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# Custom Event Loops
## Overview
go-tui has three ways to drive its event loop. `Run()` works for most apps, but when you need to integrate external event sources like LLM streaming, network I/O, or background workers, you can take control of the loop using `Open`, `Step`, `Events`, `Dispatch`, `Render`, and `Close`.
## The Standard Loop (Run)
`Run()` manages signal setup, input reading, event dispatch, dirty checking, rendering, and frame timing. Background goroutines push data into the UI through `QueueUpdate`.
```go
func runMode() {
comp := NewFeedApp("Run()")
app, err := tui.NewApp(tui.WithRootComponent(comp))
if err != nil {
fmt.Fprintf(os.Stderr, "Error: %v\n", err)
os.Exit(1)
}
defer app.Close()
// The producer must use QueueUpdate to safely mutate state from its
// goroutine. This is the only way to get external data into the UI
// when Run() owns the event loop.
go func() {
for i := 1; ; i++ {
time.Sleep(200 * time.Millisecond)
if comp.IsPaused() {
continue
}
msg := fmt.Sprintf("[%s] Message #%d", time.Now().Format("15:04:05.000"), i)
app.QueueUpdate(func() {
comp.AddMessage(msg)
})
}
}()
if err := app.Run(); err != nil {
fmt.Fprintf(os.Stderr, "Error: %v\n", err)
os.Exit(1)
}
}
```
Because `Run()` owns the event loop, all external data must funnel through `QueueUpdate` so that state mutations happen on the main goroutine.
## Owning the Frame Loop (Step)
`Open()` performs the same startup work as `Run()` (registering signals, starting the input reader, performing the initial render) but returns immediately instead of blocking.
After that, your code controls when frames happen. `Step()` combines `DispatchEvents()` and `Render()` into a single call. Between steps you can read from your own channels and mutate state directly, since you are the main goroutine.
```go
func stepMode() {
comp := NewFeedApp("Step()")
app, err := tui.NewApp(tui.WithRootComponent(comp))
if err != nil {
fmt.Fprintf(os.Stderr, "Error: %v\n", err)
os.Exit(1)
}
if err := app.Open(); err != nil {
fmt.Fprintf(os.Stderr, "Error: %v\n", err)
os.Exit(1)
}
defer app.Close()
msgCh := startProducer(comp.IsPaused)
ticker := time.NewTicker(16 * time.Millisecond)
defer ticker.Stop()
for {
// Wait for next frame (acts as a frame rate limiter)
select {
case <-ticker.C:
case <-app.StopCh():
return
}
// Drain pending messages from the producer. This runs on the
// main goroutine so we can mutate state directly.
drain:
for {
select {
case msg := <-msgCh:
comp.AddMessage(msg)
default:
break drain
}
}
if !app.Step() {
return
}
}
}
```
The ticker controls frame rate. Between ticks, the drain loop pulls all pending messages from the producer channel and updates state without `QueueUpdate`. `Step()` returns `false` when the app should exit.
`Close()` restores terminal state and is safe to call multiple times.
## Full Control with Select (Events)
`Events()` returns a read-only channel you can use in a standard Go `select`, placing your external channels alongside go-tui events as peers.
```go
func selectMode() {
comp := NewFeedApp("Select()")
app, err := tui.NewApp(tui.WithRootComponent(comp))
if err != nil {
fmt.Fprintf(os.Stderr, "Error: %v\n", err)
os.Exit(1)
}
if err := app.Open(); err != nil {
fmt.Fprintf(os.Stderr, "Error: %v\n", err)
os.Exit(1)
}
defer app.Close()
msgCh := startProducer(comp.IsPaused)
for {
select {
case ev := <-app.Events():
app.Dispatch(ev)
case msg := <-msgCh:
comp.AddMessage(msg)
case <-app.StopCh():
return
}
app.Render()
}
}
```
`Dispatch(ev)` routes the event through the key/mouse/resize dispatch system. `Render()` checks for dirty state and redraws if needed, so calling it after every select case is fine since it short-circuits when nothing changed.
This is the cleanest option when you have external event sources, because each source gets its own select case instead of funneling through `QueueUpdate`.
## When to Use Which
Start with **`Run()`**. It handles frame timing, signal setup, and event dispatch internally. If your app receives external data, `QueueUpdate` and channel watchers cover most cases without leaving `Run()`.
Use **`Step()`** when you need to control frame timing yourself, for example to implement variable frame rates, skip rendering during heavy computation, or pause the render loop entirely while waiting for a resource. It also lets you drain your own channels between frames without `QueueUpdate`, which avoids the overhead of serializing closures through the event queue.
Use **`Events()` + select** when you are building something that is driven by multiple event sources at once, like a chat client that handles keyboard input, incoming messages, and connection status changes in a single loop. Each source gets its own select case, which is the standard Go pattern for multiplexing channels.
## Complete Example
The UI component lives in `feed.gsx` and displays a scrollable message feed with pause/resume and sticky-bottom scrolling.
```gsx
package main
import (
"fmt"
"math"
tui "github.com/grindlemire/go-tui"
)
type feedApp struct {
messages *tui.State[[]string]
paused *tui.State[bool]
scrollY *tui.State[int]
stickToBottom *tui.State[bool]
content *tui.Ref
mode string
}
func NewFeedApp(mode string) *feedApp {
return &feedApp{
messages: tui.NewState([]string{}),
paused: tui.NewState(false),
scrollY: tui.NewState(0),
stickToBottom: tui.NewState(false),
content: tui.NewRef(),
mode: mode,
}
}
func (f *feedApp) scrollBy(delta int) {
el := f.content.El()
if el == nil {
return
}
_, curY := el.ScrollOffset()
_, maxY := el.MaxScroll()
newY := curY + delta
if newY < 0 {
newY = 0
} else if newY > maxY {
newY = maxY
}
f.scrollY.Set(newY)
f.stickToBottom.Set(false)
}
func (f *feedApp) KeyMap() tui.KeyMap {
return tui.KeyMap{
tui.OnStop(tui.KeyEscape, func(ke tui.KeyEvent) { ke.App().Stop() }),
tui.OnStop(tui.Rune('q'), func(ke tui.KeyEvent) { ke.App().Stop() }),
tui.OnStop(tui.Rune('p'), func(ke tui.KeyEvent) {
f.paused.Set(!f.paused.Get())
}),
tui.OnStop(tui.Rune('s'), func(ke tui.KeyEvent) {
if f.stickToBottom.Get() {
if el := f.content.El(); el != nil {
_, y := el.ScrollOffset()
f.scrollY.Set(y)
}
f.stickToBottom.Set(false)
} else {
f.stickToBottom.Set(true)
}
}),
tui.On(tui.Rune('j'), func(ke tui.KeyEvent) { f.scrollBy(1) }),
tui.On(tui.Rune('k'), func(ke tui.KeyEvent) { f.scrollBy(-1) }),
tui.On(tui.KeyUp, func(ke tui.KeyEvent) { f.scrollBy(-1) }),
tui.On(tui.KeyDown, func(ke tui.KeyEvent) { f.scrollBy(1) }),
tui.On(tui.KeyPageUp, func(ke tui.KeyEvent) { f.scrollBy(-10) }),
tui.On(tui.KeyPageDown, func(ke tui.KeyEvent) { f.scrollBy(10) }),
tui.On(tui.KeyHome, func(ke tui.KeyEvent) {
f.scrollY.Set(0)
f.stickToBottom.Set(false)
}),
}
}
func (f *feedApp) HandleMouse(me tui.MouseEvent) bool {
switch me.Button {
case tui.MouseWheelUp:
f.scrollBy(-1)
return true
case tui.MouseWheelDown:
f.scrollBy(1)
return true
}
return false
}
func (f *feedApp) AddMessage(msg string) {
f.messages.Update(func(msgs []string) []string {
return append(msgs, msg)
})
if f.stickToBottom.Get() {
f.scrollY.Set(math.MaxInt)
}
}
func (f *feedApp) IsPaused() bool {
return f.paused.Get()
}
templ (f *feedApp) Render() {
<div class="flex-col h-full border-rounded border-cyan">
<div class="flex justify-between px-1 shrink-0">
<span class="text-gradient-cyan-magenta font-bold">Event Loop Demo</span>
<div class="flex gap-1">
<span class="font-dim">mode:</span>
<span class="text-cyan font-bold">{f.mode}</span>
</div>
</div>
<hr />
<div
ref={f.content}
class="flex-col flex-grow border-single p-1"
scrollable={tui.ScrollVertical}
scrollOffset={0, f.scrollY.Get()}
>
for _, msg := range f.messages.Get() {
<span class="font-dim">{msg}</span>
}
</div>
<hr />
<div class="flex justify-between px-1 shrink-0">
<div class="flex gap-2">
<span class="font-dim">p: pause</span>
<span class="font-dim">s: sticky</span>
<span class="font-dim">j/k: scroll</span>
<span class="font-dim">q: quit</span>
</div>
</div>
</div>
}
```
The `main.go` file contains the `startProducer` helper and all three modes:
```go
package main
import (
"fmt"
"os"
"time"
tui "github.com/grindlemire/go-tui"
)
//go:generate go run ../../cmd/tui generate feed.gsx
func main() {
mode := "run"
if len(os.Args) > 1 {
mode = os.Args[1]
}
switch mode {
case "run":
runMode()
case "step":
stepMode()
case "select":
selectMode()
default:
fmt.Fprintf(os.Stderr, "Unknown mode %q. Use: run, step, or select\n", mode)
os.Exit(1)
}
}
func startProducer(paused func() bool) <-chan string {
ch := make(chan string, 10)
go func() {
for i := 1; ; i++ {
time.Sleep(200 * time.Millisecond)
if paused() {
continue
}
select {
case ch <- fmt.Sprintf("[%s] Message #%d", time.Now().Format("15:04:05.000"), i):
default:
}
}
}()
return ch
}
// ... runMode, stepMode, selectMode as shown above ...
```
Generate and run:
```bash
tui generate ./...
go run . run # Standard Run() mode
go run . step # Step-based loop
go run . select # Select-based loop
```
All three modes produce the same UI, and the difference is how the event loop is wired:
## Next Steps
- [Streaming Data](streaming) for channel watchers, auto-scroll, and the producer pattern
- [Watchers](watchers) for timers, channels, and the WatcherProvider interface
- [App Reference](../reference/app) for full documentation of Open, Step, Events, Dispatch, Render, and Close.
|