@synthesize IBOutlet property

I'm an Objective-C newbie and 开发者_开发问答I'm reading "iPhone programming" by Alasdair Allan. While reading, I found this code:

@interface RootController : UIViewController <UITableViewDataSource, UITableViewDelegate> {
    UITableView *tableView;
    NSMutableArray *cities;
}
// warning: remember this tableView
@property (nonatomic, retain) IBOutlet UITableView *tableView;

The relative implementation starts this way:

@implementation RootController
@synthesize tableView;

Now: I learnt that @synthesize is a sort of shortcut to avoid boring getters and setters.

But I've some question:

1. in the code of the implementation tableView is never explicitly called but the dealloc releases it;
2. if it never gets called explicitly why the @synthesize?

Is it mandatory for IBOutlets to be synthesized?

From Memory Management of Nib Objects,

When a nib file is loaded and outlets established, the nib-loading mechanism always uses accessor methods if they are present (on both Mac OS X and iOS). Therefore, whichever platform you develop for, you should typically declare outlets using the Objective-C declared properties feature.

For iOS, you should use:

@property (nonatomic, retain) IBOutlet UIUserInterfaceElementClass *anOutlet;

You should then either synthesize the corresponding accessor methods, or implement them according to the declaration, and (in iOS) release the corresponding variable in dealloc.

in the code of the implementation tableView is never explicitly called but the dealloc releases it;

That is because when you do assign a value to the tableView, your controller retains it, and it will need to release it when it gets dealloc'd. Don't forget, @properties declared in an interface are publicly accessible. In your case specifically, the tableView you're declaring as IBOutlet is initialized by the view controller loadView method using the connections you define in Interface Builder between the File's Owner and the UITableView.

if it never gets called explicitly why the @synthesize?

You need to provide accessors for all declared @properties. They can be @synthesized, or you could write your own.

Is it mandatory for IBOutlets to be synthesized?

No, but it's way more convenient that way. The rule enforced by the compiler is that @properties must have corresponding accessors (synthesized or not) in the implementation.

For reference: From Xcode 4.4 and LLVM Compiler 4.0 on the @synthesize directive is no longer required as it will be provided by default for @properties defined in the interface.

If you type

@property (nonatomic, retain) IBOutlet UITableView *tableView;

you tell the compiler: "Listen, there will be a getter and a setter. If appropriate, use them!" And it will use them when loading the nib.

Therefore you have to implement the getter and the setter otherwise the compiler will complain.

The IBoutlet pseudo-type is just a marker so that the InterfaceBuilder "knows" that the mentioned class-file has a handle/outlet to the UITableView instance.

When compiling IBOutlet is being removed by the preprocessor (InterfaceBuilder parses (looks at) the source files). It's similar with IBAction: it is being replaced with void by the preprocessor.

That said, you could use the reference to said instance to do stuff programmatically (Like adding/changing values of the UITableView)