On Oct 16, 2011, at 00:23 , Gerriet M. Denkmann wrote:
> I have this code:
>
> for( id aThing in someArray )
> {
> if ( [ aThing respondsToSelector: @selector(setTitle:) ] )
> {
> [ self replaceIn: aThing
> readSelector: @selector(title)
> writeSelector: @selector(setTitle:)
> withTable: tableName
> ];
> }
> }
>
>
> - (void)replaceIn: thing readSelector: (SEL)selectorIn writeSelector:
> (SEL)selectorOut withTable: (NSString *)tableName;
> {
> NSString *key = [ thing performSelector: selectorIn ];
> ...
> NSString *rep = ...
> [ thing performSelector: selectorOut withObject: rep ];
> }
>
> Now, using Arc I am told that "PerformSelector may cause a leak because its
> selector is unknown".
> What I am supposed to do about this? I really want to have code without
> warnings.
It's not always easy to directly translate code intended for one memory
management model to another. Your code for the second method is (in some sense)
unsafe, although it's easy for us to deduce its safety (in practice) from the
code in the first method.
One solution for this kind of thing is to look for a way to do this using
blocks. Often with blocks it isn't necessary to obscure types just so that you
can route execution through a generic choke point.
Or, you can look much more carefully (on a case-by-case basis) at what you're
really trying to accomplish, and look for a less hinky way. If the 'for' loop
you've shown us really *is* your code, rather than a simplified example, I'd be
inclined to do something like this:
for (id aThing in someArray)
if ([aThing respondsToSelector @selector (setTitle:)])
[aThing setTitle: [self replacementStringFor: [aThing
title] withTable: tableName];
- (NSString*) replacementStringFor: (NSString*) key
{
…
NSString* rep = …
return rep;
}
That's assuming you want unfettered polymorphism for property "title". If
things are actually a bit more organized, you might want to make a protocol
that contains the "title" @property, and make some objects conform to the
protocol, then write:
for (id<MyTitleProtocol> aThing in someArray)
if ([aThing conformsToProtocol: @protocol (MyTitleProtocol)])
aThing.title = [self replacementStringFor: aThing.title
withTable: tableName];
These are pretty much random ideas for rewrites of your code, which you can pay
no attention to. The point I'm trying to make is if you express a clearer
intention *syntactically*, the issue that ARC raises might not even come up.
> Update:
> I guess Arc has no way to know, whether "selectorIn" is something like
> "copy".
> So I am quite willing to reassure it that "selectorIn" is always something
> like title, stringValue, toolTip, etc.
> I think I should declare the SEL as __attribute__((ns_returns_not_retained)).
>
> But how exactly? There really should be some examples.
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