The relative mass density of the behavior’s dynamic items.
- iOS 7.0+
- tvOS 9.0+
A dynamic item’s relative density, along with its size, determines its effective mass when it participates in UIKit Dynamics behaviors—including friction, collisions, pushes, and so on. For example, say you have two dynamic items with the same density but different sizes: item one is
100 x 100 points and item two is
100 x 200 points. In this example, item two has twice the effective mass of item one. In an elastic collision, these items exhibit a natural conservation of momentum according to their relative masses.
100 x 100 point dynamic item with a density of
1, to which you apply a force (via a push behavior) of magnitude
1, accelerates at
100 points per second².