Detect when SKNode is added as child - objective-c

Question

I have a Polygon class that is used as data container, but for debugging purposes I want to draw polygons by adding them as children to an SKNode. When I do this I want the Polygon object to add border sprite children to itself. Because of performance reasons I only want to add those sprites when the Polygon has been added as child to another node.
Is there any way that the Polygon object itself can detect that it has been added to the scene, or do I need to tell it by making an extra createSprites call after it has been added to the scene?
I guess I can poll the parent attribute, but I'm looking for something event driven.

Answer 1

In Swift, you can define property observers for the properties of the class yours inherits from.
You could observe changes in the parent property of your custom SKNode subclass, like this:
class MyNode : SKNode {
override var parent: SKNode? {
didSet {
// parent node changed; do something
}
}
}
EDIT: Like I mentioned in the comments, in Objective-C (where you can not use property observers) you can instead use Key-Value Observing (KVO), and observe changes in SKNode's parent property:
Actual code:
- (instancetype) init
{
if (self = [super init]){
// REGISTER for KVO
[self addObserver:self
forKeyPath:#"parent"
options:NSKeyValueObservingOptionNew
context:NULL];
}
return self;
}
- (void) dealloc
{
// UNREGISTER from KVO
[self removeObserver:self forKeyPath:#"parent" context:NULL];
}
// Method that is called when any keyPath you subscribed to is modified
- (void)observeValueForKeyPath:(NSString *)keyPath
ofObject:(id)object
change:(NSDictionary *)change
context:(void *)context
{
// (Check keypath in case you are observing several different
// properties. Otherwise, remove if statement:)
if ([keyPath isEqualToString:#"parent"]) {
// parent node changed; do something
}
}

Answer 2

In Objective-C you could do it like this:
[node addObserver:self forKeyPath:#"parent" options:NSKeyValueChangeOldKey context:nil];
- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context {
//logic here
}
Or in this case simply subclass SKNode and override setParent: