Online search shows that others have reported receiving the same error message occasionally over the last few years. I have tried all of the proposed solutions whether they reportedly worked or not, without success. I have confirmed that I did not make the errors that others have reported in the past, such as failing to connect the dataSource outlet. In fact, I know that I did everything right, because my application was working yesterday.
Here's what I've tried so far. Is there anything else I should try?
1. I eyeballed the code intensively. This revealed no errors, and I see the required datasource methods right where they're supposed to be.
2. I looked at the Xcode menu item listing all methods, and the datasource methods show up, which confirms that they are implemented.
3. I moved the datasource methods to the top of the @implementation part to make sure I hadn't overlooked any intervening typing or cut-and-paste errors and rebuilt, but I get the same error.
4. I modified -awakeFromNib to call the datasource methods explicitly and reubilt. The breakpoints that I set within them are reached, so Xcode sees the datasource methods to run them.
5. In the nib file, I confirmed that the outline view's dataSource and delegate outlets are correctly connected to the file's owner, my view controller. I even disconnected and reconnected them and then rebuilt, but it makes no difference.
6. I confirmed that I had declared conformance to the datasource protocol. I even tried removing the declaration of conformance, but it makes no difference. (I also sent a -conformsToProtocol:@protocol(NSOutlineviewDataSource)] message to my controller and it reported YES, but that's only because I declared that it conforms in the @interface part.)
7. I declared the datasource methods in the @interface part even though that isn't required for datasource methods when conformance to the protocol is declared, but it makes no difference.
8. I removed the project's workspace index by removing the derived data in Xcode's Organizer and rebuilt the project to rebuild the index, but it makes no difference.
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