base/Value

godoc

import "barista.run/base/value"

Value and ErrorValue provide atomic value storage with update notifications. Mostly used in modules to store configuration and select {} changes to the configuration. For example, most modules will update the output when either the interval elapses, or the output function is changed, so a common pattern is:

type Module struct {
	outputFormat value.Value
	// ...
}

// ...

func (m *Module) Stream(sink bar.Sink) {
	// ...
	for {
		select {
			case <-m.outputFormat.Next():
				format = m.outputFormat.Get().(/* output format type */)
			case <-scheduler.Tick():
				data = /* read new data */
		}
		// output with format, data
	}
}

Value

Value provides simple atomic storage and update notifications:

• Get() interface{}: Gets the previously stored value, or nil if nothing was stored.

• Set(interface{}): Stores a new value and notifies any listeners of the update.

• Next() <-chan struct{}: Listens for the next value update. Each call to Next() creates a new channel that will be closed the next time the value changes, so once the channel notifies it is no longer useful, and Next() should be called again for the next change of interest.

• Subscribe() (<-chan struct{}, func()): Subscribes to all future updates of the value, until the cleanup func is called. This adds a listener to the value, so make sure to call the done func when updates are no longer needed.

ErrorValue

ErrorValue extends Value with support for storing errors.

• Get() (interface{}, error): Gets the previously stored value or error. At most one will be set, but both can be nil if no value or error has been stored before.

• Set(interface{}): Sets a non-error value, clearing any previous error.

• Error(error) bool: If given a nil error, returns false and does not change the value. Otherwise clears any previously set value and replaces it with the error. This is similar to bar.Sink#Error, allowing code like:

  err, raw := /* something */
  if ev.Error(err) {
    return
  }
  data := process(raw)
  ev.Set(data)
  

• SetOrError(interface{}, error) bool: Combines Set(interface{}) and Error(error) bool. Primarily useful for directly using the return values from functions that return (value, error).

• Next() and Subscribe(): Same as Value.