In a piano app, I'm assigning the coordinates of the black keys. Here is the line of code causing the error.
'blackKey' and 'whiteKey' are both customViews
blackKey.center.x = (whiteKey.frame.origin.x + whiteKey.frame.size.width);
The other answers don't exactly explain what's going on here, so this is the basic problem:
When you write blackKey.center.x
, the blackKey.center
and center.x
both look like struct member accesses, but they're actually completely different things. blackKey.center
is a property access, which desugars to something like [blackKey center]
, which in turn desugars to something like objc_msgSend(blackKey, @selector(center))
. You can't modify the return value of a function, like objc_msgSend(blackKey, @selector(center)).x = 2
— it just isn't meaningful, because the return value isn't stored anywhere meaningful.
So if you want to modify the struct, you have to store the return value of the property in a variable, modify the variable, and then set the property to the new value.