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

SourceForge Headquarters 225 Broadway Suite 1600 San Diego, CA 92101 +1 (858) 422-6466

you got the gist of it. Two things:

1. In your boolean expression, you don't have to go through the TIB
    to retrieve the function pointer. You can inline the function
    directly as a constant.
2. It may be possible that the XMLVM_VTABLE_xxx symbol isn't defined.
    This can happen if the method is not overridden. XMLVM performs
    compile-time optimizations and will not even generate a vtable
    entry for that method. In this case, the result should always
    be FALSE.

Here a re-worked version:

if (aSelector == @selector(tableView:heightForRowAtIndexPath:)) {
#ifdef 
XMLVM_VTABLE_IDX_org_xmlvm_iphone_UITableViewDelegate_heightForRowAtIndexPath___ 
  VTABLE_PTR f = ((org_xmlvm_iphone_UITableViewDelegate*) 
delegate_)->tib-> 
vtable[XMLVM_VTABLE_IDX_org_xmlvm_iphone_UITableViewDelegate_heightForRowAtIndexPath___org_xmlvm_iphone_UITableView_org_xmlvm_iphone_NSIndexPath]; 
   return f != (VTABLE_PTR) 
org_xmlvm_iphone_UITableViewDelegate_heightForRowAtIndexPath___org_xmlvm_iphone_UITableView_org_xmlvm_iphone_NSIndexPath;
#else
   return 0;
#endif

If the mailer messes up the indentation and you can't read it, I'll send 
it as an attachment.

Arno

On 5/6/11 2:50 AM, Markus Heberling wrote:
> 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
>