Override -openDocumentWithContentsOfURL:
1. If you've already got a document open, send -canCloseDocument… to the open
document, supplying the callback info
2. Either:
A) Return nil and an NSUserCancelled error.
B) Return the existing document.
3. When you get the callback from -canCloseDocument… call super's
implementation of -openDocumentWithContentsOfURL:
On 5 Jan 2010, at 16:45, Martin Stanley wrote:
> I have a Core-Date application that uses the Cocoa Document architecture
> (NSPersistentDocument) and would like to ensure that the user only can have 1
> document open at a time. Think of this application as similar to Mail.app or
> Addressbook.app, etc. except that I would like the user to be able to open
> different persistent stores at will, just not simultaneously.
>
> (As an aside, the reason for this is because my document has many related
> windows and I think it would be confusing for the user. It would not be
> obvious which auxiliary window relates to which document. I may fix this in
> the future by using the concept of Inspectors, but at this point I'm not sure
> if this is the correct model.)
>
> I searched extensively and came up with the recommendation that I subclass
> NSDocumentController and override:
> - (id)openDocumentWithContentsOfURL:display:error:
> This was recommended over simply trapping the open menu item(s).
>
>
> This seemed very promising until I ran into a stumbling block. In
> openDocumentWithContentsOfURL:display:error: I check to see if there is an
> already open document and if so call:
> canCloseDocumentWithDelegate:shouldCloseSelector:contextInfo:
> with an appropriate delegate and selector. The problem is that this returns
> immediately, even in the case when the document is dirty and it presents a
> modal dialog to the user. Because of this, the 2nd document is opened before
> I have a chance to deal with the first one.
>
> All of this is making me wonder if I'm taking the wrong approach.
>
> Should I figure out a way to prevent the 2nd document from opening before the
> first one is either saved or abandoned: (override
> canCloseDocumentWithDelegate:shouldCloseSelector:contextInfo: in my
> NSPersistentDocument subclass)? This feels like the solution is getting
> uglier by the minute.
>
> Or is there a much easier way to accomplish what I want to do? Or should I be
> looking at a entirely different approach for my application?
>
> Thanks,
> Martin
>
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