03 - MVU and messages

FlowUI uses a small Model-View-Update loop:

Role	Signature	Responsibility
Model	Any application value	Business state
Msg	A closed set of message types	What happened
Update	func(*M, Msg) Cmd[Msg]	Apply state changes and effects
View	func(*Context, M, Send[Msg]) Widget	Render the current state

Closed message sets

A private marker method keeps the message set explicit and makes type switches easy to review:

type Msg interface{ msg() }

type NameChanged struct{ Value string }
type Submitted struct{}

func (NameChanged) msg() {}
func (Submitted) msg() {}

Update handles messages and returns an optional command:

func Update(m *Model, msg Msg) ui.Cmd[Msg] {
    switch msg := msg.(type) {
    case NameChanged:
        m.Name = msg.Value
    case Submitted:
        return submitCmd(m.Name)
    }
    return nil
}

Single-window programs

Use NewProgram when the initial model is a fixed value:

ui.Run(ui.NewProgram(Model{}, Update, View),
    ui.Title("FlowUI app"),
    ui.Size(800, 600),
)

Use a full Program value when initialization returns a command or must create fresh reference-backed state:

ui.Run(ui.Program[Model, Msg]{
    Init: func() (Model, ui.Cmd[Msg]) {
        return Model{}, loadInitial()
    },
    Update: Update,
    Subscriptions: func(Model) []ui.Subscription[Msg] { return nil },
    View: View,
})

Nested modules

Child modules can keep their own model and messages. The parent wraps child messages with ui.MapCmd and maps the child's Send callback into a parent message. The complete pattern is shown in examples/modules.

Actions and commands

Cmd represents asynchronous effects such as network requests, file IO, and timers. Action and Command describe reusable menu, toolbar, and shortcut actions. They are related but serve different purposes.

Continue with Commands and subscriptions.