commands: add switch-account command for composer

Switch accounts when in the composer mode. When switching accounts, the
From header, the crypto status and the address completer will be
updated.

Accounts can be switched with :switch-account <account-name>. The
completions for the switch-account command will list the available
accounts. If switch-account is run without arguments, the current
account name with the correct usage is displayed.

Fixes: https://todo.sr.ht/~rjarry/aerc/72
Signed-off-by: Koni Marti <koni.marti@gmail.com>
Acked-by: Robin Jarry <robin@jarry.cc>

commands/compose/switch.go

@@ -0,0 +1,47 @@
+package compose
+
+import (
+	"errors"
+	"fmt"
+
+	"git.sr.ht/~rjarry/aerc/widgets"
+)
+
+type AccountSwitcher interface {
+	SwitchAccount(*widgets.AccountView) error
+}
+
+type SwitchAccount struct{}
+
+func init() {
+	register(SwitchAccount{})
+}
+
+func (SwitchAccount) Aliases() []string {
+	return []string{"switch-account"}
+}
+
+func (SwitchAccount) Complete(aerc *widgets.Aerc, args []string) []string {
+	return aerc.AccountNames()
+}
+
+func (SwitchAccount) Execute(aerc *widgets.Aerc, args []string) error {
+	if len(args) != 2 {
+		name := ""
+		if acct := aerc.SelectedAccount(); acct != nil {
+			name = fmt.Sprintf("Current account: %s. ", acct.Name())
+		}
+		return errors.New(name + "Usage: switch-account <account-name>")
+	}
+
+	switcher, ok := aerc.SelectedTabContent().(AccountSwitcher)
+	if !ok {
+		return errors.New("this tab cannot switch accounts")
+	}
+
+	if acct, err := aerc.Account(args[1]); err != nil {
+		return err
+	} else {
+		return switcher.SwitchAccount(acct)
+	}
+}