发布网点的最佳和最安全的做法是什么？

Question

The iOS documentation says that your view controllers should release (and nil, since that's good practice and actually a must for 2.x compatibility) any retained outlets in -dealloc.

The documentation also says that you should do the same for your outlets in -viewDidUnload if you want to allow them being cleaned up when the view is not visible and a memory warning was issued.

I wonder about two things:

1. Does this mean that I should duplicate all my outlet releasing code? Put it all in -dealloc and in -viewDidUnload? That's a real pain to maintain and really easy to forget!
2. Am I guaranteed that all my outlets will be re-populated when the view is reloaded after a -viewDidUnload, including my outlets that reference non-view objects in my nib?

The ideal answer answers both questions above and if there is a distinction between which properties should optimally be released in -dealloc VS -viewDidUnload, clarifies this distinction in detail.

Answer 1

-viewDidUnload is strictly used for releasing IBOutlets with retain properties.

The reason for this has to do with the fact that UIViewController has a view property which it retains. That view property itself retains references to all of its subviews. These subviews are exactly what you are retaining inside these outlet properties. The problem lies in that these subviews have an "extra" retain on them.

The goal of -viewDidUnload is to clear up unnecessary memory usage. When -viewDidUnload is called, the view property has already been released, which releases the top level UIView along with all its subviews. Since we have retained some of these subviews however, they linger in memory, and we want to release them since they will no longer be used. New copies of these subviews will be created when (if) the view is reloaded. The properties are also set to nil, strictly so we don't have pointers pointing to deallocated memory.

In -dealloc all retained properties and instance variables should be released. In the case where the -viewDidUnload just executed, you will be sending a harmless [nil release]; to the IBOutlet retained properties you just set to nil.

Answer 2

To add to the excellent answers:

With the Apple LLVM 3.0 compiler (and ARC enabled), the need for -dealloc disappears (or at least, the need to release your outlets in it), leaving just -viewDidUnload to deal with. As a result, no more duplicate code.

Answer 3

Nick answered the part 1 of the question perfectly so I skip to part 2 right away.

You are guaranteed, indeed! When your view gets reloaded from a nib file, everything on that nib file gets reloaded. There is not method to partially load a nib file's contents so if that nib gets reloaded, everything inside will get reloaded, too. But everything loaded from a nib has an autorelease on it. So you have to retain them. If you have Outlets which retains these freshly loaded classes and you connected those Outlets as outlets on nib, you are good to go. On a view controller example controller itself is the file's owner of that nib, view is the root element on nib, view being retained by file's owner outlet "view", all other child elements retained by view itself. If you have multiple root elements or elements that root view does not retain then you have to be sure there is outlets for this elements to retain them if nib loading by system automatically, if you are loading nib by a method then you must be aware all those classes are autoreleased when they arrived to your code.

Simply views are classes too and when a nib file gets loaded, everything inside it gets loaded, not some part of it. It is your responsibility to manage outlets.