material-components_material-components-ios/components/TextFields/docs/usage.md

A text field that conforms to MDCTextInput can be added to a view hierarchy the same way UITextField and UIView are. But to achieve the animations and presentations defined by the guidelines (floating placeholders, character counts), a controller that conforms to protocol MDCTextInputController must be initialized to manage the text field.

NOTE: Expect to interact with both the text field (for the traditional API) and the controller (for changes affecting the presentation and state).