# Runtime Considerations for Flow Screen Components

When you’re configuring components for flows, review these considerations for defining attributes and flow runtime behavior.

- **Handle `null` default input values.**

  If a flow admin doesn't set a default input value for a custom screen component, Flow Builder passes a `null` value at component run time. To set a default value at run time, add a `null` handler.

- **Handle hidden component output values.**

  If a screen component is hidden, all output attributes are set to `null` when the user navigates away from the screen.

- **Notify runtime of attribute changes.**

  Lightning web components use events to report changes from components to flows. To notify the flow runtime of attribute changes, a component fires the `FlowAttributeChangeEvent` event. For example, use this event when using conditional visibility to render a screen component when conditions are met or when mapping the value of output attributes to flow variables. For code samples, see `lightning-flow-support`.

- **Consider the default attribute behavior.**

  All flow screen component input attributes are also output attributes by default.

**See Also**

- [*Salesforce Help*: Flow Builder](https://help.salesforce.com/articleView?id=flow_builder.htm\&type=5)
- [Component Reference: lightning-flow-support](https://developer.salesforce.com/docs/platform/lightning-component-reference/guide/lightning-flow-support)