为什么自动生成的iOS Apple代码使用实例变量而不是属性？

Question

I did a lot of iOS development pre-ARC. Now with ARC I've noticed that Apple generated code is using instance variables instead of properties. Why is that?

For instance a master-detail view controller - on the DetailView we see this:

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

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

Why not self.detailItem = newDetailItem;? When I first learned iOS dev the use of properties was the recommended practice.

Answer 1

It would create a infinitive recursive loop. Since self.detailItem = obj; calls:

- (void)setDetailItem:(id)newDetailItem

Regarding the "_" it's how the variable is named when you don't syntersize it. You can change it if you want, although i see no reason to do that.

Answer 2

self.detailItem = newDetailItem; IS [self setDetailItem:newDetailItem];

So that would cause a stack overflow.

When you declare a property it already defines the getter and the setter for an instance variable (unless it's in a category). So if you have to customize either the getter or the setter then you need to access the instance variable directly.