Watchers Reference

Overview

Watchers connect Go's concurrency primitives to the go-tui event loop. A watcher is a goroutine that listens for something (a timer tick, a channel message) and queues a handler function on the main event loop when data arrives.

Because handlers run on the event loop, they can safely call state.Set(), state.Update(), and any other state mutation without synchronization. The watcher goroutine itself never touches UI state directly.

Watchers start when their owning component mounts (or when the element tree is set as the app root) and stop when the component unmounts or the app shuts down.

Watcher Interface

type Watcher interface {
    Start(eventQueue chan<- func(), stopCh <-chan struct{})
}

All watcher types implement this single-method interface.

Parameters:

You rarely call Start yourself. The framework calls it during component mounting after walking the element tree with WalkWatchers. If you're building a custom watcher, implement this interface and launch a goroutine that selects on both your data source and stopCh.

type myWatcher struct {
    handler func()
}

func (w *myWatcher) Start(eventQueue chan<- func(), stopCh <-chan struct{}) {
    go func() {
        for {
            select {
            case <-stopCh:
                return
            case <-someCondition:
                select {
                case eventQueue <- w.handler:
                case <-stopCh:
                    return
                }
            }
        }
    }()
}

The double-select pattern (first waiting for data, then trying to enqueue) prevents the goroutine from blocking on a full event queue when the app is shutting down.

OnTimer

func OnTimer(interval time.Duration, handler func()) Watcher

Creates a watcher that fires handler at a fixed interval. The handler runs on the main event loop.

Under the hood, OnTimer uses time.NewTicker. The ticker is cleaned up automatically when stopCh closes.

func (s *stopwatch) Watchers() []tui.Watcher {
    return []tui.Watcher{
        tui.OnTimer(time.Second, func() {
            s.elapsed.Update(func(v int) int { return v + 1 })
        }),
    }
}

The interval is approximate. The handler won't fire faster than the app's frame rate, and a backed-up event queue can introduce extra delay. For UI animation, intervals below 16ms (60fps) rarely produce visible improvement.

ChannelWatcher

NewChannelWatcher

func NewChannelWatcher[T any](ch <-chan T, fn func(T)) *ChannelWatcher[T]

Creates a watcher that reads from a Go channel and calls fn for each value received. The handler runs on the main event loop, not in the goroutine that reads the channel.

When the channel closes, the watcher exits its goroutine. When stopCh closes (app shutdown), the watcher also exits.

dataCh := make(chan string)
w := tui.NewChannelWatcher(dataCh, func(s string) {
    messages.Update(func(list []string) []string {
        return append(list, s)
    })
})

Watch

func Watch[T any](ch <-chan T, handler func(T)) Watcher

A convenience wrapper around NewChannelWatcher. Returns the Watcher interface directly, which is what Watchers() expects.

func (f *feed) Watchers() []tui.Watcher {
    return []tui.Watcher{
        tui.Watch(f.dataCh, func(item DataItem) {
            f.items.Update(func(list []DataItem) []DataItem {
                return append(list, item)
            })
        }),
    }
}

Use NewChannelWatcher when you need the concrete *ChannelWatcher[T] type; use Watch when you just need a Watcher to return from Watchers().

Start

func (w *ChannelWatcher[T]) Start(eventQueue chan<- func(), stopCh <-chan struct{})

Launches the goroutine that reads from the channel. Called automatically by the framework. The goroutine exits when either the channel closes or stopCh is closed.

OnChange

func OnChange[T any](state *State[T], handler func(T)) Watcher

Creates a watcher that calls handler when a State[T] value changes. The handler also fires once at startup with the current value.

Unlike OnTimer and Watch, this watcher doesn't spawn a goroutine for its main work. It uses State.Bind internally, so the handler runs synchronously during State.Set(). A small goroutine waits for the stop signal and removes the binding on shutdown.

func (w *myApp) Watchers() []tui.Watcher {
    return []tui.Watcher{
        tui.OnChange(w.selectedTab, func(tab string) {
            w.contentArea.ScrollTo(0)
        }),
    }
}

Use OnChange for side effects that should happen in response to state changes but don't belong in Render(): scrolling, logging, triggering secondary state updates, or kicking off background work via channels.

Start

func (w *stateWatcher[T]) Start(eventQueue chan<- func(), stopCh <-chan struct{})

Called automatically by the framework. Fires the handler once with the current state value, binds to future changes, and launches a goroutine that waits for stopCh to close before unbinding.

WatcherProvider Interface

type WatcherProvider interface {
    Watchers() []Watcher
}

Implement this on a struct component to attach watchers to its lifecycle. The framework calls Watchers() after the component mounts and starts each returned watcher.

When the component unmounts, the framework closes the stop channel, which signals all watchers to exit.

type dashboard struct {
    elapsed  *tui.State[int]
    messages *tui.State[[]string]
    dataCh   <-chan string
}

func Dashboard(dataCh <-chan string) *dashboard {
    return &dashboard{
        elapsed:  tui.NewState(0),
        messages: tui.NewState([]string{}),
        dataCh:   dataCh,
    }
}

func (d *dashboard) Watchers() []tui.Watcher {
    return []tui.Watcher{
        tui.OnTimer(time.Second, func() {
            d.elapsed.Update(func(v int) int { return v + 1 })
        }),
        tui.Watch(d.dataCh, func(msg string) {
            d.messages.Update(func(list []string) []string {
                return append(list, msg)
            })
        }),
    }
}

Element-Level Watchers

Watchers can also be attached directly to elements, outside the component model.

AddWatcher

func (e *Element) AddWatcher(w Watcher)

Attaches a watcher to a specific element. The watcher starts when the element tree is set as the app root.

Watchers

func (e *Element) Watchers() []Watcher

Returns all watchers attached to the element.

WalkWatchers

func (e *Element) WalkWatchers(fn func(Watcher))

Recursively walks the element tree and calls fn for each attached watcher. Skips hidden elements and their subtrees. The framework uses this during applyRoot to discover and start all watchers.

Lifecycle

Watchers follow this lifecycle:

1. Watchers are created in Watchers() or attached via AddWatcher.
2. The framework calls Start(eventQueue, stopCh) after the component mounts or the root is set. A goroutine begins running.
3. The goroutine loops, sending handler functions to the event queue.
4. When the component unmounts, the root changes, or the app stops, the framework closes stopCh. The goroutine detects the closed channel and exits.

Each time the root changes (via SetRoot, SetRootView, or SetRootComponent), the previous stop channel is closed and a new one is created. This ensures old watchers don't leak.

Thread Safety

Watcher handlers run on the main event loop. This makes them safe for state mutation:

• Call state.Set() and state.Update() freely inside handlers.
• Access element properties without locks.
• Emit events via Events[T].Emit().

Note: OnChange handlers run synchronously during State.Set() rather than being queued through the event loop. Since Set() must be called from the main loop, the same safety guarantees apply.

If you have a background goroutine that is not a watcher, use app.QueueUpdate to safely run code on the event loop:

go func() {
    result := expensiveComputation()
    app.QueueUpdate(func() {
        data.Set(result)
    })
}()

See Also

• State Reference — reactive state that watcher handlers typically modify
• App Reference — app lifecycle, QueueUpdate, component mounting
• Events Reference — keyboard and mouse event handling