context.WithValueFunc(...)

with_value_func.go

package context

import (
	"context"
	"sync"
)

func WithValueFunc(ctx context.Context, key any, valFn func() any) context.Context {
	return &valFunc{Context: ctx, key: key, valFn: valFn}
}

type valFunc struct {
	context.Context
	key   any
	once  sync.Once
	valFn func() any
	val   any
}

var _ context.Context = &valFunc{}

func (v *valFunc) Value(key any) any {
	if key != v.key {
		return v.Context.Value(key)
	}
	v.once.Do(func() {
		v.val = v.valFn()
		v.valFn = nil
	})
	return v.val
}