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//go:build go1.19
package xsync
import (
"sync/atomic"
)
// Watchable contains a value. It is similar to an atomic.Pointer[T] but allows notifying callers
// that a new value has been set.
type Watchable[T any] struct {
p atomic.Pointer[watchableInner[T]]
}
type watchableInner[T any] struct {
t T
c chan struct{}
}
// Set sets the value in w and notifies callers of Value() that there is a new value.
func (w *Watchable[T]) Set(t T) {
newInner := &watchableInner[T]{
t: t,
c: make(chan struct{}),
}
oldInner := w.p.Swap(newInner)
if oldInner != nil {
close(oldInner.c)
}
}
// Value returns the current value inside w and a channel that will be closed when w is Set() to a
// newer value than the returned one.
//
// If called before the first Set(), returns the zero value of T.
//
// Normal usage has an observer waiting for new values in a loop:
//
// for {
// v, changed := w.Value()
//
// // do something with v
//
// <-changed
// }
//
// Note that the value in w may have changed multiple times between successive calls to Value(),
// Value() only ever returns the last-set value. This is by design so that slow observers cannot
// block Set(), unlike sending values on a channel.
func (w *Watchable[T]) Value() (T, chan struct{}) {
inner := w.p.Load()
if inner == nil {
// There's no inner, meaning w has not been Set() yet. Try filling it with an empty inner,
// so that we have a channel to listen on.
c := make(chan struct{})
emptyInner := &watchableInner[T]{
c: c,
}
// CompareAndSwap so we don't accidentally smash a real value that got put between our Load
// and here.
if w.p.CompareAndSwap(nil, emptyInner) {
var zero T
return zero, c
}
// If we fell through to here somebody Set() while we were trying to do this, so there's
// definitely an inner now.
inner = w.p.Load()
}
return inner.t, inner.c
}
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