Question

I’m was playing around with the standard sample split view that gets created when you select a split view application in Xcode, and after adding a few fields i needed to add a few fields to display them in the detail view.

and something interesting happend
in the original sample, the master view sets a “detailItem” property in the detail view and the detail view displays it.

- (void)setDetailItem:(id) newDetailItem
{
if (_detailItem != newDetailItem) {
    _detailItem = newDetailItem;

    // Update the view.
    [self configureView];
}

i understand what that does and all, so while i was playing around with it. i thought it would be the same if instead of _detailItem i used self.detailItem, since it’s a property of the class.

however, when i used

self.detailItem != newDetailItem

i actually got stuck in a loop where this method is constantly called and i cant do anything else in the simulator.

my question is, whats the actual difference between the underscore variables(ivar?) and the properties?
i read some posts here it seems to be just some objective C convention, but it actually made some difference.

Answer 1

In the course of your experiment, you’ve set up an endless loop which is why the simulator goes non-responsive.

Calling self.detailItem within the scope of setDetailItem: calls setDetailItem: recursively since your class implements a custom setter method for the property detailItem.

I would refer you to the Apple documentation on declared properties for the scoop on properties, ivars, etc; but briefly, declared properties are a simplified way of providing accessor methods for your class. Rather than having to write your own accessor methods (as we had to do before Objective-C 2.0) they are now generated for you through the property syntax.