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

The reason this does not work is due to the mixing of two syntaxes.
First you have “.” as a shortcut for calling the accessor functions (an Objective-C feature).
So

• a.b becomes [a getB];
• a.b = 5 becomes [a setB:5];

And then theres “.” as direct struct member access (pure C). So

• a.b really is a.b;
• a.b really is a.b = 5;

Combining this in a set-value-case like this, doesn’t work.
Because …
If you could call

myView.frame.origin.x = 25.0;

• The “myView.frame” part equals [myView getFrame] and you get a copied CGRect frame (a C struct)

• The “myView.frame.origin” gives you a CGPoint origin (also a struct) of the copied CGRect

• The “myView.frame.origin.x = 25.0” gives you a CGFloat x of the origin and now you want to assign something to it and here comes the problem…

You try to set a variable of a struct of a struct, which is ok, but there is no pointer from the UIView to the struct, so it is copied instead. So you copy and then you set and then you expect that the set action is somehow forwarded through the initial get to the UIView, well and this just doesn’t work.

Of course you could wonder why Apple hasn’t just created a shortcut, so that in the end your copied frame is automatically reinjected into a auto appended setFrame call, I guess you just have to live with how it is.

So remember, it would work if you’d get a pointer to the frame, but you don’t, you get a copied struct instead.
So if you expect myView.frame.origin.x = 25.0 to work you indirectly expect your call to be automagically translated into some sort of
[myView setFrame:[myView getFrame:frame].origin.x = 25.0].
Well I guess you can admit that this looks wrong.

Also imagine if you’d get a direct pointer to the CGRect frame and you’d change something through that pointer, how would the UIView know that it’s size has changed and that it has to update itself ? If on the other hand a [myView setFrame:newFrame] call is made, then UIView can do all the necessary readjusting itself.