# Step sequencer

> A grid of steps like FL Studio's channel rack for Python — click or paint steps, rows with names, beat grouping and a moving playhead.

Source: https://albakore.github.io/pyzed/audio/components/step-sequencer/

(Image: A five-row drum pattern with the playhead running across sixteen steps)

**In plain words:**

Each row is an instrument and each square a moment in the bar. Lit squares play, dark ones
don't. Click a square to switch it, or drag across a row to paint several at once.

```python
class Beat(pz.App):
    pattern = ["x...x...x...x...", "....x.......x...", "x.x.x.x.x.x.x.x."]
    step = 0

    def view(self):
        return ui.audio.StepSequencer(
            pattern=self.pattern, labels=["Kick", "Snare", "Hat"], current=self.step,
            on_change=lambda e: setattr(self, "pattern", e.value),
        )
```

## Patterns

`pattern` is a list of rows. Each row is either a list (`True`/`False`, or a velocity from 0
to 1 that sets how bright the step is) or text where `x` (or `1`) is a step that plays and any
other character is a rest: `"x...x...x...x..."` is a kick on every beat.

`change` gives the whole new pattern as a list of lists, ready to store. A drag starting on an
empty step paints steps on; one starting on a lit step erases.

## Playhead

`current` is the step being played: the sequencer frames that column and outlines its steps.
Drive it from a timer (`pz.every`) or from the engine's sequencer (`node.state["step"]`). Steps
are shaded in groups of `group` (4 by default), so beats are easy to count.

| Property | Default | What it does |
| --- | --- | --- |
| `pattern` | `()` | Rows of steps. |
| `labels` | `()` | A name per row (shown on the left). |
| `current` | `-1` | The step being played (`-1` for none). |
| `color` | `"accent"` | Color of the steps that play. |
| `group` | `4` | Steps per shaded group. |
| `cell` | `22` | Size of a step in pixels. |

Event: `on_change` with the new pattern.

**Tip:**

The [drum machine demo](https://albakore.github.io/pyzed/audio/demos/drum-machine/) joins a step sequencer, a row of pads and the engine's
`drum` module: the pads light with `lit=` as the steps play.