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# inputProperty

A particle property that provides input values for this property controller’s animation.

```
var inputProperty: SCNParticleSystem.ParticleProperty? { get set }
```

## Discussion

This property applies only when the controller’s [`inputMode`](/documentation/SceneKit/SCNParticlePropertyController/inputMode) value is [`SCNParticleInputMode.overOtherProperty`](/documentation/SceneKit/SCNParticleInputMode/overOtherProperty).

Use this option to animate one property in response to changes in one of each particle’s other properties. For example, the following code animates particles’ size as a function of their velocity, causing particles to become larger when they move faster:

```objc
CABasicAnimation *animation = [CABasicAnimation animation];
animation.fromValue = @0.1;
animation.toValue = @10.0;
 
SCNParticlePropertyController *sizeController =
    [SCNParticlePropertyController controllerWithAnimation:animation];
sizeController.inputMode = SCNParticleInputModeOverOtherProperty;
sizeController.inputProperty = SCNParticlePropertyVelocity;
sizeController.inputScale = 0.1;
 
particleSystem.propertyControllers = @{ SCNParticlePropertySize : sizeController };
```

To refine the relationship between a range of property values and a range of input values for the controller’s animation, use the [`inputBias`](/documentation/SceneKit/SCNParticlePropertyController/inputBias) and [`inputScale`](/documentation/SceneKit/SCNParticlePropertyController/inputScale) properties.

If you specify a vector property (such as acceleration) as the input property, SceneKit uses that vector’s length for the input value.

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