Re: Need advice about NSTextField, performClick IBAction, and memory management

[Riccardo Mottola]

Hi Bertrand,

Josh Freeman wrote:
>    You can set the textfield to only send its action when Enter's 
> pressed (instead of every time editing finishes): In Interface 
> Builder, select the textfield, then show the Inspector panel. Under 
> the Inspector's Attributes tab, switch the textfield's 'Send Action' 
> mode from "End editing" to "Enter only". (You can also do this 
> programmatically by sending the textfield's cell a 
> setSendsActionOnEndEditing:NO message.)

You have the same option in Gorm. Select the TextField, go to the 
inspector and in the attributes you have "Send action on" Enter only 
(what you probably want) or "End Editing" which is essentially always 
when your control looses focus.

> These three lines in createNewTask: cause the issue:
>
>         NSString *string = [[NSString alloc] init];
>         string = [textField stringValue];
>     ...
>     [string release];// With this, I've got an "exec_BAD_ACCESS" error 
> when calling three times in a row createNewTask with the same string 
> in textField

You are trying to release the textField value, not the empty one you 
allocated. By assigning the the value you overwrite your own object.
Actually, this code will leak memory: the first allocated string.

>    The first line sets 'string' to a retained, empty string object. 
> The next line sets 'string' to the object returned by [textField 
> stringValue], leaking the previous string object (its address was 
> forgotten while it was still retained). 'String' then points to an 
> object that wasn't retained by your code, so sending it a release 
> message will cause it to deallocate while it's still referenced 
> elsewhere (segfaulting if it's accessed after that).
>
>    You can fix the issue by removing the first & third lines 
> (empty-string allocation, release call), and moving the 'string' var 
> definition to the second line:
>
>         NSString *string = [textField stringValue];

Another way to explain what Josh is saying:
You need to alloc+init a string if you want to work on it, but in this 
case, you declatr *string just as a pointer to get the value of the 
textField, so you don't own the value and have no need to free it.
Conversely, if you need to "work" on the value retturned by the 
textField for more than a run-loop, or pass it to a thread or to be 
blunt "for a long time", then you need to either retain or copy it. E.g. 
you get a value from a window, then close the window but want to 
continue working on that. If you retain or copy your object, then you 
need to release it.


Riccardo