> ## Documentation Index
> Fetch the complete documentation index at: https://docs.metabind.ai/llms.txt
> Use this file to discover all available pages before exploring further.

# onChange

> Runs an action when a watched value changes.

export const PlatformStatuses = ({statuses}) => {
  const StatusBadge = ({status, label}) => {
    const styles = {
      green: {
        backgroundColor: '#dcfce7',
        color: '#166534'
      },
      orange: {
        backgroundColor: '#fed7aa',
        color: '#9a3412'
      },
      red: {
        backgroundColor: '#fecaca',
        color: '#991b1b'
      },
      gray: {
        backgroundColor: '#f3f4f6',
        color: '#4b5563'
      }
    };
    const baseStyle = {
      display: 'inline-flex',
      alignItems: 'center',
      padding: '0.125rem 0.625rem',
      borderRadius: '9999px',
      fontSize: '0.875rem',
      fontWeight: '500'
    };
    const colorStyle = styles[status] || styles.green;
    return <span style={{
      ...baseStyle,
      ...colorStyle
    }}>
        {label || status}
      </span>;
  };
  const STATUS_CONFIG = {
    supported: {
      label: "Supported",
      color: "green"
    },
    partial: {
      label: "Partial",
      color: "orange"
    },
    "not-implemented": {
      label: "Not Implemented",
      color: "gray"
    }
  };
  const renderCard = (platform, value) => {
    if (!value) return null;
    const {status, note} = typeof value === "string" ? {
      status: value
    } : value;
    const config = STATUS_CONFIG[status];
    if (!config) return null;
    const titleMap = {
      ios: "SwiftUI",
      android: "Jetpack Compose",
      web: "Web"
    };
    return <Card key={platform} title={titleMap[platform] || platform}>
          <StatusBadge status={config.color} label={config.label} />
          {note && <div style={{
      marginTop: '0.5rem',
      fontSize: '0.875rem',
      color: '#6b7280'
    }}>
              {note}
            </div>}
      </Card>;
  };
  if (statuses == null) {
    return null;
  }
  return <Columns cols="3">
      {Object.entries(statuses).map(([platform, value]) => renderCard(platform, value))}
    </Columns>;
};

export const ComposeJS = ({code, name, height}) => {
  const encodedCode = useMemo(() => {
    if (!code) return "";
    try {
      return btoa(code);
    } catch (e) {
      console.error("Failed to encode code", e);
      return "";
    }
  }, [code]);
  if (!encodedCode) {
    return null;
  }
  return <iframe src={`https://www.metabind.ai/embed?code=${encodedCode}&name=${name ?? 'Example'}`} loading="lazy" style={{
    width: "100%",
    height: height || '350px',
    border: "1px solid #e5e7eb",
    borderRadius: "var(--rounded-2xl,1rem)",
    overflow: "hidden"
  }} title="ComposeJS Preview" />;
};

```typescript theme={null}
.onChange<V>(value: V, action: (values: [V, V]) => void): Component
```

<ParamField path="value" type="V" required>
  The value to observe for changes.
</ParamField>

<ParamField path="action" type="(values: [V, V]) => void" required>
  A callback that receives a tuple of `[newValue, oldValue]` when the watched value changes.
</ParamField>

## Support

<PlatformStatuses
  statuses={{
ios: { status: "supported" },
android: "not-implemented",
web: { status: "supported" },
}}
/>

## Usage

### Watching a state value

```typescript theme={null}
const body = () => {
    const [name, setName] = useState("")

    return TextField({ text: name, setText: setName })
        .onChange(name, ([newVal, oldVal]) => {
            console.log("Changed from " + oldVal + " to " + newVal)
        })
}
```

### Reacting to selection changes

```typescript theme={null}
const body = () => {
    const [selected, setSelected] = useState("home")

    return VStack([
        Picker("Tab", selected, setSelected, [
            Text("Home").tag("home"),
            Text("Profile").tag("profile")
        ])
    ]).onChange(selected, ([newVal]) => {
        console.log("Selected: " + newVal)
    })
}
```

### Triggering side effects

```typescript theme={null}
const body = () => {
    const [count, setCount] = useState(0)

    return VStack([
        Text(String(count)),
        Button("Increment", () => setCount(count + 1))
    ]).onChange(count, ([newVal]) => {
        if (newVal >= 10) {
            console.log("Reached limit")
        }
    })
}
```

## Notes

* The action only fires when the value actually changes, not on initial render. Use `.onAppear()` for initial setup.
* The callback receives both the new and old values as a tuple `[newValue, oldValue]`, allowing comparison logic.

## See Also

* [onAppear](/bindjs/modifiers/onAppear) -- runs an action on first appearance
* [onDisappear](/bindjs/modifiers/onDisappear) -- runs an action on removal