Re: presentError:

FROM : Ali Ozer
DATE : Sun Apr 20 00:36:41 2008

>> "In rare cases, you may want to display a caution icon in your 
>> alert, badged with the application icon as shown in Figure 14-48. A 
>> badged alert is appropriate only if the user is performing a task, 
>> such as installing software, and a possible side effect of that 
>> task would be the inadvertent destruction of data. Don’t use a 
>> caution icon for tasks whose only purpose is to overwrite or remove 
>> data, such as Save or Empty Trash; too-frequent use of the caution 
>> icon dilutes its significance."

>
> Yet presentError: *always* chooses to use a badged caution icon, 
> AFAICT, even though there's no good reason to assume that an error 
> in general implies the possible "inadvertent destruction of data". 
> For example, NSDocumentController reports (via presentError:, I 
> assume) a failure to create or open a document with a caution icon, 
> and in neither case is it likely that data destruction is imminent.


Yes, it would be good to be able to easily customize the badge on the 
alert.  I don't think there is a straightforward way to do this.

>> "The alert displays information found in the NSError object 
>> anError; this information can include error description, recovery 
>> suggestion, failure reason, and button titles (all localized)."

>
> In fact, AFAICT the failure reason is never included in the alert -- 
> the secondary (smaller) text always comes from the recovery 
> suggestion and the failure reason is ignored.


Actually, the failure and the reason are both included in the default 
presentError: call, assuming the error has them. For instance, trying 
to open a read-protected file with NSData will result in:

Bold text: The file “foo.txt” could not be opened because you do not 
have appropriate access privileges.
Smaller text: To view or change access privileges, select the item in 
Finder and choose File > Get Info.

If you call individual methods on that NSError, you will see:

localizedDescription: The file “foo.txt” could not be opened because 
you do not have appropriate access privileges.
localizedFailureReason: You do not have appropriate access privileges.
localizedRecoverySuggestion: To view or change access privileges, 
select the item in Finder and choose File > Get Info.

> (3) According to the NSResponder class reference, a responder passes 
> presentError to the next error responder and:
>

>> "if there is no next responder, it passes the error object to 
>> NSApp, which displays a document-modal error alert"

>
> I had hoped this meant that NSApp cleverly displayed the error as a 
> sheet on the document window and waited for it to be dismissed. In 
> fact, if you send presentError: to a NSDocument you get an 
> application-modal alert.


Actually, the comment for presentError: says "Present an error alert 
to the user, as an application-modal panel"; it's 
presentError:modalForWindow:delegate:didPresentSelector:contextInfo: 
that puts up a doc-modal panel.

Ali