Objective-C和iOS开发

Question

I just started developing iOS using objective-c, and I've had some trouble getting used to the syntax. I'm finally beginning to understand the semantics- even the reason for them, but one thing is puzzling me.

When defining the outlets and actions for an iOS app: why are the outlets defined within the interface declaration, versus the actions and other declarations defined outside the interface declaration?

Answer 1

Because outlets have no implementation. They are just plain old instance variables.

Actions, on the other hand, are methods (object functions) and they need to be implemented.

---

Methods are declared actually, but outside the curly brackets, while ivars happen to be declared inside the curly brackets. Everything between @interface and @end is in the interface.

---

@interface MyClass {
   instance variables here. outlets are also instance variables
}

methods here. actions are also methods
@end

There is no reason why . This simply is the syntax. :)

Answer 2

IBOutlet and IBAction are not data-types; they're just for Interface Builder to know which variables are outlets and which methods are actions.

So, when you write this:

@interface AClass
{
    IBOutlet UIButton *someButton;
}

- (IBAction) buttonTap:(id)sender;

@end

It actually ends up being something like this when compiled:

@interface AClass
{
   UIButton *someButton;
}

- (void) buttonTap:(id)sender;

@end

Answer 3

You actually have the option of defining the outlets in a property declaration if you're more comfortable:

@interface SomeViewController : UIViewController 
{
    UITextField*    aTextField;

}
@property (nonatomic, retain) IBOutlet UITextField* aTextField;

@end

IBAction and IBOutlet are really just clues to interface builder, here are the actual definitions:

#ifndef IBOutlet
#define IBOutlet
#endif

#ifndef IBOutletCollection
#define IBOutletCollection(ClassName)
#endif

#ifndef IBAction
#define IBAction void
#endif