Question

I see some examples sometimes would declare a property as well as variable other times they do not .e.g. some time I see code like this

@interface Test : NSObject
{
  UIProgressView* _progressView;
}
@property (nonatomic,retain)UIProgressView* progressView;

@end

at other times I will come across

@interface Test : NSObject

@property (nonatomic,retain)UIProgressView* progressView;

@end

Why what are the reasons ? I am learning and almost always use property and variable both.

I have used UIProgressView just as example.

Answer 1

It depends whether the property is synthesized against an iVar or derived in some other way (or against another iVar).

IF we have an instance of the class – i.e:

Test *myTest = [[Test alloc] init];

Then basically the property declaration

@property (nonatomic,retain)UIProgressView* progressView;

is telling anyone interested in using the interface that they can access the following two functions on an instance of this class:

[myTest progressBar];
[myTest setProgressBar:aProgressBar];

And objective C also lets you use shorthand notation:

myTest.progressBar =
xxx = myTest.progressBar

which does exactly the same thing.

It is not necessary for these two methods to be implemented via an iVar of the same name as the property, or even via an iVar at all (they could do even do a database fetch or derive the value).

If you @synthesize the property (which means you want the precompiler to generate the above methods for you) and don’t explicitly specify an iVar on the @synthesize directive, then the methods described above will automatically be generated (due to the synthesize method) to set or get the value to/from an iVar of the same name as the property (and the implementation will include retain/release logic depending on the property directive.

If you don’t @synthesize the property then you provide your own implementation and it can be anything you want. It is also possible to @synthesize the property but include a directive to use a different iVar in the method definition:

@synthesize progressBar=someOtheriVar;

in which case you will not see an iVar of the same name as the property in the header file either.