# SCNParticleModifierBlock The signature for blocks called by SceneKit to modify particle properties on each frame of simulation, used by the [`addModifier(forProperties:at:modifier:)`](/documentation/SceneKit/SCNParticleSystem/addModifier(forProperties:at:modifier:)) method. ``` typealias SCNParticleModifierBlock = (UnsafeMutablePointer, UnsafeMutablePointer, Int, Int, Float) -> Void ``` ## Discussion The block takes the following parameters: - data: An array of floating-point values containing stripes of property data for the system’s particles. The width and format of each data stripe depend on the properties you specify when calling the [`addModifier(forProperties:at:modifier:)`](/documentation/SceneKit/SCNParticleSystem/addModifier(forProperties:at:modifier:)) method. - dataStride: An array identifying the offset, in bytes, of each property’s value in the data stripe for each particle. The order of offsets in this array corresponds to the order of the `properties` array you specify when calling the [`addModifier(forProperties:at:modifier:)`](/documentation/SceneKit/SCNParticleSystem/addModifier(forProperties:at:modifier:)) method. - start: The index of the first particle’s data stripe in the `data` array. - end: The index of the last particle’s data stripe in the `data` array. - deltaTime: The elapsed time, in seconds, since the last frame of simulation. Use this block to change properties of individual particles on each frame of simulation. > Important: > Running your own code to update particle properties every frame can have a severe impact on rendering performance. If the behavior over time that you want for your particle system can be described more declaratively, use the ``doc://com.apple.scenekit/documentation/SceneKit/SCNParticleSystem/propertyControllers`` property and ``doc://com.apple.scenekit/documentation/SceneKit/SCNParticlePropertyController`` class instead. If you need to change particle properties only at certain times (rather than continuously), add a handler block for an event using the ``doc://com.apple.scenekit/documentation/SceneKit/SCNParticleSystem/handle(_:forProperties:handler:)`` method. The following example illustrates setting up a modifier block that alters particle’s position and velocity: ```objc [system addModifierForProperties:@[SCNParticlePropertyPosition, SCNParticlePropertyVelocity] atStage:SCNParticleModifierStagePostDynamics withBlock:^(void **data, size_t *dataStride, NSInteger start, NSInteger end, float deltaTime) { // For each particle to be processed, // calculate pointers in the data to each property's value: for (NSInteger i = start; i < end; ++i) { // SCNParticlePropertyPosition (float3) float *pos = (float *)((char *)data[0] + dataStride[0] * i); // pos[0..2] are the xyz components of the particle's position. // SCNParticlePropertyVelocity (float3) float *vel = (float *)((char *)data[1] + dataStride[1] * i); // vel[0..2] are the xyz components of the particle's position. // Now, compute a new position and velocity (not shown). } }]; ``` --- Copyright © 2026 Apple Inc. All rights reserved. | [Terms of Use](https://www.apple.com/legal/internet-services/terms/site.html) | [Privacy Policy](https://www.apple.com/privacy/privacy-policy)