Question

I saw a sample code with this code:

in the .h file:

  CALayer *_animationLayer;
  @property (nonatomic, retain) CALayer *animationLayer;

And in the .m file:

  @synthesize animationLayer = _animationLayer;

I am guessing it is related to the retain count?

Can someone explain that?

Answer 1

The code in the .h file is declaring two things, a variable called _animationLayer which is of type CALayer*, and a property called animationLayer which is also of type CALayer*. The code in the .m file is instructing the objective-c compiler to automatically generate getters and setters for the animationLayer property, using the _animationLayer variable to hold the actual value that is set.

So for instance:

_animationLayer = nil;  //initialize the variable to nil
self.animationLayer = [[CALayer alloc] init];  //assign a value to the property
NSLog("_animationLayer is:  %@", _animationLayer);  //it's not set to nil anymore
_animationLayer = nil;  //set it to nil again
NSLog("self.animationLayer is:  %@", self.animationLayer);  //now the property is nil

And you’re correct, this does have some relationship to the object’s retainCount (so it’s not quite correct to think of the variable as a direct alias for the property). In essence, setting the value directly using the _animationLayer variable does not retain the new value or release the old value. Setting the value using the property accessor does. For example:

_animationLayer = [[CALayer alloc] init];  //retain count is 1
self.animationLayer = nil;                 //retain count is 0

self.animationLayer = [[CALayer alloc] init];  //oops, retain count is 2
self.animationLayer = nil;  //retain count is 1, the object leaked

_animationLayer = [[CALayer alloc] init];  //retain count is 1
[_animationLayer release];               //retain count is 0
self.animationLayer = nil;               //oops, just released it again