myView.frame.origin.x = value; does not work - But why?

Question

I know that I can't use this:

myView.frame.origin.x = 25.0;

and that I have to use this instead:

CGRect myFrame = myView.frame;
myFrame.origin.x = 25.0;
myView.frame = myFrame;

And I'm doing it all the time, but I don't know why I must do it that way. I would like to fill that gap in my understanding. Can someone explain ?

Nowadays Xcode gives you "Expression not assignable". Some time ago you got a compile error "Lvalue required as left operand of assignment".

Answer 1

There are two distinct dot syntaxes being used here. They look the same, but they do different things depending on what they are operating on and what is being done with it:

• The first myView.frame is a shorthand for [myView frame], a method call that returns a CGRect struct by value.
• myFrame.origin.x is accessing ordinary struct members in the traditional C fashion.
• The second myView.frame is again a shorthand, but because the statement is an assignment it translates to calling a different method, [myView setFrame:myFrame].

In your single-line top example, you get a copy of the rect and set its x, but never copy it back to the view. You have to explicitly differentiate between the method calls, the dot syntax sugar can't magic them into a single call.