How Apple manage @private var and @properties?

I take for example the UIButton interface.

Here the first rows of @private definition :

  @private
    CFMutableDictionaryRef _contentLookup;
    UIEdgeInsets           _contentEdgeInsets;
    UIEdgeInsets           _titleEdgeInsets;

And here 2 of these ivar, that are defined as properties:

@property(nonatomic) UIEdgeInsets contentEdgeInsets; 
@property(nonatomic) UIEdgeInsetstitleEdgeIns开发者_如何转开发ets;

However these 2 properties are not defined on the ivars i found in private method (which have suffix _).

I'm not sure to understand how could be implemented setter and getter for these 2 properties to refer to the private ivars.

And a second question... i used to create properties for ivar, thus, if i have an ivar FOO i can create a @property for FOO. Is it a normal behavior create property for a non existing ivar ? (in this case contentEdgeInsets is not an attribute for this class... on the contrary _contentEdgeInset is defined in @interface and this's a valid ivar). Ok what i missed with this argument ?

When you @synthesize these properties you do so like

@synthesize contentEdgeInsets = _contentEdgeInsets;
            ^property name      ^iVar name

Check out the Property Implementation Directives section in the documentation.

By default, a property will use the ivar whose name is the same as that of the property, but it's also possible to specify an ivar of a different name. Do this in your @synthesize statement in the class implementation.

In the modern runtime, used pretty much everywhere at this point, you don't actually have to declare the ivar at all -- if you synthesize accessors for a property and there's no matching ivar, the runtime will provide one.

Finally, properties with @dynamic rather than @synthesized accessors don't necessarily need an ivar at all -- you're providing the accessors in this case, so you're free to derive the value of the property however you like.