Gotcha with Obj-C properties
Properties in Objective-C can be quite useful sometimes, but you have to be careful because they don’t always work how you think they should. Here is a common gotcha among people new to Obj-C and properties.
Consider we have a class called Sprite, which has a property called position.
@interface Sprite : NSObject {
CGPoint position;
}@property (readwrite) CGPoint position;
@end
CGPoint is a struct with 2 int variables, x and y.
Lets create one:
Sprite mySprite* = [[Sprite alloc] init];
Now consider the situation of getting and setting the position.
Getting
When you call this:
CGPoint pos = mySprite.position;
It is actually the same as:
CGPoint pos = [mySprite position];
Here its clear that pos is a copy of the position (its not a reference
to the instance variable).
Setting
When you can call:
mySprite.position = pos;
It is actually the same as:
[mySprite setPosition:pos];
OK this is probably fairly clear so far!
The Gotcha
Now this is where the issue happens.
When you call this:
mySprite.position.y = 99;
It is actually the same as this:
[mySprite position].y = 99;
In this instance, the position variable of mySprite is not changed at all.
So when using properties, try to be aware of what Objective-C is actually doing behind the scenes :)
Posted: November 1st, 2008 under Mac, Objective C, iPhone.
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