Why does [SKAction scaleXTo: duration:] not invoke the xScale setter?

Question

I have a custom node that has an overridden xScale setter. E.g.:

-(void)setXScale:(CGFloat)xScale
{
    [super setXScale:xScale];
    NSLog(@"Scale changed to: %f", xScale);
}

If I set the xScale property manually, then I can see the setter being triggered.

However, if I scale the node via an action, e.g.:

[myCustomNode runAction:[SKAction scaleXTo:2.0f duration:0.5f]];

then even though the node gets correctly scaled, the setter is never invoked.

Does anybody know why not? How does the SKScale action change the node's scale?

Answer 1

Internally Sprite Kit uses a C++ render engine. My observation is that SKAction completely bypasses the Objective-C layer and instead will change properties directly on the C++ classes. Likely to improve the performance of actions.

If you want to observe such changes use a customActionWithDuration:actionBlock: action that sets the node's property via the block. Or you could subclass the node, add a previousScale property and periodically check if the original scale has changed by comparing it with the previousScale property.