generated) and injected into 
> the C wrappers. Whoever writes the wrapper for UITableViewDelegate 
> (probably me) has to do the 'magic' with respondsToSelector as Panayotis 
> explained.
> 
> Note that quite a while ago (sometime last year) we had a longer 
> discussion on how to represent iOS API in Java when it comes to optional 
> methods. Fact is, that we are constricted by the Java syntax and 
> semantics and such an XMLVM-specific annotation can't help here.
> 
> Arno
> 
> 
> On 5/5/11 12:24 PM, Markus Heberling wrote:
>> Hi.
>> 
>> shouldn't this be done automagically by xmlvm?
>> 
>> Like adding an @Optional annotation, that can be put on optional methods in java classes, and xmlvm will generate the correct respondsToSelector?
>> 
>> 
>> Regards,
>> Markus
>> 
>> Am 05.05.2011 um 18:19 schrieb Arno Puder:
>> 
>>> 
>>> 
>>> On 5/5/11 6:30 AM, Panayotis Katsaloulis wrote:
>>>> 
>>>> On 05 Μαϊ 2011, at 11:35 π.μ., Markus Heberling wrote:
>>>> 
>>>>> Hi,
>>>>> 
>>>>> I'd like to use a UITableViewDelegate with the C Backend. Some methods
>>>>> in that class are optional. XMLVM generates stubs for the, that throw
>>>>> the XMLVM_NOT_IMPLEMENTED Error:
>>>>> 
>>>>> JAVA_FLOAT
>>>>> org_xmlvm_iphone_UITableViewDelegate_heightForRowAtIndexPath___org_xmlvm_iphone_UITableView_org_xmlvm_iphone_NSIndexPath(JAVA_OBJECT
>>>>> me, JAVA_OBJECT n1, JAVA_OBJECT n2)
>>>>> {
>>>>> //XMLVM_BEGIN_WRAPPER[org_xmlvm_iphone_UITableViewDelegate_heightForRowAtIndexPath___org_xmlvm_iphone_UITableView_org_xmlvm_iphone_NSIndexPath]
>>>>> XMLVM_NOT_IMPLEMENTED();
>>>>> //XMLVM_END_WRAPPER
>>>>> }
>>>>> 
>>>>> I can override all of the methods, of course, but I rather would leave
>>>>> them out. What would be the correct way to handle optional methods?
>>>>> 
>>>>> Markus
>>>> 
>>>> I can't help you with the C backend, but I'll describe a bit what I did
>>>> in the same case with the ObjC backend, so that the C Gurus here could
>>>> advise the correct answer.
>>>> 
>>>> The problem is, there is no "default" value for these (and all similar)
>>>> methods. Moreover, the iOS would behave different, when these methods
>>>> are present, and do something else, when these methods are missing.
>>>> 
>>>> What I did on the ObjC backend was to do something similar to what (I
>>>> believe) Apple does.
>>>> First of all, *all* ObjC delegate methods exist and all point to their
>>>> Java counterpart.
>>>> Then the method that answers to the ObjC runtime if this method exists
>>>> (i.e. respondsToSelector) is overridden. If an actual Java-based
>>>> implementation exists, then respondsToSelector responds that the actual
>>>> ObjC delegate selector exists (although the ObjC selector *always*
>>>> exist). If it doesn't then it replies that this method does not exist.
>>>> 
>>>> WIth such a trick it is possible to properly inform the UITableView that
>>>> an optional *Java* method exist, and have the desired/correct behavior.
>>> 
>>> You can do the same with the C backend. The key is to implement
>>> respondsToSelector in UITableViewDelegate in such a way that it figures
>>> out if a Java method has been overridden or not. This can easily be done
>>> with the help of the vtable: if a method is overridden in a derived
>>> class, the vtable entry for that method will point to the implementation
>>> in the derived class. If the method has not been overridden, the vtable
>>> entry will still point to the implementation of the base class. The same
>>> trick is used in org_xmlvm_iphone_UIResponder.m (for the C backend) to
>>> check if methods such as touchesBegan: have been overridden. Here the
>>> relevant code sniplet from that file:
>>> 
>>>     VTABLE_PTR func =
>>> thiz->tib->vtable[XMLVM_VTABLE_IDX_org_xmlvm_iphone_UIResponder_touchesBegan___java_util_Set_org_xmlvm_iphone_UIEvent];
>>>     if (func == (VTABLE_PTR)
>>> org_xmlvm_iphone_UIResponder_touchesBegan___java_util_Set_org_xmlvm_iphone_UIEvent)
>>> {
>>>         // vtable index for this method still points to the
>>> implementation of
>>>         // class UIResponder. Delegate event to the next responder
>>> 		[[self nextResponder] touchesBegan:touches withEvent:event];
>>> 
>>> If the if-condition is true, the method has not been overridden.
>>> 
>>> Hope this helps,
>>> Arno
>>> 
>>> ------------------------------------------------------------------------------
>>> WhatsUp Gold - Download Free Network Management Software
>>> The most intuitive, comprehensive, and cost-effective network
>>> management toolset available today.  Delivers lowest initial
>>> acquisition cost and overall TCO of any competing solution.
>>> http://p.sf.net/sfu/whatsupgold-sd
>>> _______________________________________________
>>> xmlvm-users mailing list
>>> xml...@li...
>>> https://lists.sourceforge.net/lists/listinfo/xmlvm-users
>> 
> 
> ------------------------------------------------------------------------------
> WhatsUp Gold - Download Free Network Management Software
> The most intuitive, comprehensive, and cost-effective network 
> management toolset available today.  Delivers lowest initial 
> acquisition cost and overall TCO of any competing solution.
> http://p.sf.net/sfu/whatsupgold-sd
> _______________________________________________
> xmlvm-users mailing list
> xml...@li...
> https://lists.sourceforge.net/lists/listinfo/xmlvm-users