Swift UIKit dynamics - Vary UIPushBehavior force magnitude by distance from center of source

Question

Can someone suggest how I can vary the UIPushBehavior magnitude force by distance from source. To mirror the affect of the force of wind from a fan on another object. So the closer the object is to the fan the stronger the force.

if String(describing: identifier) == "fan1" {
                self.push = UIPushBehavior(items: [self.object], mode: .instantaneous)
                self.push.setAngle(.pi/4, magnitude: 1)
                self.animator.addBehavior(self.push)
                self.push.addItem(self.object)
                
            }

Answer 1

Use a UIFieldBehaviour to create a linear gravitational field in the direction of the fan. Then you can specify a falloff:

var forceField: UIFieldBehaviour!

// ...

forceField = UIFieldBehavior.linearGravityField(direction: CGVector(dx: 1, dy: -5))

// example values for a "fan" on the bottom left blowing mostly upwards:
forceField.position = CGPoint(x: view.bounds.minX, y: view.bounds.maxY)
forceField.region = UIRegion(radius: 3000)
forceField.minimumRadius = 100
forceField.falloff = 5
forceField.strength = 10
forceField.addItem(view1)
forceField.addItem(view2)
animator.addBehavior(forceField)

Have fun playing around with these values!

Adding collision, another gravity behaviour, and a dynamic item behaviour to two views, we get the following effect:

That feels like a fan on the bottom left to me!

You can also choose a radial gravitational field positioned at where the fan is, if your fan is in a corner and blows "radially", but note that you should use a negative value for strength in that case to say that the field repels rather than attracts.