Bizarre Animation with UIDynamics After Delay

Question

The Details:

• I am using DispatchQueue.main.asyncAfter(deadline: ... to update the snapPoint of a UISnapBehavior, which causes the view associated with the snap behavior to snap (with animation) to the new snap point.
• I want to update the snapPoint of three circular views each of which are stacked one on top of the other.
• I want to update the snapPoint of each consecutively. In other words, I want view A to start animating into position at t0, view B to start animating at t1, and view C to start animating at t2.

The Problem:

The timing of the animations is bizarre. For example:

• If the difference between t0, t1, and t2 is 100 milliseconds, they all snap to their respective snap points at exactly the same time.
• However, if the difference between t0, t1, and t2 is 200 milliseconds, then view A and view B snap to their respective positions at the same time, but view C snaps to its new snap point at a totally different time later than the first two.
• Even more bizarre, if the difference between t0, t1, and t2 reach 400 milliseconds, then view A animates on its own, and views B and C animate together.
• Finally, if the difference reaches 500 milliseconds, each view animates at a different time.

What is going on here?

Answer 1

Using Timer (Swift) or NSTimer (Objective-C) instead of GCD (Grand Central Dispatch) is allowing me to make these nuanced delays in animations with expected behavior.

However, I do not know why this works. I greatly welcome explanations as to what is happening under the hood or if it is possible to achieve the same effect with GCD.