The self.username will call the setter of the username that’s why the breakpoint jumps to the synthesize statement.
When you a _variable, then the property can be accessed using the _variable.
And in your case:
self.username stores the value to ivar _username and _username = @"admin"; is also stores the value to _username ivar. Means both saves to same ivar _username.
Listing 4-2 Using @synthesize
You can use the form property=ivar to indicate that a particular
instance variable should be used for the property, for example:@synthesize firstName, lastName, age=yearsOld;
This specifies that the accessor methods for firstName, lastName, and
age should be synthesized and that the property age is represented by
the instance variable yearsOld. Other aspects of the synthesized
methods are determined by the optional attributes
Please refer http://developer.apple.com/library/ios/#documentation/cocoa/conceptual/objectivec/chapters/ocproperties.html
solved in the ViewController.m, what’s the difference between self.username with _username [duplicate]