I am developing a game using iOS SpriteKit. I am trying to make an object in this game that will pull things towards it and the force will get greater as objects come closer to it, think of a magnet or a black hole. I've been having a difficult time figuring out what properties to change to get this nodes physicsBody to attract other nodes as they pass by.
In iOS 8 and OS X 10.10, SpriteKit has SKFieldNode for creating forces that apply to bodies in an area. This is great for things like buoyancy, area-specific gravity, and "magnets".
Watch out, though — the magneticField you get from that class is probably not what you want for the kind of "magnets" gameplay you might be looking for. A magnetic field behaves as per real-world physics at the micro level... that is, it deflects moving, charged bodies. What we usually think of as magnets — the kind that stick to your fridge, pick up junked cars, or make a hoverboard fly — is a higher-level effect of that force.
If you want a field that just attracts anything (or some specific things) nearby, a radialGravityField is what you're really after. (To attract only specific things, use the categoryBitMask on the field and the fieldBitMask on the bodies it should/shouldn't interact with.)
If you want a field that attracts different things more or less strongly, or attracts some things and repels others, the electricField is a good choice. You can use the charge property of physics bodies to make them attracted or repelled (negative or positive values) or more or less strongly affected (greater or less absolute value) by the field.
Prior to iOS 8 & OS X 10.10, SpriteKit's physics simulation doesn't include such kinds of force.
That doesn't keep you from simulating it yourself, though. In your scene's update: method you can find the distances between bodies, calculate a force on each proportional to that distance (and to whatever strength of magnetic field you're simulating), and apply forces to each body.