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

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

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
> 
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