What does "unidirectional data flow" mean?

Unidirectional data flow is a principle in which data in an application moves in only one direction, usually top-down: Model → ViewModel → View (or State → UI).

This means that:

• data is created or changed in one place (for example, in the state model),
• changes flow down the chain,
• the View does not change the state directly but sends "signals" (events) that are handled by the logic, creating a new flow of data.

This approach makes the application's behavior predictable: at any moment, the interface is simply a function of the current state, not the result of random changes from different places.