Events

Register a Go function to get a handler ID, then reference that ID from an element. Handlers run in order on the goroutine that calls DispatchEvents, never during painting.

Registration	Use it with
app.OnClick(fn)	Node.OnClick
app.OnEvent(fn)	Node.Interact, BindKey, OnAction
app.OnTextChange(fn)	TextInputOptions.OnChange
app.RemoveHandler(id)	Removes a handler. Events already queued for it become harmless

Each function has the signature func(context.Context, *client.Event) error. A handler returning an error stops DispatchEvents, which returns that error. Handler IDs are never reused.

Event fields

Every *client.Event carries:

Field	Description
Sequence	Position in the event stream
Window, View	Handles of the source window and view
Revision	The view revision the event was captured from
ElementId	The element that reported the event
HandlerId	The handler it was dispatched to

Payloads by kind

Read the payload with the getter matching the kind. Getters for other kinds return nil, so one handler can serve several event kinds.

Getter	Reported by	Fields
GetClick()	OnClick	Position, Button, Modifiers, ClickCount
GetKeyDown()	Interaction.KeyDown	Key, Modifiers, Held
GetKeyUp()	Interaction.KeyUp	Key, Modifiers, Held
GetMouseDown()	Interaction.MouseDown	Position, Button, Modifiers
GetMouseUp()	Interaction.MouseUp	Position, Button, Modifiers
GetMouseMove()	Interaction.MouseMove	Position, Button, Modifiers, Dragging
GetScroll()	Interaction.Scroll	Position, Modifiers, Delta in pixels
GetFocusChanged()	Interaction.FocusChanged	Focused
GetFileDrop()	Interaction.FileDrop	Paths
GetDataDrop()	Interaction.DataDrop	Kind, Payload, Preview
GetAction()	Key bindings, OnAction	Name, Payload
GetTextChanged()	TextInputOptions.OnChange	Value, Epoch, EditRevision, Composing

Modifiers has the booleans Control, Alt, Shift and Platform, where Platform is the Command key on macOS. Button is client.MouseLeft, client.MouseRight or client.MouseMiddle.

One handler receiving three actions sent with Window.DispatchAction, reading ElementId and GetAction().

Handling several kinds

inspect := app.OnEvent(func(ctx context.Context, event *client.Event) error {
	switch kind := event.Kind.(type) {
	case *client.EventMouseDown:
		fmt.Println("down at", kind.MouseDown.Position.X, kind.MouseDown.Position.Y)
	case *client.EventScroll:
		fmt.Println("scrolled", kind.Scroll.Delta.Y)
	}
	return nil
})

Delivery

Events are held until the next NextEvents call acknowledges them, so canceling a pending pull can be retried without losing any. Acknowledgement means the batch was received, not that every handler succeeded.