Possible to retrieve the parameters used to create an SKPhysicsBody object? - ios

Question

Is it possible to retrieve the parameters used to create an SKPhysicsBody object without keeping them around separately? In other words, is there a way to get the body type (i.e. circle, rectangle, polygon) and related information (i.e. radius, size, path) from the following objects after creation:
SKPhysicsBody *circleBody = [SKPhysicsBody bodyWithCircleOfRadius:100.0];
SKPhysicsBody *rectangleBody = [SKPhysicsBody bodyWithRectangleOfSize:CGSizeMake(100.0, 100.0)];
There don't appear to be any properties or methods in the SKPhysicsNode class reference along the lines of:
SKPhysicsBodyType type = circleBody.bodyType; // Doesn't exist
CGFloat radius = circleBody.radius; // Doesn't exist
I'm surprised that there is information you can pass to a SKPhysicsBody object at creation that isn't at least available through a read-only property later on. Any ideas?

Answer 1

One way you can do this is by analyzing the description string. For example logging a circle body prints this:
<SKPhysicsBody> type:<Circle> representedObject:[(null)]
A regular expression search will do the job, provided that the description string is consistent across shape types and future Sprite Kit versions. Potentially brittle solution, but legal.
The only other way is to use the ObjC runtime to read from properties or ivars. Though this may constitute a use of private APIs and may get the app rejected if you're doing this on a live app.
This code logs all of the private PKPhysicsBody class' properties and ivars.
SKPhysicsBody* circle = [SKPhysicsBody bodyWithCircleOfRadius:10];
NSLog(#"%#", circle);
NSLog(#"%#", NSStringFromClass([circle class])); // log the 'true' class name
unsigned int num;
objc_property_t* properties = class_copyPropertyList(NSClassFromString(#"PKPhysicsBody"), &num);
for (unsigned i = 0; i < num; i++)
{
objc_property_t property = properties[i];
NSLog(#"property: %s", property_getName(property));
}
Ivar* ivars = class_copyIvarList(NSClassFromString(#"PKPhysicsBody"), &num);
for (unsigned i = 0; i < num; i++)
{
Ivar ivar = ivars[i];
NSLog(#"ivar: %s", ivar_getName(ivar));
}
From this it looks like the _shapeType ivar will give you what you're looking for.

Answer 2

I've found a simple way of tracking this information, assuming every body gets bound statically to a single node (as is the case in my application):
SKPhysicsBody *body = [SKPhysicsBody bodyWithCircleOfRadius:10.0];
node.physicsBody = body;
node.userData = [NSMutableDictionary dictionary];
[node.userData setObject:#"circle" forKey:#"shapeType");
[node.userData setObject:[NSNumber numberWithFloat:10.0] forKey:#"radius"];