# Single/Periodic Burst

Menu Path:

- **Spawn > Single Burst**
- **Spawn > Periodic Burst**

The Single/Periodic Burst Block spawns a number of particles instantly either once, or periodically using a delay. When this Block triggers a burst of particles to spawn, it increments the [spawnCount](https://docs.unity3d.com/Documentation/ScriptReference/VFX.VFXSpawnerState-spawnCount.html) instantly. If you change the **Repeat** setting to **Periodic**, this Block changes its name to **Periodic Burst** and spawns bursts of particles after a delay.

## Block compatibility

This Block is compatible with the following Contexts:

- [Spawn](Context-Spawn.md)

## Block settings

| **Setting**    | **Type** | **Description**                                              |
| -------------- | -------- | ------------------------------------------------------------ |
| **Repeat**     | Enum     | **(Inspector)** The mode this Block uses to determine whether to trigger the birth only once or repeat it after a delay. The options are:<br/>&#8226; **Single**: The Block spawns a single burst of particles.<br/>&#8226; **Periodic**: The Block spawns multiple bursts of particles. After it spawns a burst of particles, it waits for the time you specify in **Delay** and spawns another burst. |
| **Spawn Mode** | Enum     | The mode this Block uses to determine how many particles to spawn. The options are:<br/>&#8226; **Constant**: This Block spawns a specific number of particles. You can set this number in the **Count** property.<br/>&#8226; **Random**: This Block spawns a random number of particles between two boundaries. If you select this mode, the **Count** property becomes a Vector 2 where the x-axis represents one boundary value and the y-axis represents the other boundary value. |
| **Delay Mode** | Enum     | The mode this Block uses to determine how to calculate a delay time for use between bursts. The options are:<br/>&#8226; **Constant**: The delay time is of a specific duration. You can set the duration in the **Delay** property.<br/>&#8226; **Random**: The delay time is of a random duration between two boundaries. If you select this mode, the **Delay** property becomes a Vector 2 where the x-axis represents one boundary value and the y-axis represents the other boundary value. |

## Block properties

| **Input** | **Type**      | **Description**                                              |
| --------- | ------------- | ------------------------------------------------------------ |
| **Count** | float/Vector2 | The number of particles to spawn.<br/>If you set **Spawn Mode** to **Constant**, this property is a float type and the value you set is the specific number of particles to spawn.<br/>If you set **Spawn Mode** to **Random**, this property is a Vector 2 where the x-axis represents one boundary value and the y-axis represents the other boundary value. When spawning a burst of particles, this Block uses a random number between these two values to determine how many particles to spawn. |
| **Delay** | float/Vector2 | The duration of the delay this Block waits for before it spawns a burst of particles.<br/>If you set **Delay Mode** to **Constant**, this property is a float type and the value you set is the specific delay duration this Block waits for between particle spawns.<br/>If you set **Delay Mode** to **Random**, this property is a Vector 2 where the x-axis represents one boundary value and the y-axis represents the other boundary value. When the Block initiates a delay, it uses a random number between these two values to determine the delay duration. |

## Remarks
You can emulate the single burst behavior with the following equivalent custom spawner callback implementation:
```C#
class SingleBurstEquivalent : VFXSpawnerCallbacks
{
    public class InputProperties
    {
        public float Count = 1.0f;
        public float Delay = 2.0f;
    }

    static private readonly int countID = Shader.PropertyToID("Count");
    static private readonly int delayID = Shader.PropertyToID("Delay");

    public sealed override void OnPlay(VFXSpawnerState state, VFXExpressionValues vfxValues, VisualEffect vfxComponent)
    {
    }

    private float m_NextBurstTime;
    private bool m_Sleeping;

    public sealed override void OnUpdate(VFXSpawnerState state, VFXExpressionValues vfxValues, VisualEffect vfxComponent)
    {
        if (state.newLoop)
        {
            m_NextBurstTime = vfxValues.GetFloat(delayID);
            m_Sleeping = false;
        }

        if (!m_Sleeping && state.playing && state.totalTime >= m_NextBurstTime)
        {
            state.spawnCount += vfxValues.GetFloat(countID);
            m_Sleeping = true;
        }
    }

    public sealed override void OnStop(VFXSpawnerState state, VFXExpressionValues vfxValues, VisualEffect vfxComponent)
    {
    }
}
```

You can emulate the periodic burst behavior with the following equivalent custom spawner callback implementation:
```C#
class PeriodicBurstEquivalent : VFXSpawnerCallbacks
{
    public class InputProperties
    {
        public float Count = 1.0f;
        public float Delay = 2.0f;
    }

    static private readonly int countID = Shader.PropertyToID("Count");
    static private readonly int delayID = Shader.PropertyToID("Delay");

    public sealed override void OnPlay(VFXSpawnerState state, VFXExpressionValues vfxValues, VisualEffect vfxComponent)
    {
    }

    private float m_NextBurstTime;

    public sealed override void OnUpdate(VFXSpawnerState state, VFXExpressionValues vfxValues, VisualEffect vfxComponent)
    {
        if (state.newLoop)
        {
            m_NextBurstTime = vfxValues.GetFloat(delayID);
        }

        if (state.playing && state.totalTime >= m_NextBurstTime)
        {
            m_NextBurstTime += vfxValues.GetFloat(delayID);
            state.spawnCount += vfxValues.GetFloat(countID);
        }
    }

    public sealed override void OnStop(VFXSpawnerState state, VFXExpressionValues vfxValues, VisualEffect vfxComponent)
    {
    }
}
```