JsonView.render_custom_fields

You're seeing just the function render_custom_fields, go back to JsonView module for more information.
Link to this function

render_custom_fields(struct, view \\ nil, fields)

View Source

Render field with custom render function View module must defines render_field/2 function to render each custom field

use JsonView

def render_field(:is_success, item) do
  item.state > 3
end

# then this invoke above function
render_custom_fields(struct, __MODULE__, [:is_success])