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// Package cfg implements a simple wrapper around go's flag package, in order to
// implement initialization hooks.
package cfg
import (
"context"
"flag"
"os"
)
// Cfger is a component which can be used with Cfg to setup its initialization.
type Cfger interface {
SetupCfg(*Cfg)
}
// Cfg is a wrapper around the stdlib's FlagSet and a set of initialization
// hooks.
type Cfg struct {
*flag.FlagSet
hooks []func(ctx context.Context) error
args []string
}
// New initializes and returns a new instance of *Cfg.
func New() *Cfg {
return &Cfg{
FlagSet: flag.NewFlagSet("", flag.ExitOnError),
args: os.Args[1:],
}
}
// OnInit appends the given callback to the sequence of hooks which will run on
// a call to Init.
func (c *Cfg) OnInit(cb func(context.Context) error) {
c.hooks = append(c.hooks, cb)
}
// Init runs all hooks registered using OnInit, in the same order OnInit was
// called. If one returns an error that error is returned and no further hooks
// are run.
func (c *Cfg) Init(ctx context.Context) error {
if err := c.FlagSet.Parse(c.args); err != nil {
return err
}
for _, h := range c.hooks {
if err := h(ctx); err != nil {
return err
}
}
return nil
}
// SubCmd should be called _after_ Init. Init will have consumed all arguments
// up until the first non-flag argument. This non-flag argument is a
// sub-command, and is returned by this method. This method also resets Cfg's
// internal state so that new options can be added to it.
//
// If there is no sub-command following the initial set of flags then this will
// return empty string.
func (c *Cfg) SubCmd() string {
c.args = c.FlagSet.Args()
if len(c.args) == 0 {
return ""
}
subCmd := c.args[0]
c.FlagSet = flag.NewFlagSet(subCmd, flag.ExitOnError)
c.hooks = nil
c.args = c.args[1:]
return subCmd
}
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